THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES LECTURES ON MATHEMATICS 1 THE EYANSTON COLLOQUIUM LECTURES ON MATHEMATICS DELIVERED FROM AUG. 28 TO SEPT. 9, 1893 BEFORE MEMBERS OF THE CONCHESS OF MA THEM A TICS HELD IN CONNECTION WITH THE WORLD'S FAIR IN CHICAGO AT NORTHWESTERN UNIVERSITY EVANSTON, ILL. BY FELIX KLEIN REPORTED BY ALEXANDER ZIWET REPUBLISHED BY THE AMERICAN MATHEMATICAL SOCIETY NEW YORK 1911 All rights reserved COPYRIGHT, 1893, BY MACMILLAN AND CO. NortoooU J. S. Gushing & Co. Berwick & Smith. Boston, Mass., U.S.A. Sciences Library 1 To reproduce after a lapse of seventeen years lectures which at the time they were delivered were in such close contact with the most recent work of that day, may well call for a word of justification. Has mathematics not advanced since then, and are the questions here treated of first importance at the present time ? I reply by asking : What is important in the develop- ment of mathematics ? Is it solely the attainment of new re- sults of potential value, or must not an essential part of the best scientific efforts of each new age be devoted to possessing itself of the heritage of the age that has just preceded it ? It is a noble example that Professor Klein has set all ages of mathematicians that, beginning his activity at a time when the contributions of the immediate past were so rich and so unre- lated, he was able to uncover the essential bonds that connect them and to discern the fields to whose development the new methods were best adapted. His instinct for that which is vital in mathematics is sure, and the light with which his treatment illumines the problems here considered may well serve as a guide for the youth who is approaching the study of the problems of a later day. WILLIAM F. OSGOOD. CAMBRIDGE, December 31, 1910. 918576 PREFACE. THE Congress of Mathematics held under the auspices of the World's Fair Auxiliary in Chicago, from the 2ist to the 26th of August, 1893, was attended by Professor Felix Klein of the University of Gottingen, as one of the commissioners of the German university exhibit at the Columbian Exposition. After the adjournment of the Congress, Professor Klein kindly consented to hold a colloquium on mathematics with such mem- bers of the Congress as might wish to participate. The North- western University at Evanston, 111., tendered the use of rooms for this purpose and placed a collection of mathematical books from its library at the disposal of the members of the collo- quium. The following is a list of the members attending the colloquium : W. W. BEMAN, A.M., professor of mathematics, University of Michigan. E. M. BLAKE, Ph.D., instructor in mathematics, Columbia College. O. BOLZA, Ph.D., associate professor of mathematics, University of Chicago. H. T. EDDY, Ph.D., president of the Rose Polytechnic Institute. A. M. ELY, A.B., professor of mathematics, Vassar College. F. FRANKLIN, Ph.D., professor of mathematics, Johns Hopkins University. T. F. HOLGATE, Ph.D., instructor in mathematics, Northwestern University. L. S. HULBURT, A.M., instructor in mathematics, Johns Hopkins University. F. H. LOUD, A.B., professor of mathematics and astronomy, Colorado College. J. McMAHON, A.M., assistant professor of mathematics, Cornell University. H. MASCHKE, Ph.D., assistant professor of mathematics, University of Chicago. E. H. MOORE, Ph.D., professor of mathematics, University of Chicago. viii PREFACE. J. E. OLIVER, A.M., professor of mathematics, Cornell University. A. M. SAWIN, Sc.M., Evanston. W. E. STORY, Ph.D., professor of mathematics, Clark University. E. STUDY, Ph.D., professor of mathematics, University of Marburg. H. TABER, Ph.D., assistant professor of mathematics, Clark University. H. W. TYLER, Ph.D., professor of mathematics. Massachusetts Institute of Technology. J. M. VAN VLECK, A.M., LL.D., professor of mathematics and astronomy, Wesleyan University. E. B. VAN VLECK, Ph.D., instructor in mathematics, University of Wis- consin. C. A. WALDO, A.M., professor of mathematics, De Pauw University. H. S. WHITE, Ph.D., associate professor of mathematics, Northwestern Uni- versity. M. F. WINSTON, A.B., honorary fellow in mathematics, University of Chicago. A. ZIWET, assistant professor of mathematics, University of Michigan. The meetings lasted from August 28th till September 9th ; and in the course of these two weeks Professor Klein gave a daily lecture, besides devoting a large portion of his time to personal intercourse and conferences with those attending the meetings. The lectures were delivered freely, in the English language, substantially in the form in which they are here given to the public. The only change made consists in oblit- erating the conversational form of the frequent questions and discussions by means of which Professor Klein understands so well to enliven his discourse. My notes, after being written out each day, were carefully revised by Professor Klein him- self, both in manuscript and in the proofs. As an appendix it has been thought proper to give a transla- tion of the interesting historical sketch contributed by Professor Klein to the work Die deutschen Universitaten. The translation was prepared by Professor H. W. Tyler, of the Massachusetts Institute of Technology. It is to be hoped that the proceedings of the Chicago Con- gress of Mathematics, in which Professor Klein took a leading PREFACE. ix part, will soon be published in full. The papers presented to this Congress, and the discussions that followed their reading, form an important complement to the Evanston colloquium. Indeed, in reading the lectures here published, it should be kept in mind that they followed immediately upon the adjournment of the Chicago meeting, and were addressed to members of the Congress. This circumstance, in addition to the limited time and the informal character of the colloquium, must account for the incompleteness with which the various subjects are treated. In concluding, the editor wishes to express his thanks to Professors W. W. Beman and H. S. White for aid in preparing the manuscript and correcting the proofs. ALEXANDER ZIWET. ANN ARBOR, MICH., November, 1893. The new edition is printed from the original plates. The only changes in the text consist in the correction of a few misprints. A. Z. ANN ARBOR, January, 1911. CONTENTS. LECTURE PAGE I. Clebsch i II. Sophus Lie 9 III. Sophus Lie .......... 18 IV. On the Real Shape of Algebraic Curves and Surfaces . . 25 V. Theory of Functions and Geometry . ... . . -33 VI. On the Mathematical Character of Space-Intuition, and the Relation of Pure Mathematics to the Applied Sciences . . 41 VII. The Transcendency of the Numbers e and TT .... 51 VIII. Ideal Numbers 58 IX. The Solution of Higher Algebraic Equations .... 67 X. On Some Recent Advances in Hyperelliptic and Abelian Func- tions 75 XI. The Most Recent Researches in Non-Euclidean Geometry . 85 XII. The Study of Mathematics at Gottingen 94 The Development of Mathematics at the German Universities . 99 LECTURES ON MATHEMATICS. LECTURE I. : CLEBSCH. (August 28, 1893.) IT will be the object of our Colloquia to pass in review some of the principal phases of the most recent development of math- ematical thought in Germany. A brief sketch of the growth of mathematics in the German universities in the course of the present century has been con- tributed by me to the work Die deutschen Universitdten, com- piled and edited by Professor Lexis (Berlin, Asher, 1893), for the exhibit of the German universities at the World's Fair.* The strictly objective point of view that had to be adopted for this sketch made it necessary to break off the account about the year 1870. In the present more informal lectures these restrictions both as to time and point of view are abandoned. It is just the period since 1870 that I intend to deal with, and I shall speak of it in a more subjective manner, insisting par- ticularly on those features of the development of mathematics in which I have taken part myself either by personal work or by direct observation. The first week will be devoted largely to Geometry, taking this term in its broadest sense ; and in this first lecture it will surely be appropriate to select the celebrated geometer Clebsch * A translation of this sketch will be found in the Appendix, p. 99. 2 LECTURE 1. as the central figure, partly because he was one of my principal teachers, and also for the reason that his work is so well known in this country. Among mathematicians in general, three main categories may be distinguished ; and perhaps the names logicians, formalists, and intuitionists may serve to characterize them, (i) The word logician is here used, of course, without reference to the mathe- matical logic of Boole, Peirce, etc. ; it is only intended to indi- cate that the main strength of the men belonging to this class lies in their logical and critical power, in their ability to give strict definitions, and to derive rigid deductions therefrom. The great . and wholesome influence exerted in Germany by Weierstrass in this direction is well known. (2) The formalists among the mathematicians excel mainly in the skilful formal treatment of a given question, in devising for it an "algorithm." Gordan, or let us say Cayley and Sylvester, must be ranged in this group. (3) To the intuitionists, finally, belong those who lay particular stress on geometrical intuition (Anschauung), not ir; pure geometry only, but in all branches of mathematics. What Benjamin Peirce has called " geometrizing a mathematical question " seems to express the same idea. Lord Kelvin and von Staudt may be mentioned as types of this category. Clebsch must be said to belong both to the second and third of these categories, while I should class myself with the third, and also the first. For this reason my account of Clebsch's work will be incomplete ; but this will hardly prove a serious drawback, considering that the part of his work characterized by the second of the above categories is already so fully appre- ciated here in America. In general, it is my intention here, not so much to give a complete account of any subject, as to supplement the mathematical views that I find prevalent in this country. CLEBSCH. 3 As the first achievement of Clebsch we must set down the introduction into Germany of the work done previously by Cayley and Sylvester in England. But he not only trans- planted to German soil their theory of invariants and the inter- pretation of projective geometry by means of this theory; he also brought this theory into live and fruitful correlation" with the fundamental ideas of Riemann's theory of functions. In the former respect, it may be sufficient to refer to Clebsch's Vorlesungen iiber Geometric, edited and continued by Linde- mann ; to his Bindre algebraische Formen, and in general to what he did in co-operation with Gordan. A good historical account of his work will be found in the biography of Clebsch published in the Math. Annalen, Vol. 7. Riemann's celebrated memoir of 1857* presented the new ideas on the theory of functions in a somewhat startling novel form that prevented their immediate acceptance and recogni- tion. He based the theory of the Abelian integrals and their inverse, the Abelian functions, on the idea of the surface now so well known by his name, and on the corresponding funda- mental theorems of existence (Existenztheoreme). Clebsch, by taking as his starting-point an algebraic curve defined by its equation, made the theory more accessible to the mathema- ticians of his time, and added a more concrete interest to it by the geometrical theorems that he deduced from the theory of Abelian functions. Clebsch's paper, Ueber die Anwendung der AbeV scJten Functionen in der Geometrie,^ and the work of Clebsch and Gordan on Abelian functions,^ are well known to American mathematicians ; and in accordance with my plan, I proceed to give merely some critical remarks. * Theorie der AbeFschen Functionen, Journal fur reine und angewandte Mathe- matik, Vol. 54 (1857), pp. 115-155; reprinted in Riemann's Werke, 1876, pp. 81-135. t Journal fur reine und angewandte Mathematik, Vol. 63 (1864), pp. 189-24.3. \ Theori' der AbePschen Functionen, Leipzig, Teubner, rR66. 4 LECTURE I. However great the achievement of Clebsch's in making the work of Riemann more easy of access to his contempo- raries, it is my opinion that at the present time the book of Clebsch is no longer to be considered as the standard work for an introduction to the study of Abelian functions. The chief objections to Clebsch's presentation are twofold : they can be briefly characterized as a lack of mathematical rigour on the one hand, and a loss of intuitiveness, of geometrical perspicuity, on the other. A few examples will explain my meaning. (a) Clebsch bases his whole investigation on the considera- tion of what he takes to be the most general type of an algebraic curve, and this general curve he assumes as having only double points, but no other singularities. To obtain a sure foundation for the theory, it must be proved that any algebraic curve can be transformed rationally into a curve having only double points. This proof was not given by Clebsch ; it has since been supplied by his pupils and follow- ers, but the demonstration is long and involved. See the papers by Brill and Nother in the Math. Annalen, Vol. 7 (1874),* and by Nother, ib., Vol. 23 (1884)-! Another defect of the same kind occurs in connection with the determinant of the periods of the Abelian integrals. This determinant never vanishes as long as the curve is irredu- cible. But Clebsch and Gordan neglect to prove this ; and however simple the proof may be, this must be regarded as an inexactness. The apparent lack of critical spirit which we find in the work of Clebsch is characteristic of the geometrical epoch in which * Ueber die algebraischen Functional und ihre Anivendung in der Geometric^ pp. 269-310. t Rationale Aus/uhrung der Operationen in der Theorie der algebraischen Func- tionen, pp. 311-358. CLEBSCH. 5 he lived, the epoch of Steiner, among others. It detracts in no- wise from the merit of his work. But the influence of the theory of functions has taught the present generation to be more exacting. (b) The second objection to adopting Clebsch's presentation lies in the fact that, from Riemann's point of view, many points of the theory become far more simple and almost self-evident, whereas in Clebsch's theory they are not brought out in all their beauty. An example of this is presented by the idea of the deficiency p. In Riemann's theory, where p represents the order of connectivity of the surface, the invariability of p under any rational transformation is self-evident, while from the point of view of Clebsch this invariability must be proved by means of a long elimination, without affording the true geometrical insight into its meaning. For these reasons it seems to me best to begin the theory of Abelian functions with Riemann's ideas, without, however, neglecting to give later the purely algebraical developments. This method is adopted in my paper on Abelian functions ; * it is also followed in the work Die elliptischen Modulfunctionen, Vols. I. and II., edited by Dr. Fricke. A general account of the historical development of the theory of algebraic curves in con- nection with Riemann's ideas will be found in my (lithographed) lectures on Riemannsche Flachen, delivered in 1891-92.! If this arrangement be adopted, it is interesting to follow out the true relation that the algebraical developments bear to Riemann's theory. Thus in Brill and Nother's theory, the so-called fundamental theorem of Nother is of primary impor- * Zur Theorie der AbeVschen Functionen, Math. Annalen, Vol. 36 (1890), pp. l-8 3 . t My lithographed lectures frequently give only an outline of the subject, omit- ting details and long demonstrations, which are supposed to be supplied by the student by private reading and a study of the literature of the subject. 6 LECTURE I. tance. It gives a rule for deciding under what conditions an algebraic rational integral function f of x and y can be put into the form where and i|r are likewise rational algebraic functions. Each point of intersection of the curves < = o and i/r=o must of course be a point of the curve f=o. But there remains the question of multiple and singular points ; and this is disposed of by Nother's theorem. Now it is of great interest to in- vestigate how these relations present themselves when the starting-point is taken from Riemann's ideas. One of the best illustrations of the utility of adopting Riemann's principles is presented by the very remarkable advance made recently by Hurwitz, in the theory of algebraic curves, in particular his extension of the theory of algebraic correspondences, an account of which is given in the second volume of the Elliptische Modulfunctionen. Cayley had found as a fundamental theorem in this theory a rule for determining the number of self-corresponding points for algebraic corre- spondences of a simple kind. A whole series of very valuable papers by Brill, published in the Math. Annalen,* is devoted to the further investigation and demonstration of this theorem. Now Hurwitz, attacking the problem from the point of view of Riemann's ideas, arrives not only at a more simple and quite general demonstration of Cayley's rule, but proceeds to a complete study of all possible algebraic correspondences. He finds that while for general curves the correspondences consid- * Ueber z-wei Beruhrungsprobleme, Vol. 4 (1871), pp. 527-549. Ueber Ent- sprechen von Punktsystemen auf einer Curve, Vol. 6 (1873), pp. 33-65. Ueber die Correspondenzformel, Vol. 7 (1874), pp. 607-622. Ueber algebraische Correspon- denten, Vol. 31 (1888), pp. 374-409. Ueber algebraische Correspondenzen. Zweite Abhandlung: Specialgruppen von Punkten einer algebraischen Curve, Vol. 36 (1890), pp. 321-360. CLEBSCH. 7 ered by Cayley and Brill are the only ones that exist, in the case of singular curves there are other correspondences which also can be treated completely. These singular curves are characterized by certain linear relations with integral coeffi- cients, connecting the periods of their Abelian integrals. Let us now turn to that side of Clebsch's method which appears to me to be the most important, and which certainly must be recognized as being of great and permanent value ; I mean the generalization, obtained by Clebsch, of the whole theory of Abelian integrals to a theory of algebraic functions with several variables. By applying the methods he had developed for functions of the form f(x, y) o, or in homo- geneous co-ordinates, f(x^ x.^ x^) = o, to functions with four homogeneous variables f(x^, x^, x s , x^)o, he found in 1868, that there also exists a number / that remains invariant under all rational transformations of the surface f o. Clebsch arrives at this result by considering double integrals belonging to the surface. It is evident that this theory could not have been found from Riemann's point of view. There is no difficulty in conceiving a four-dimensional Riemann space corresponding to an equation f(x, y, z)=o. But the difficulty would lie in proving the "theorems of existence" for such a space; and it may even be doubted whether analogous theorems hold in such a space. While to Clebsch is due the fundamental idea of this grand generalization, the working out of this theory was left to his pupils and followers. The work was mainly carried on by Nother, who showed, in the case of algebraic surfaces, the existence of more than one invariant number / and of corresponding moduli, i.e. constants not changed by one-to-one transformations. Italian and French mathematicians, in partic- ular Picard and Poincar6, have also contributed largely to the further development of the theory. 8 LECTURE I. If the value of a man of science is to be gauged not by his general activity in all directions, but solely by the fruitful new ideas that he has first introduced into his science, then the theory just considered must be regarded as the most valuable work of Clebsch. In close connection with the preceding are the general ideas put forth by Clebsch in his last memoir,* ideas to which he himself attached great importance. This memoir implies an application, as it were, of the theory of Abelian functions to the theory of differential equations. It is well known that the central problem of the whole of modern mathematics is the study of the transcendental functions defined by differential equations. Now Clebsch, led by the analogy of his theory of Abelian integrals, proceeds somewhat as follows. Let us con- sider, for example, an ordinary differential equation of the first order f(x, y, y')=o, where f represents an algebraic function. Regarding y 1 as a third variable z, we have the equation of an algebraic surface. Just as the Abelian integrals can be classi- fied according to the properties of the fundamental curve that remain unchanged under a rational transformation, so Clebsch proposes to classify the transcendental functions defined by the differential equations according to the invariant properties of the corresponding surf aces f=o under rational one-to-one transformations. The theory of differential equations is just now being culti- vated very extensively by French mathematicians ; and some of them proceed precisely from this point of view first adopted by Clebsch. * Ueber tin neues Grundgebilde der analytischen Gtometrie der Ebene, Math. Annalen, Vol. 6 (1873), pp. 203-215. LECTURE II.: SOPHUS LIE. (August 29, 1893.) To fully understand the mathematical genius of Sophus Lie, one must not turn to the books recently published by him in collaboration with Dr. Engel, but to his earlier memoirs, written during the first years of his scientific career. There Lie shows himself the true geometer that he is, while in his later publi- cations, finding that he was but imperfectly understood by the mathematicians accustomed to the analytical point of view, he adopted a very general analytical form of treatment that is not always easy to follow. Fortunately, I had the advantage of becoming intimately acquainted with Lie's ideas at a very early period, when they were still, as the chemists say, in the "nascent state," and thus most effective in producing a strong reaction. My lecture to-day will therefore be devoted chiefly to his paper, " Ueber Complexe, insbesondere Linien- und Kugel-Complexe, mil Anw en- dung auf die Theorie partieller Differentialgleichungen." * To define the place of this paper in the historical develop- ment of geometry, a word must be said of two eminent geome- ters of an earlier period: Pliicker (1801-68) and Monge (1746- 1818). Plucker's name is familiar to every mathematician, through his formulae relating to algebraic curves. But what is of importance in the present connection is his generalized idea * Math. Annalen, Vol. 5 (1872), pp. 145-256. 9 I0 LECTURE II. of the space-element. The ordinary geometry with the point as element deals with space as three-dimensioned, conformably to the three constants determining the position of a point. A dual transformation gives the plane as element ; space in this case has also three dimensions, as there are three independent con- stants in the equation of the plane. If, however, the straight line be selected as space-element, space must be considered as four-dimensional, since four independent constants determine a straight line. Again, if a quadric surface F% be taken as element, space will have nine dimensions, because every such element requires nine quantities for its determination, viz. the nine independent constants of the surface F 2 ; in other words, space contains 9 quadric surfaces. This conception of hyper- spaces must be clearly distinguished from that of Grassmann and others. Pliicker, indeed, rejected any other idea of a space of more than three dimensions as too abstruse. The work of Monge that is here of importance, is his Application de r analyse a la gtomttrie, 1809 (reprinted 1850), in which he treats of ordinary and partial differential equations of the first and second order, and applies these to geometrical questions such as the curvature of surfaces, their lines of curvature, geodesic lines, etc. The treatment of geometrical problems by means of the differential and integral calculus is one feature of this work ; the other, perhaps even more important, is the con- verse of this, viz. the application of geometrical intuition to questions of analysis. Now this last feature is one of the most prominent character- istics of Lie's work; he increases its power by adopting Pliicker's idea of a generalized space-element and extending this funda- mental conception. A few examples will best serve to give an idea of the character of his work ; as such an example I select (as I have done elsewhere before) Lie's sphere-geometry (Kugel- geometrie). SOPHUS LIE. 1 1 Taking the equation of a sphere in the form x 2 -\-y* -f- zr 2 Bx 2 Cy 2 Dz -f E = o, the coefficients, j?, C, D, E, can be regarded as the co-ordinates of the sphere, and ordinary space appears accordingly as a manifoldness of four dimensions. For the radius, R, of the sphere we have as a relation connecting the fifth quantity, R, with the four co- ordinates, B, C, D, E. To introduce homogeneous co-ordinates, put B=*, C= c -, D= d -, E= e -, R= r -; a a a a a then a : b :c : d: e are the five homogeneous co-ordinates of the sphere, and the sixth quantity r is related to them by means of the homogeneous equation of the second degree, ? 3 = &* + <*+ K ~ K = oo 10 substitutions. By adopting this defi- nition we obtain point-transformations of a simple character. The geometrical meaning of equation (2) is that the radius is zero. Every sphere of vanishing radius, i.e. every point, is therefore transformed into a point. Moreover, as the polar 2bl>' + 2cc' + 2dd' ae' a'e = o remains likewise unchanged in the transformation, it follows that orthogonal spheres are transformed into orthogonal spheres. Thus the group of the elementary sphere-geometry is character- ized as the conformal group, well known as that of the trans- formation by inversion (or reciprocal radii) and through its applications in mathematical physics. Darboux has further developed this elementary sphere- geometry. Any equation of the second degree F(a, b, c, d, e) = o, taken in connection with the relation (2) represents a point- surface which Darboux has called cyclide. From the point of view of ordinary projective geometry, the cyclide is a surface of the fourth order containing the imaginary circle common to all spheres of space as a double curve. A careful investigation ders-Universitat zu Erlangen. Erlangen, Deichert, 1872. For an English transla- tion, by Haskell, see the Bulletin of the New York Mathematical Society, Vol. 2 (1893), PP- 215-249. SOPHUS LIE. I3 of these cyclides will be found in Darboux's Lemons sur la thtorie ge"nerale des surfaces et les applications ge'ome'triqiies du calcnl infinitesimal, and elsewhere. As the ordinary surfaces of the second degree can be regarded as special cases of cyclides, we have here a method for generalizing the known properties of quadric surfaces by extending them to cyclides. Thus Mr. M. Bocher, of Harvard University, in his dissertation,* has treated the extension of a problem in the theory of the poten- tial from the known case of a body bounded by surfaces of the second degree to a body bounded by cyclides. A more extended publication on this subject by Mr. Bocher will appear in a few months (Leipzig, Teubner). In the higher sphere-geometry of Lie, the six homogeneous co-ordinates a:b:c:d:e:r are connected, as mentioned above, by the homogeneous equation of the second degree, The corresponding group is selected as the group of the linear substitutions transforming this equation into itself. We have thus a group of oo 36 " 2 ^^ 15 substitutions. But this is not a group of point-transformations ; for a sphere of radius zero becomes a sphere whose radius is in general different from zero. Thus, putting for instance B' = , C' = C, D' = D, E'=E, R' = R + const., it appears that the transformation consists in a mere dilatation or expansion of each sphere, a point becoming a sphere of given radius. The meaning of the polar equation z bb' + 2 cc ] + 2 dd 1 2 rr' ae' a'e = o * Ueber die Reihenenhuickelungen der Potentialtheorie, gekronte Preisschrift, Gottingen, Dieterich, 1891. I4 LECTURE II. remaining invariant for any transformation of the group, is evi- dently that the spheres originally in contact remain in contact. The group belongs therefore to the important class of contact- transformations, which will soon be considered more in detail. In studying any particular geometry, such as Lie's sphere- geometry, two methods present themselves. (i) We may consider equations of various degrees and inquire what they represent. In devising names for the different con- figurations so obtained, Lie used the names introduced by Pliicker in his line-geometry. Thus a single equation, F(a, b, c, d, e, r) = o, is said to represent a complex of the first, second, etc., degree, according to the degree of the equation ; a complex contains, therefore, oo 3 spheres. Two such equations, represent a congruency containing oo 2 spheres. Three equations, F! = o, F 2 = o, F 3 = o, may be said to represent a set of spheres, the number being oo 1 . It is to be noticed that in each case the equation of the second degree, p + c 2 + 13 , /> 14 , / 34 , /42 /23 b the usual six homogeneous co-ordinates, where /= p K . Then we have the identity /12/34 +/13/42 4-/14/23 = > and we take as group the QO 15 linear substitutions transforming this equation into itself. This group corresponds to the totality of collineations and reciprocations, i.e. to the projective group. The reason for this lies in the fact that the polar equation expresses the intersection of the two lines /, p'-. T 6 LECTURE II. Now Lie has instituted a comparison of the highest interest between the line-geometry of Pliicker and his own sphere- geometry. In each of these geometries there occur six homo- geneous co-ordinates connected by a homogeneous equation of the second degree. The discriminant of each equation is differ- ent from zero. It follows that we can pass from either of these geometries to the other by linear substitutions. Thus, to trans- form PvtPu + /13/42 into it is sufficient to assume, say, pu = b + iV, p K = d+ r, pu = -a, It follows from the linear character of the substitutions that the polar equations are likewise transformed into each other. Thus we have the remarkable result that two spheres that touch correspond to two lines that intersect. It is worthy of notice that the equations of transformation involve the imaginary unit / ; and the law of inertia of quadratic forms shows at once that this introduction of the imaginary cannot be avoided, but is essential. To illustrate the value of this transformation of line-geometry into sphere-geometry, and vice versa, let us consider three linear equations, ^l = o, ^, = 0, ^3 = 0, the variables being either line co-ordinates or sphere co-ordi- nates. In the former case the three equations represent a set of lines ; i.e. one of the two sets of straight lines of a hyper- boloid of one sheet. It is well known that each line of either set intersects all the lines of the other. Transforming to sphere- SOPHUS LIE. i 7 geometry, we obtain a set of spheres corresponding to each set of lines ; and every sphere of either set must touch every sphere of the other set. This gives a configuration well known in geometry from other investigations ; viz. all these spheres envelop a surface known as Dupin's cyclide. We have thus found a noteworthy correlation between the hyper- boloid of one sheet and Dupin's cyclide. Perhaps the most striking example of the fruitfulness of this work of Lie's is his discovery that by means of this transfor- mation the lines of curvature of a surface are transformed into asymptotic lines of the transformed surface, and vice versa. This appears by taking the definition given above for the lines of curvature and translating it word for word into the language of line-geometry. Two problems in the infinitesimal geome- try of surfaces, that had long been regarded as entirely distinct, are thus shown to be really identical. This must certainly be regarded as one of the most elegant contributions to differential geometry made in recent times. LECTURE III.: SOPHUS LIE. (August 30, 1893.) THE distinction between analytic and algebraic functions, so important in pure analysis, enters also into the treatment of geometry. Analytic functions are those that can be represented by a power series, convergent within a certain region bounded by the so-called circle of convergence. Outside of this region the analytic function is not regarded as given a priori ; its continuation into wider regions remains a matter of special investigation and may give very different results, according to the particular case considered. On the other hand, an algebraic function, w = Alg. (z), is supposed to be known for the whole complex plane, having a finite number of values for every value of z. Similarly, in geometry, we may confine our attention to a limited portion of an analytic curve or surface, as, for instance, in constructing the tangent, investigating the curvature, etc. ; or we may have to consider the whole extent of algebraic curves and surfaces in space. Almost the whole of the applications of the differential and integral calculus to geometry belongs to the former branch of geometry ; and as this is what we are mainly concerned with in the present lecture, we need not restrict ourselves to algebraic functions, but may use the more general analytic functions confining ourselves always to limited portions of space. I 18 SOPHUS LIE. l g thought it advisable to state this here once for all, since here in America the consideration of algebraic curves has perhaps been too predominant. The possibility of introducing new elements of space has been pointed out in the preceding lecture. To-day we shall use again a new space-element, consisting of an infinitesimal portion of a surface (or rather of its tangent plane) with a definite point in it. This is called, though not very properly, a surface-element (Flachenelement), and may perhaps be likened to an infinitesi- mal fish-scale. From a more abstract point of view it may be defined as simply the combination of a plane with a point in it. As the equation of a plane passing through a point (x, y, z) can be written in the form x \y 'y z 1 being the current co-ordinates, we have x, y, z, p, q as the co-ordinates of our surface-element, so that space becomes a fivefold manifoldness. If homogeneous co-ordinates be used, the point (*j, x v x^ x^ and the plane (tt lt 2 > M & u ^i passing through it are connected by the condition = O expressing their united position ; and the number of indepen- dent constants is 3 + 31=5, as before. Let us now see how ordinary geometry appears in this representation. A point, being the locus of all surface-elements passing through it, is represented as a manifoldness of two dimensions, let us say for shortness, an M 2 . A curve is repre- sented by the totality of all those surface-elements that have their point on the curve and their plane passing through the tangent ; these elements form again an M 2 . Finally, a surface is given by those surface-elements that have their point on the 20 LECTURE III. surface and their plane coincident with the tangent plane of the surface ; they, too, form an M z . Moreover, all these M z 's have an important property in common : any two consecutive surface-elements belonging to the same point, curve, or surface always satisfy the condition dz pdx qdy = o, which is a simple case of a Pfaffian relation ; and conversely, if two surface-elements satisfy this condition, they belong to the same point, curve, or surface, as the case may be. Thus we have the highly interesting result that in the geome- try of surface-elements points as well as curves and surfaces are brought under one head, being all represented by twofold mani- foldnesses having the property just explained. This definition is the more important as there are no other M z 's having the same property. We now proceed to consider the very general kind of trans- formations called by Lie contact-transformations. They are transformations that change our element (x, y, z, p, q) into (x',y' t z',p', q'} by such substitutions x' = (x,y, z,p, q), y' = (x,y,z,p,q), z' = --, /=, ?'=, as will transform into itself the linear differential equation dz pdx qdy = o. The geometrical meaning of the transformation is evidently that any M z having the given property is changed into an M^ having the same property. Thus, for instance, a surface is transformed generally into a surface, or in special cases into a point or a curve. Moreover, let us consider two manifoldnesses M 2 having a contact, i.e. having a surface-element in common ; these M z 's are changed by the transformation into two other M^s> having SOPHUS LIE. 21 also a contact. From this characteristic the name given by Lie to the transformation will be understood. Contact-transformations are so important, and occur so fre- quently, that particular cases attracted the attention of geome- ters long ago, though not under this name and from this point of view, i.e. not as contact-transformations, so that the true insight into their nature could not be obtained. Numerous examples of contact-transformations are given in my (lithographed) lectures on Hohere Geometrie, delivered during the winter-semester of 1892-93. Thus, an example in two dimensions is found in the problem of wheel-gearing. The outline of the tooth of one wheel being given, it is here required to find the outline of the tooth of the other wheel, as I explained to you in my lecture at the Chicago Exhibition, with the aid of the models in the German university exhibit. Another example is found in the theory of perturbations in astronomy ; Lagrange's method of variation of parameters as applied to the problem of three bodies is equivalent to a contact-transformation in a higher space. The group of oo 15 substitutions considered yesterday in line-geometry is also a group of contact-transformations, both the collineations and reciprocations having this character. The reciprocations give the first well-known instance of the transformation of a point into a plane (i.e. a surface), and a curve into a developable (i.e. also a surface). These trans- formations of curves will here be considered as transforming the elements of the points or curves into the elements of the surface. Finally, we have examples of contact-transformations, not only in the transformations of spheres discussed in the last lecture, but even in the general transition from the line- geometry of Pliicker to the sphere-geometry of Lie. Let us consider this last case somewhat more in detail. 22 LECTURE III. First of all, two lines that intersect have, of course, a surface-element in common ; and as the two corresponding spheres must also have a surface-element in common, they will be in contact, as is actually the case for our transformation. It will be of interest to consider more closely the correlation between the surface-elements of a line and those of a sphere, although it is given by imaginary formulae. Take, for instance, the totality of the surface-elements belonging to a circle on one of the spheres ; we may call this a circular set of elements. In line-geometry there corresponds the set of surface-elements along a generating line of a skew surface ; and so on. The theorem regarding the transformation of the curves of curva- ture into asymptotic lines becomes now self-evident. Instead of the curve of curvature of a surface we have here to con- sider the corresponding elements of the surface which we may call a ctirvature set. Similarly, an asymptotic line is replaced by the elements of the surface along this line ; to this the name osculating set may be given. The correspondence between the two sets is brought out immediately by considering that two consecutive elements of a curvature set belong to the same sphere, while two consecutive elements of an osculating set belong to the same straight line. One of the most important applications of contact-transforma- tions is found in the theory of partial differential equations ; I shall here confine myself to partial differential equations of the first order. From our new point of view, this theory assumes a much higher degree of perspicuity, and the true meaning of the terms "solution," "general solution," "com- plete solution," " singular solution," introduced by Lagrange and Monge, is brought out with much greater clearness. Let us consider the partial differential equation of the first order f(x, y, z, p, q) = o. SOPHUS LIE. 23 In the older theory, a distinction is made according to the way in which / and q enter into the equation. Thus, when / and q enter only in the first degree, the equation is called linear. If/ and q should happen to be both absent, the equation would not be regarded as a differential equation at all. From the higher point of view of Lie's new geometry, this distinction disappears entirely, as will be seen in what follows. The number of all surface elements in the whole of space is of course oo 6 . By writing down our equation we single out from these a manifoldness of four dimensions, J/ 4 , of oo 4 ele- ments. Now, to find a " solution " of the equation in Lie's sense means to single out from this M a twofold manifoldness, M y of the characteristic property ; whether this M% be a point, a curve, or a surface, is here regarded as indifferent. What Lagrange calls finding a "complete solution" consists in dividing the J/ 4 into oo 2 M 2 's. This can of course be done in an infinite number of ways. Finally, if any singly infinite set be taken out of the oo 2 M 2 's, we have in the envelope of this set what Lagrange calls a "general solution." These formulations hold quite generally for all partial differential equations of the first order, even for the most specialized forms. To illustrate, by an example, in what sense an equation of the form f(x, y, z)=Q may be regarded as a partial differ- ential equation and what is the meaning of its solutions, let us consider the very special case 2 = 0. While in ordinary co-ordinates this equation represents all the points of the xy- plane, in Lie's system it represents of course all the surface- elements whose points lie in the plane. Nothing is so simple as to assign a "complete solution" in this case; we have only to take the oo 2 points of the plane themselves, each point being an M 2 of the equation. To derive from this the " general solu- tion," we must take all possible singly infinite sets of points in the plane, i.e. any curve whatever, and form the envelope 24 LECTURE III. of the surface-elements belonging to the points ; in other words, we must take the elements touching the curve. Finally, the plane itself represents of course a "singular solution." Now, the very high interest and importance of this simple illustration lies in the fact that by a contact-transformation every partial differential equation of the first order can be changed into this particular form z=o. Hence the whole dis- position of the solutions outlined above holds quite generally. A new and deeper insight is thus gained through Lie's theory into the meaning of problems that have long been regarded as classical, while at the same time a full array of new problems is brought to light and finds here its answer. It can here only be briefly mentioned that Lie has done much in applying similar principles to the theory of partial differential equations of the second order. At the present time Lie is best known through his theory of continuous groups of transformations, and at first glance it might appear as if there were but little connection between this theory and the geometrical considerations that engaged our attention in the last two lectures. I think it therefore desira- ble to point out here this connection. It has been the final aim of Lie from the beginning to make progress in the theory of differential equations ; and as subsidiary to this end may be regarded both the geometrical developments considered in these lectures and the theory of continuous groups. For further particulars concerning the subjects of the present as well as the two preceding lectures, I may refer to my (litho- graphed) lectures on Hohere Geometric, delivered at Gottingen, in 1892-93. The theory of surface-elements is also fully devel- oped in the second volume of the Theorie der Transformations- gruppen, by Lie and Engel (Leipzig, Teubner, 1890). LECTURE IV. : ON THE REAL SHAPE OF ALGE- BRAIC CURVES AND SURFACES. (August 31, 1893.) WE turn now to algebraic functions, and in particular to the question of the actual geometric forms corresponding to such functions. The question as to the reality of geometric forms and the actual shape of algebraic curves and surfaces was some- what neglected for a long time. Otherwise it would be difficult to explain, for instance, why the connection between Cayley's theory of projective measurement and the non-Euclidean geom- etry should not have been perceived at once. As these ques- tions are even now less well known than they deserve to be, I proceed to give here an historical sketch of the subject, without, however, attempting completeness. It must be counted among the lasting merits of Sir Isaac Newton that he first investigated the shape of the plane curves of the third order. His Enumeratio linearum tertii ordinis* shows that he had a very clear conception of projective geometry ; for he says that all curves of the third order can be derived by central projection from five fundamental types (Fig. i). But I wish to direct your particular attention to the paper by Mobius, Ueber die Grundformen der Linien der dritten Ordnung,^ where the forms of the cubic curves are derived by * First published as an appendix to Newton's Opticks, 1 704. f Abhandlungen der Konigl. Sachsischen Gesellschaft der Wissenschaften, math.- phys. Klasse, Vol. I (1852), pp. 1-82; reprinted in Mobius' Gesammelte Werke, Vol. II (1886), pp. 89-176. 26 LECTURE IV. purely geometric considerations. Owing to its remarkable elegance of treatment, this paper has given the impulse to all the subsequent researches in this line that I shall have to mention. In 1872 we considered, in Gottingen, the question as to the shape of surfaces of the third order. As a particular case, Clebsch at this time constructed his beautiful model of the Fig. 1. diagonal surface, with 27 real lines, which I showed to you at the Exhibition. The equation of this surface may be written in the simple form = o which shows that the surface can be transformed into itself by the 1 20 permutations of the xs. It may here be mentioned as a general rule, that in select- ing a particular case for constructing a model the first pre- requisite is regularity. By selecting a symmetrical form for the model, not only is the execution simplified, but what is of more importance, the model will be of such a character as to impress itself readily on the mind. Instigated by this investigation of Clebsch, I turned to the general problem of determining all possible forms of cubic sur- ALGEBRAIC CURVES AND SURFACES. 27 faces.* I established the fact that by the principle of continu- ity all forms of real surfaces of the third order can be derived from the particular surface having four real conical points. This surface, also, I exhibited to you at the World's Fair, and pointed out how the diagonal surface can be derived from it. But what is of primary importance is the completeness of enumeration resulting from my point of view ; it would be of comparatively little value to derive any number of special forms if it cannot be proved that the method used exhausts the subject. Models of the typical cases of all the principal forms of cubic surfaces have since been constructed by Rodenberg for Brill's collection. In the 7th volume of the Math. Annalen (1874) Zeuthenf has discussed the various forms of plane curves of the fourth order (Q). He considers in particular the reality of the double tangents on these curves. The number of such tangents is 28, and they are all real when the curve consists of four sepa- rate closed portions (Fig. 2). What is of par- ticular interest is the relation of Zeuthen's researches on quartic curves to my own re- p . _ searches on cubic surfaces, as explained by Zeuthen himself.} It had been observed before, by Geiser, that if a cubic surface be projected on a plane from a point on the surface, the contour of the projection is a quartic curve, and that every quartic curve can be generated in this way. If a surface with four conical points be chosen, the resulting quartic has four double points ; that is, it breaks up into two conies * See my paper Ueber Flachen driller Ordnung, Math. Annalen, Vol. 6 (1873), pp. 551-581. t Sur les differenles formes des courbes planes du qualrieme ordre, pp. 410-432. J tudes des proprieles de situation des surfaces cubiques, Math. Annalen, Vol. 8 (1875), pp. 1-30. d D 28 LECTURE IV. (Fig. 3). By considering the shaded portions in the figure it will readily be seen how, by the principle of continuity, the four ovals of the quartic (Fig. 2) are obtained. This corresponds exactly to the derivation of the diagonal surface from the cubic surface having four conical points. The attempts to extend this application of the principle of continuity so as to gain an insight into the shape of curves of the nth order have hitherto proved futile, as far as a general classification and an enu- meration of all fundamental forms is concerned. Still, some important results have been obtained. A paper by Harnack* and a more recent one by Hilbertf are here to be mentioned. Harnack finds that, if / be the deficiency of the curve, the maximum number of separate branches the curve can have is /+i; and a curve with p+i branches actually exists. Hil- bert's paper contains a large number of interesting special results which from their nature cannot be included in the present brief summary. I myself have found a curious relation between the numbers of real singularities.^: Denoting the order of the curve by , the class by k, and considering only simple singularities, we may have three kinds of double points, say d' ordinary and d" isolated real double points, besides imaginary double points ; then there may be r 1 real cusps, besides imaginary cusps ; and similarly, by the principle of duality, /' ordinary, /" isolated * Ueber die Vieltheiligkeit der ebcnen algebraischen Curven, Math. Annalen, Vol. 10 (1876), pp. 189-198. f Ueber die reellen ZUge algebraischer Curven, Math. Annalen, Vol. 38 (1891), pp. 115-138. J Eine neue Relation zwischen den Singularitdten einer algebraischen Curve, Math. Annalen, Vol. 10 (1876), pp. 199-209. ALGEBRAIC CURVES AND SURFACES. 29 real double tangents, besides imaginary double tangents ; also w' real inflexions, besides imaginary inflexions. Then it can be proved by means of the principle of continuity, that the following relation must hold : n + w' + 2 1" = k + r 1 + 2 d". This general law contains everything that is known as to curves of the third or fourth orders. It has been somewhat extended in a more algebraic sense by several writers. More- over, Brill, in Vol. 16 of the Math. Annalen (1880),* has shown how the formula must be modified when higher singularities are involved. As regards quartic surfaces, Rohn has investigated an enor- mous number of special cases ; but a complete enumeration he has not reached. Among the special surfaces of the fourth order the Kum- mer surface with 16 conical points is one of the most important. The models constructed by Plucker in connection with his theory of com- plexes of lines all represent special cases of the Kummer surface. Some types of this surface are also included in the Brill collection. But all these models are now of less importance, since Rohn found the following in- teresting and comprehensive result. Fig. 4. Imagine a quadric surface with four generating lines of each set (Fig. 4). According to the character of the surface and the reality, non-reality, or coincidence of these lines, a large number of special cases is possible ; all these cases, however, must be * Ueber Singular it aten ebener algebraischer Curven und eine neue Curvenspecies, pp. 348-408. 30 LECTURE IV. treated alike. We may here confine ourselves to the case of an hyperboloid of one sheet with four distinct lines of each set. These lines divide the surface into 16 regions. Shading the alternate regions as in the figure, and regarding the shaded regions as double, the unshaded regions being disregarded, we have a surface consisting of eight separate closed portions hang- ing together only at the points of intersection of the lines ; and this is a Kummer surface with 16 real double points. Rohn's researches on the Kummer surface will be found in the Math. Annalen, Vol. 18 (1881);* his more general investigations on quartic surfaces, ib., Vol. 29 (1887)-! There is still another mode of dealing with the shape of curves (not of surfaces), viz. by means of the theory of Rie- mann. The first problem that here presents itself is to estab- lish the connection between a plane curve and a Riemann sur- face, as I have done in Vol. 7 of the Math. Annalen (1874)4 Let us consider a cubic curve ; its deficiency is /= I. Now it is well known that in Riemann's theory this deficiency is a measure of the connectivity of the corresponding Riemann sur- face, which, therefore, in the present case, must be that of a torus, or anchor-ring. The question then arises : what has the anchor-ring to do with the cubic curve ? The connection will best be understood by considering the curve of the third class whose shape is represented in Fig. 5. It is easy to see that of the three tangents that can be drawn to this curve from any point in its plane, all three will be real if the point be selected outside the oval branch, or inside the triangular branch ; but that only one of the three tangents will be real for any point in the shaded region, while the other two tangents are imaginary. As * Die verschiedenen Gestalten der Kummer 'schen Flache, pp. 99-159. t Die Flachen vierter Ordnung hinsichtlich ihrer Knotenpunkte und ihrer Ge- staltung, pp. 81-96. \ Ueber eine neue Art der Riemann'' schen Flachen, pp. 558566. ALGEBRAIC CURVES AND SURFACES. 31 there are thus two imaginary tangents corresponding to each point of this region, let us imagine it covered with a double leaf ; along the curve the two leaves must, of course, be regarded as joined. Thus we obtain a surface which can be considered as a Riemann surface belonging to the curve, each point of the surface corresponding to a single tangent of the curve. Here, then, we have our anchor-ring. If on such a sur- face we study integrals, they will be of double periodicity, and the true reason is thus disclosed for the connection of elliptic Fig. 5. integrals with the curves of the third class, and hence, owing to the relation of duality, with the curves of the third order. To make a further advance, I passed to the general theory of Riemann surfaces. To real curves will of course correspond symmetrical Riemann surfaces, i.e. surfaces that reproduce themselves by a conformal transformation of the second kind (i.e. a transformation that inverts the sense of the angles). Now it is easy to enumerate the different symmetrical types belonging to a given /. The result is that there are altogether />+i " diasymmetric " and \ " orthosymmetric " cases. If we denote as a line of symmetry any line whose points 32 LECTURE IV. remain unchanged by the conformal transformation, the dia- symmetric cases contain respectively /,/!, 2, I, o lines of symmetry, and the orthosymmetric cases contain /+ I, / i, / 3)-" such lines. A surface is called diasymmetric or ortho- symmetric according as it does not or does break up into two parts by cuts carried along all the lines of symmetry. This enumeration, then, will contain a general classification of real curves, as indicated first in my pamphlet on Riemann's theory.* In the summer of 1892 I resumed the theory and developed a large number of propositions concerning the reality of the roots of those equations connected with our curves that can be treated by means of the Abelian integrals. Compare the last volume of the Math. Annalen^ and my (lithographed) lectures on Riemanrische Flachen, Part II. In the same manner in which we have to-day considered ordinary algebraic curves and surfaces, it would be interesting to investigate all algebraic configurations so as to arrive at a truly geometrical intuition of these objects. In concluding, I wish to insist in particular on what I regard as the principal characteristic of the geometrical methods that I have discussed to-day : these methods give us an actual mental image of the configuration under discussion, and this I consider as most essential in all true geometry. For this reason the so-called synthetic methods, as usually developed, do not appear to me very satisfactory. While giving elaborate constructions for special cases and details they fail entirely to afford a general view of the configurations as a whole. * Ueber Riemanri's Theorie der algebraischen Functionen und ihrer Integrate, Leipzig, Teubner, 1882. An English translation by Frances Hardcastle (London, Macmillan) has just appeared. f Ueber Realitatsverhaltnisse bei der einem beliebigen Geschlechte zugehorigcn Normalcurve der , Vol. 42 (1893), pp. 1-29. LECTURE V. : THEORY OF FUNCTIONS AND GEOMETRY. (September i, 1893.) A GEOMETRICAL representation of a function of a complex variable w=f(z), where w = u + iv and z=x + iy, can be ob- tained by constructing models of the two surfaces u = (x,y), v=*ty-(x,y). This idea is realized in the models constructed by Dyck, which I have shown to you at the Exhibition. Another well-known method, proposed by Riemann, consists in representing each of the two complex variables in the usual way in a plane. To every point in the ^-plane will correspond one or more points in the w-plane ; as z moves in its plane, w describes a corresponding curve in the other plane. I may refer to the work of Holzmiiller* as a good elementary intro- duction to this subject, especially on account of the large number of special cases there worked out and illustrated by drawings. In higher investigations, what is of interest is not so much the corresponding curves as corresponding areas or regions of the two planes. According to Riemann's fundamental theorem concerning conformal representation, two simply con- nected regions can always be made to correspond to each other conformally, so that either is the conformal representation * Einfuhrung in die Theorie der isogonalen Verwandtschaften und der conformen Abbildungen, verbunden mit Anwendungen auf mathematische Physik, Leipzig, Teubner, 1882. 33 34 LECTURE V. (Abbildung) of the other. The three constants at our disposal in this correspondence allow us to select three arbitrary points on the boundary of one region as corresponding to three arbi- trary points on the boundary of the other region. Thus Riemann's theory affords a geometrical definition for any func- tion whatever by means of its conformal representation. This suggests the inquiry as to what conclusions can be drawn from this method concerning the nature of transcen- dental functions. Next to the elementary transcendental func- tions the elliptic functions are usually regarded as the most important. There is, however, another class for which at least equal importance must be claimed on account of their numerous applications in astronomy and mathematical physics ; these are the hypergeometric functions, so called owing to their connection with Gauss's hypergeometric series. The hypergeometric functions can be defined as the integrals of the following linear differential equation of the second order: i-X'-X", v w z b zc \(za)(zb}(zc) where 2= a, b, c are the three singular points and X', X" ; ///, /A" ; v', v" are the so-called exponents belonging respectively to a, b, c. If W-L be a particular solution, w 2 another, the general solution can be put in the form aw^^w^ where a, $ are arbitrary con- stants ; so that and YI represent a pair of general solutions. THEORY OF FUNCTIONS AND GEOMETRY. 35 If we now introduce the quotient ^ = 77 (2) as a new variable, ^v ft its most general value is an d contains there- fore three arbitrary constants. Hence 77 satisfies a differential equation of the third order which is readily found to be "i-X* - a) in which the left-hand member has the property of not being changed by a linear substitution, and is therefore called a differ- ential invariant. Cayley has named this function the Schwar- zian derivative ; it has formed the starting-point for Sylvester's investigations on reciprocants. In the right-hand member, X = X'-X", /t = /*'-/*", v = v'-v". As to the conformal representation (Fig. 6), it can be shown that the upper half of the ^-plane, with the points a, b, c on Fig. 6. the real axis and X, /*, v assumed as real, is transformed for each branch of the ^-function into a triangular area abc bounded by 36 LECTURE V. three circular arcs ; let us call such an area a circular triangle (Kreisbogendreieck}. The angles at the vertices of this triangle are \TT, /ATT, VTT. This, then, is the geometrical representation we have to take as our basis. In order to derive from it conclusions as to the nature of the transcendental functions defined by the differential equation, it will evidently be necessary to inquire what are the forms of such circular triangles in the most general case. For it is to be noticed that there is no restric- tion laid upon the values of the constants \, ft, v, so that the angles of our triangle are not necessarily acute, nor even convex ; in other words, in the general case the vertices will- be branch-points. The triangle itself is here to be regarded as something like an extensible and flexible membrane spread out between the circles forming the boundary. I have investigated this question in a paper published in the Math. Annalen, Vol. 37.* It will be convenient to project the plane containing the circular triangle stereographically on a sphere. The question then is as to the most general form of spherical triangles, taking this term in a generalized meaning as denoting any triangle on the sphere bounded by the inter- sections of three planes with the sphere, whether the planes intersect at the centre or not. This is really a question of elementary geometry ; and it is interesting to notice how often in recent times higher re- search has led back to elementary problems not previously settled. The result in the present case is that there are two, and only two, species of such generalized triangles. They are obtained from the so-called elementary triangle by two distinct operations : (a) lateral, (b) polar attachment of a circle. * Ueber die Nullstellen df.r hypergeometrischen Reihe, pp. 573-590. THEORY OF FUNCTIONS AND GEOMETRY. 37 Let abc (Fig. 7) be the elementary spherical triangle. Then the operation of lateral attachment consists in attaching to the area abc the area enclosed by one of the sides, say be, this side being produced so as to form a complete circle. The process can, of course, be repeated any number of times and applied to each side. If one circular area be attached at be, the angles at b and c are increased each by TT ; if the whole sphere be attached, by 2ir, etc. The vertices in this way become branch-points. A triangle so obtained I call a triangle of the first species. Fig. 7. A triangle of the second species is produced by the process of polar attachment of a circle, say at bc\ the whole area bounded by the circle be is, in this case, connected with the original triangle along a branch-cut reaching from the vertex a to some point on be. The point a becomes a branch-point, its angle being increased by 2?r. Moreover, lateral attach- ments can be made at ab and ac. The two species of triangles are now characterized as follows : the first species may have any number of lateral attachments at any or all of the three sides, while the second has a polar attachment to one vertex and the opposite side, and may have lateral attachments to the other two sides. 38 LECTURE V. Analytically the two species are distinguished by inequali- ties between the absolute values of the constants \, p, v. For the first species, none of the three constants is greater than the sum of the other two, i.e. for the second species, where A, refers to the pole. For the application to the theory of functions, it is impor- tant to determine, in the case of the second species, the number of times the circle formed by the side opposite the vertex is passed around. I have found this number to be E (- -), where E denotes the greatest positive integer contained in the argument, and is therefore always zero when this argument happens to be negative or fractional. Let us now apply these geometrical ideas to the theory of hypergeometric functions. I can here only point out one of the results obtained. Considering only the real values that ?7=a/ 1 /w 2 can assume between a and b, the question presents itself as to the shape of the i;-curve between these limits. Let us consider for a moment the curves w-^ and w 2 . It is well known that, if w oscillates between a and b from one side of the axis to the other, w 2 will also oscillate ; their quotient rj = zv 1 /zv z is represented by a curve that consists of separate branches extending from oo to +00, somewhat like the curve j/ = tan.r. Now it appears as the result of the investigation that the number of these branches, and therefore the number of the oscillations of zt> l and w 2 , is given precisely by the number of circuits of the point c ; that is to say, it is E\~ -} This is a result of importance for all V 2 / THEORY OF FUNCTIONS AND GEOMETRY. 39 applications of hypergeometric functions which was derived only later (by Hurwitz) by means of Sturm's methods. I wish to call your particular attention not so much to the result itself, however interesting it may be, as to the geometrical method adopted in deriving it. More advanced researches on a similar line of thought are now being carried on at Gottingen by myself and others. When a differential equation with a larger number of singular points than three is the object of investigation, the triangles must be replaced by quadrangles and other polygons. In my lithographed lectures on Linear Differential Equations, delivered in 1890-91, I have thrown out some suggestions regarding the treatment of such cases. The difficulty arising in these generalizations is, strange to say, merely of a geometrical nature, viz. the difficulty of obtaining a general view of the possible forms of the polygons. Meanwhile, Dr. Schoenflies has published a paper on recti- linear polygons of any number of sides* while Dr. Van Vleck has considered such rectilinear polygons together with the functions they define, the polygons being defined in so general a way as to admit branch-points even in the interior. Dr. Schoenflies has also treated the case of circular quadrangles, the result being somewhat complicated. In all these investigations the singular points of the ^-plane corresponding to the vertices of the polygons are of course assumed to be real, as are also their exponents. There remains the still more general question how to represent by conformal correspondence the functions in the case when some of these elements are complex. In this direction I have to mention the name of Dr. Schilling who has treated the case of the ordinary hypergeometric function on the assumption of complex exponents. * Ueber Kreisbogenpolygone, Math. Annalen, Vol. 42, pp. 377-408. 40 LECTURE V. This treatment of the functions defined by linear differential equations of the second order is of course only an example of the general discussion of complex functions by means of geometry. I hope that many more interesting results will be obtained in the future by such geometrical methods. LECTURE VI.: ON THE MATHEMATICAL CHAR- ACTER OF SPACE-INTUITION AND THE RELATION OF PURE MATHEMATICS TO THE APPLIED SCIENCES. (September 2, 1893.) IN the preceding lectures I have laid so much stress on geometrical methods that the inquiry naturally presents itself as to the real nature and limitations of geometrical intuition. In my address before the Congress of Mathematics at Chi- cago I referred to the distinction between what I called the naive and the refined intuition. It is the latter that we find in Euclid ; he carefully develops his system on the basis of well- formulated axioms, is fully conscious of the necessity of exact proofs, clearly distinguishes between the commensurable and incommensurable, and so forth. The na'fve intuition, on the other hand, was especially active during the period of the genesis of the differential and integral calculus. Thus we see that Newton assumes without hesitation the existence, in every case, of a velocity in a moving point, without troubling himself with the inquiry whether there might not be continuous functions having no derivative. At the present time we are wont to build up the infinitesi- mal calculus on a purely analytical basis, and this shows that we a*re living in a critical period similar to that of Euclid. It is my private conviction, although I may perhaps not be able to fully substantiate it with complete proofs, that Euclid's 41 42 LECTURE VI. period also must have been preceded by a "naive" stage of development. Several facts that have become known only quite recently point in this direction. Thus it is now known that the books that have come down to us from the time of Euclid constitute only a very small part of what was then in existence ; moreover, much of the teaching was done by oral tradition. Not many of the books had that artistic finish that we admire in Euclid's " Elements " ; the majority were in the form of improvised lectures, written out for the use of the students. The investigations of Zeuthen* and Allman f have done much to clear up these historical conditions. If we now ask how we can account for this distinction between the na'fve and refined intuition, I must say that, in my opinion, the root of the matter lies in the fact that the naive intuition is not exact, while the refined intuition is not properly intuition at all, but arises through the logical develop- ment from axioms considered as perfectly exact. To explain the meaning of the first half of this statement it is my opinion that, in our na'fve intuition, when thinking of a point we do not picture to our mind an abstract mathemati- cal point, but substitute something concrete for it. In imagin- ing a line, we do not picture to ourselves "length without breadth," but a strip of a certain width. Now such a strip has of course always x a tangent (Fig. 9) ; i.e. we can always imagine a straight strip having a small portion (element) in common with the curved strip ; similarly with respect to the osculating circle. The definitions in this case are regarded as holding only approximately, or as far as may be necessary. * Die Lehre von den Kegelschnitten im Altertum, ubersetzt von R. v. Fischer- Benzon, Kopenhagen, Host, 1886. f Greek geometry from Thales to Euclid, Dublin, Hodges, 1889. MATHEMATICAL CHARACTER OF SPACE-INTUITION. 43 The " exact " mathematicians will of course say that such definitions are not definitions at all. But I maintain that in ordinary life we actually operate with such inexact definitions. Thus we speak without hesitancy of the direction and curvature of a river or a road, although the " line" in this case has certainly considerable width. As regards the second half of my proposition, there actually are many cases where the conclusions derived by purely logical reasoning from exact definitions can no more be verified by intuition. To show this, I select examples from the theory of automorphic functions, because in more common geometrical illustrations our judgment is warped by the familiarity of the ideas. Let any number of non-intersecting circles I, 2, 3, 4, , be given (Fig. 10), and let every circle be reflected (i.e. transformed Fig. 10. by inversion, or reciprocal radii vectores) upon every other circle ; then repeat this operation again and again, ad infinitum. The question is, what will be the configuration formed by the totality 44 LECTURE VI. of all the circles, and in particular what will be the position of the limiting points. There is no difficulty in answering these questions by purely logical reasoning ; but the imagination seems to fail utterly when we try to form a mental image of the result. Again, let a series of circles be given, each circle touching the following, while the last touches the first (Fig. 1 1). Every circle is now reflected upon every other just as in the preceding exam- ple, and the process is repeated indefinitely. The special case when the original points of contact happen to lie on a circle Fig. 11. being excluded, it can be shown analytically that the continuous curve which is the locus of all the points of contact is not an analytic curve. The points of contact form a manifoldness that is everywhere dense on the curve (in the sense of G. Cantor), although there are intermediate points between them. At each of the former points there is a determinate tangent, while there is none at the intermediate points. Second deriv- atives do not exist at all. It is easy enough to imagine a strip covering all these points ; but when the width of the strip is reduced beyond a certain limit, we find undulations, and it seems impossible to clearly picture to the mind the final outcome. It is to be noticed that we have here an example of a curve MATHEMATICAL CHARACTER OF SPACE-INTUITION. 45 with indeterminate derivatives arising out of purely geometrical considerations, while it might be supposed from the usual treatment of such curves that they can only be defined by artificial analytical series. Unfortunately, I am not in a position to give a full account of the opinions of philosophers on this subject. As regards the more recent mathematical literature, I have presented my views as developed above in a paper published in 1873, and since reprinted in the Math. Annalen* Ideas agreeing in general with mine have been expressed by Pasch, of Giessen, in two works, one on the foundations of geometry,! the other on the principles of the infinitesimal calculus.:}: Another author, Kopcke, of Hamburg, has advanced the idea that our space-intuition is exact as far as it goes, but so limited as to make it impossible for us to picture to ourselves curves with- out tangents. On one point Pasch does not agree with me, and that is as to the exact value of the axioms. He believes and this is the traditional view that it is possible finally to discard intuition entirely, basing the whole science on the axioms alone. I am of the opinion that, certainly, for the purposes of research it is always necessary to combine the intuition with the axioms. I do not believe, for instance, that it would have been possible to derive the results discussed in my former lectures, the splendid researches of Lie, the continuity of the shape of algebraic curves and surfaces, or the most general forms of triangles, without the constant use of geometrical intuition. * Ueber den allgemeincn Functionsbegriff und dessen Darstellung durch tine willkurliche Curve, Math. Annalen, Vol. 22 (1883), pp. 249-259. f Vorlesungen iiber neuere Geomttrie, Leipzig, Teubner, 1882. J Einleitung in die Differential- und Integralrechnung, Leipzig, Teubner, 1882. Ueber Differentiirbarkeit und Anschaulichkeit der stetigen Functionen, Math. Annalen, Vol. 29 (1887), pp. 123-140. 46 LECTURE VI. Pasch's idea of building up the science purely on the basis of the axioms has since been carried still farther by Peano, in his logical calculus. Finally, it must be said that the degree of exactness of the intuition of space may be different in different individuals, per- haps even in different races. It would seem as if a strong naive space-intuition were an attribute pre-eminently of the Teutonic race, while the critical, purely logical sense is more fully developed in the Latin and Hebrew races. A full investi- gation of this subject, somewhat on the lines suggested by Francis Galton in his researches on heredity, might be inter- esting. What has been said above with regard to geometry ranges this science among the applied sciences. A few general remarks on these sciences and their relation to pure mathe- matics will here not be out of place. From the point of view of pure mathematical science I should lay particular stress on the heuristic value of the applied sciences as an aid to discov- ering new truths in mathematics. Thus I have shown (in my little book on Riemann's theories) that the Abelian integrals can best be understood and illustrated by considering electric currents on closed surfaces. In an analogous way, theorems concerning differential equations can be derived from the con- sideration of sound-vibrations ; and so on. But just at present I desire to speak of more practical mat- ters, corresponding as it were to what I have said before about the inexactness of geometrical intuition. I believe that the more or less close relation of any applied science to mathematics might be characterized by the degree of exactness attained, or attainable, in its numerical results. Indeed, a rough classifi- cation of these sciences could be based simply on the number of significant figures averaged in each. Astronomy (and some branches of physics) would here take the first rank ; the num- MATHEMATICS AND THE APPLIED SCIENCES. 47 her of significant figures attained may here be placed as high as seven, and functions higher than the elementary transcendental functions can be used to advantage. Chemistry would probably be found at the other end of the scale, since in this science rarely more than two or three significant figures can be relied upon. Geometrical drawing, with perhaps 3 to 4 figures, would rank between these extremes ; and so we might go on. The ordinary mathematical treatment of any applied science substitutes exact axioms for the approximate results of experi- ence, and deduces from these axioms the rigid mathematical conclusions. In applying this method it must not be forgotten that mathematical developments transcending the limit of exact- ness of the science are of no practical value. It follows that a large portion of abstract mathematics remains without finding any practical application, the amount of mathematics that can be usefully employed in any science being in proportion to the degree of accuracy attained in the science. Thus, while the astronomer can put to good use a wide range of mathemati- cal theory, the chemist is only just beginning to apply the first derivative, i.e. the rate of change at which certain processes are going on ; for second derivatives he does not seem to have found any use as yet. As examples of extensive mathematical theories that do not exist for applied science, I may mention the distinction between the commensurable and incommensurable, the investigations on the convergency of Fourier's series, the theory of non-analytical functions, etc. It seems to me, therefore, that Kirchhoff makes a mistake when he says in his Spectral-Analyse that absorption takes place only when there is exact coincidence between the wave-lengths. I side with Stokes, who says that absorption takes place in the vicinity of such coincidence. Similarly, when the astronomer says that the periods of two planets must be exactly commensurable to admit the possibility of a collision, 48 LECTURE VI. this holds only abstractly, for their mathematical centres ; and it must be remembered that such things as the period, the mass, etc., of a planet cannot be exactly denned, and are changing all the time. Indeed, we have no way of ascertaining whether two astronomical magnitudes are incommensurable or not ; we can only inquire whether their ratio can be expressed approxi- mately by two small integers. The statement sometimes made that there exist only analytic functions in nature is in my opinion absurd. All we can say is that we restrict ourselves to analytic, and even only to simple analytic, functions because they afford a sufficient degree of approximation. Indeed, we have the theorem (of Weierstrass) that any continuous function can be approximated to, with any required degree of accuracy, by an analytic function. Thus if $(x) be our continuous func- tion, and & a small quantity representing the given limit of exactness (the width of the strip that we substitute for the curve), it is always possible to determine an analytic function f(x) such that <(*) =/(*) +e, where | e | < | 8 |, within the given limits. All this suggests the question whether it would not be pos- sible to create a, let us say, abridged system of mathematics adapted to the needs of the applied sciences, without passing through the whole realm of abstract mathematics. Such a system would have to include, for example, the researches of Gauss on the accuracy of astronomical calculations, or the more recent and highly interesting investigations of Tchebycheff on interpolation. The problem, while perhaps not impossible, seems difficult of solution, mainly on account of the somewhat vague and indefinite character of the questions arising. I hope that what I have here said concerning the use of mathematics in the applied sciences will not be interpreted MATHEMATICS AND THE APPLIED SCIENCES. 49 as in any way prejudicial to the cultivation of abstract mathe- matics as a pure science. Apart from the fact that pure mathematics cannot be supplanted by anything else as a means for developing the purely logical powers of the mind, there must be considered here as elsewhere the necessity of the presence of a few individuals in each country developed in a far higher degree than the rest, for the purpose of keeping up and gradually raising the general standard. Even a slight raising of the general level can be accomplished only when some few minds have progressed far ahead of the average. Moreover, the "abridged" system of mathematics referred to above is not yet in existence, and we must for the present deal with the material at hand and try to make the best of it. Now, just here a practical difficulty presents itself in the teaching of mathematics, let us say of the elements of the differential and integral calculus. The teacher is confronted with the problem of harmonizing two opposite and almost con- tradictory requirements. On the one hand, he has to consider the limited and as yet undeveloped intellectual grasp of his students and the fact that most of them study mathematics mainly with a view to the practical applications ; on the other, his conscientiousness as a teacher and man of science would seem to compel him to detract in nowise from perfect mathe- matical rigour and therefore to introduce from the beginning all the refinements and niceties of modern abstract mathe- matics. In recent years the university instruction, at least in Europe, has been tending more and more in the latter direc- tion ; and the same tendencies will necessarily manifest them- selves in this country in the course of time. The second edition of the Cours d 'analyse of Camille Jordan may be regarded as an example of this extreme refinement in laying the foundations of the infinitesimal calculus. To place a work of this character in the hands of a beginner must necessarily 50 LECTURE VI. have the effect that at the beginning a large part of the sub- ject will remain unintelligible, and that, at a later stage, the student will not have gained the power of making use of the principles in the simple cases occurring in the applied sciences. It is my opinion that in teaching it is not only admissible, but 'absolutely necessary, to be less abstract at the start, to have constant regard to the applications, and to refer to the refinements only gradually as the student becomes able to understand them. This is, of course, nothing but a universal pedagogical principle to be observed in all mathematical instruction. Among recent German works I may recommend for the use of beginners, for instance, Kiepert's new and revised edition of Stegemann's text-book ; * this work seems to combine sim- plicity and clearness with sufficient mathematical rigour. On the other hand, it is a matter of course that for more advanced students, especially for professional mathematicians, the study of works like that of Jordan is quite indispensable. I am led to these remarks by the consciousness of a growing danger in the higher educational system of Germany, the danger of a separation between abstract mathematical science and its scientific and technical applications. Such separation could only be deplored ; for it would necessarily be followed by shallowness on the side of the applied sciences, and by isolation on the part of pure mathematics. * Grundriss der Differential- und Integral- Rechnung, 6te Auflage, herausgegeben von Kiepert, Hannover, Helvving, 1892. LECTURE VII. : THE TRANSCENDENCY OF THE NUMBERS e AND TT. (September 4, 1893.) LAST Saturday we discussed inexact mathematics ; to-day we shall speak of the most exact branch of mathematical science. It has been sb^pwn by G. Cantor that there are two kinds of infinite manifolcraegses : (a) countable (abzalilbare) manifold- nesses, whose quantities^Q&x be numbered or enumerated so that to each quantity a definite pl^je can be assigned in the system ; and (b) non-countable manifoldness'&Sj^for which this is not possi- ^^^KJ ble. To the former group belong not oo^uthe rational numbers, but also the so-called algebraic numbers, z. + /> = o. Now it can be proved that the coefficients of #, d^, a 2 ,"-a n in /^ are all integers, that p can be so selected as to make P l different from zero, and that at the same time p can be taken so large as to make P 2 as small as we please. Thus, equation (i) will be reduced to the impossible form (3). We proceed to prove these properties of P l and P z . The integral J is readily seen to be an integer divisible by p!, owing to the well-known relation \ ^e~*dz-=p\. Similarly, by substituting z = s'+i, z = z' + 2, 2=2' + n, it can be shown that e ( ,>o, which have to be considered separately, I select the former as being more simple. Both cases were treated in my lectures referred to before. The following elementary geometrical interpretation of the binary quadratic form was given by Gauss, who was k much inclined to using geometrical considerations in all branches of mathematics. Construct a parallelogram (Fig. 13) with two Fig. 13. adjacent sides equal to Va, Ve, respectively, and the included angle such that cos < = . As ^ 4 ac < o, a and c have 2 ^/ac necessarily the same sign ; we here assume that a and c are IDEAL NUMBERS. 6l both positive ; the case when they are both negative can readily be treated by changing the signs throughout. Next produce the sides of the parallelogram indefinitely, and draw parallels so as to cover the whole plane by a network of equal parallelograms. I shall call this a line-lattice (Parallel- fitter). We now select any one of the intersections, or vertices, as origin O, and denote every other vertex by the symbol (x, y), x being the number of sides V#, y that of sides vV, which must be traversed in passing from O to (x, y). Then every value that the form f takes for integral values of x, y evidently represents the square of the distance of the point (x, y) from O. Thus the lattice gives a complete geometrical representa- tion of the binary quadratic form. The discriminant D has also a simple geometrical interpretation, the area of each paral- lelogram being =1V D. Now, in the theory of numbers, two forms f=ax* + bxy + f y i and /' = a '*' 2 + b '*'/ + ''/* are regarded as equivalent if one can be derived from the other by a linear substitution whose determinant is i, say x' = ax + fly, y' = yx + Sy, where S $7=1, , ft, 7, B being integers. All forms equiva- lent to a given one are said to compose a class of quadratic forms ; these forms have all the same discriminant. What corresponds to this equivalence in our geometrical representa- tion will readily appear if we fix our attention on the vertices only (Fig. 14) ; we then obtain what I propose to call a point- lattice (Punktgitter). Such a network of points can be con- nected in various ways by two sets of parallel lines ; i.e. the point-lattice represents an infinite number of line-lattices. Now it results from an elementary investigation that the point- 62 LECTURE VIII. lattice is the geometrical image of the class of binary quad- ratic forms, the infinite number of line-lattices contained in the point-lattice corresponding exactly to the infinite number of binary forms contained in the class. Fig. 14. It is further known from the theory of numbers that to every value of D belongs only a finite number of classes ; hence to every D will correspond a finite number of point- lattices, which we shall afterwards consider together. Among the different classes belonging to the same value of D, there is one class of particular importance, which I call the principal class. It is defined as containing the form x i -\Df when D = o (mod. 4), and the form when D = i (mod. 4). It is easy to see that the correspond- ing lattices are very simple. When D = o(mod. 4), the principal lattice is rectangular, the sides of the elementary parallelo- IDEAL NUMBERS. 63 gram being i and V \D. For /)= i (mod. 4), the parallelogram becomes a rhombus. For the sake of simplicity, I shall here consider only the former case. Let us now define complex numbers in connection with the principal lattice of the rectangular type (Fig. 15). The point Fig. 15. (x, y) of the lattice will represent simply the complex number such numbers we shall call principal numbers. In any system of numbers the laws of multiplication are of prime importance. For our principal numbers it is easy to prove that the product of any two of them always gives a principal number; i.e. the system of principal numbers is, for multiplication, complete in itself. We proceed next to the consideration of lattices of discrimi- nant D that do not belong to the principal class ; let us call them secondary lattices (Nebengitter}. Before investigating the laws of multiplication of the corresponding numbers, I must call attention to the fact that there is one feature of arbitrari- ness in our representation that has not yet been taken into account ; this is the orientation of the lattice, which may be regarded as given by the angles, -ty and ^, made by the sides 6 4 LECTURE VIII. Vtf, V*:, respectively, with some fixed initial line (Fig. 16). For the angle of the parallelogram we have evidently = ^ -^. The point (x, y) of the lattice will thus give the complex number -y, which we call a secondary number. The definition of a secondary number is therefore indeterminate as long as -/r or ^ is not fixed. Now, by determining ty properly for every secondary point- lattice, it is always possible to bring about the important result Fig. 16. that the product of any two complex numbers of all our lattices taken together will again be a complex number of the system, so that the totality of these complex numbers forms, likewise, for multiplication, a complete system. Moreover, the multiplication combines the lattices in a definite way; thus, if any number belonging to the lattice L 1 be multiplied into any number of the lattice L v we always obtain a number belonging to a definite lattice L s . These properties will be seen to correspond exactly to the characteristic properties of Gauss's composition of algebraic fonns. For Gauss's law merely asserts that the product of IDEAL NUMBERS. 65 two ordinary numbers that can be represented by two primitive forms /j, / 2 of discriminant D is always representable by a definite primitive form / 3 of discriminant D. This law is included in the theorem just stated, inasmuch as the values of V/i, V/Cj, V^ represent the distances of the points in the lattices from the origin. At the same time we notice that Gauss's law is not exactly equivalent to our theorem, since in the multiplication of our complex numbers, not only the distances are multiplied, but the angles are added. It is not impossible that Gauss himself made use of similar considerations in deducing his law, which, taken apart from this geometrical illustration, bears such an abstruse character. It now remains to explain what relation these investigations have to the ideal numbers of Kummer. This involves the question as to the division of our complex numbers and their resolution into primes. In the ordinary theory of real numbers, every number can be resolved into primes in only one way. Does this fundamental law hold for our complex numbers ? In answering this question we must distinguish between the system formed by the totality of all our complex numbers and the system of principal numbers alone. For the former system the answer is : yes, every com- plex number can be decomposed into complex primes in only one way. We shall not stop to consider the proof which is directly contained in the ordinary theory of binary quadratic forms. But if we proceed to the consideration of the system of principal numbers alone, the matter is different. There are cases when a principal number can be decomposed in more than one way into prime factors, i.e. principal numbers not decomposable into principal factors. Thus it may happen that we have m l m 2 = n l n 2 ; m v m 2 , n v ;/ 2 being principal primes. The reason is, that these principal numbers are no longer primes 66 LECTURE VIII. if we adjoin the secondary numbers, but are decomposable as follows : a > @, 7, 8 being primes in the enlarged system. In investigating the laws of division it is therefore not convenient to consider the principal system, by itself ; it is best to introduce the secondary systems. Kummer, in studying these questions, had originally at his disposal only the principal system ; and noticing the imperfection of the resulting laws of division, he introduced by definition his ideal numbers so as to re-establish the ordinary laws of division. These ideal numbers of Kummer are thus seen to be nothing but abstract representatives of our secondary numbers. The whole difficulty encountered by every one when first attacking the study of Kummer's ideal numbers is there- fore merely a result of his mode of presentation. By introduc- ing from the beginning the secondary numbers by the side of the principal numbers, no difficulty arises at all. It is true that we have here spoken only of complex numbers containing square roots, while the researches of Kummer him- self and of his followers, Kronecker and Dedekind, embrace all possible algebraic numbers. But our methods are of universal application ; it is only necessary to construct lattices in spaces of higher dimensions. It would carry us too far to enter into details. LECTURE IX.: THE SOLUTION OF HIGHER ALGE- BRAIC EQUATIONS. (September 6, 1893.) FORMERLY the " solution of an algebraic equation " used to mean its solution by radicals. All equations whose solutions cannot be expressed by radicals were classed simply as insoluble, although it is well known that the Galois groups belonging to such equations may be very different in character. Even at the present time such ideas are still sometimes found prevail- ing; and yet, ever since the year 1858, a very different point of view should have been adopted. This is the year in which Hermite and Kronecker, together with Brioschi, found the solution of the equation of the fifth degree, at least in its fundamental ideas. This solution of the quintic equation is often referred to as a "solution by elliptic functions"; but this expression is not accurate, at least not as a counterpart to the "solution by radicals." Indeed, the elliptic functions enter into the solution of the equation of the fifth degree, as logarithms might be said to enter into the solution of an equation by radicals, because the radicals can be computed by means of logarithms. The solution of an equation will, in the present lecture, be regarded as consisting in its reduction to certain algebraic normal equa- tions. That the irrationalities involved in the latter can, in the case of the quintic equation, be computed by means of tables of elliptic functions (provided that the proper tables of 6 7 68 LECTURE IX. the corresponding class of elliptic functions were available) is an additional point interesting enough in itself, but not to be considered by us to-day. I have simplified the solution of the quintic, and think that I have reduced it to the simplest form, by introducing the icosahedron equation as the proper normal equation.* In other words, the icosahedron equation determines the typical irra- tionality to which the solution of the equation of the fifth degree can be reduced. This method is capabje of being so generalized as to embrace a whole theory of the solution of higher algebraic equations ; and to this I wish to devote the present lecture. It may be well to state that I speak here of equations with coefficients that are not fixed numerically ; the equations are considered from the point of view of the theory of functions, the coefficients corresponding to the independent variables. In saying that an equation is solvable by radicals we mean that it is reducible by algebraic processes to so-called pure equations, ri n = z, where z is a known quantity ; then only the new question arises, how i^=-\Tz can be computed. Let us compare from this point of view the icosahedron equation with the pure equation. The icosahedron equation is the following equation of the 6oth degree : ^ 3 0?) = , 1 7287*07) where H is a numerical expression of the 2Oth, f one of the 1 2th degree, while z is a known quantity. For the actual * See my work Vorlesungen tiber das Ikosaeder und die Auflosung der Gleichun- gen vom fiinften Grade, Leipzig, Teubner, 1884.* SOLUTION OF HIGHER ALGEBRAIC EQUATIONS. 69 forms of H and f as well as other details I refer you to the Vorlesungen iiber das Ikosaeder ; I wish here only to point out the characteristic properties of this equation. (i) Let 77 be any one of the roots ; then the 60 roots can all be expressed as linear functions of 77, with known coeffi- cients, such as for instance, ** 2 v &) 2 , instead of the two Jacobian arguments. This was a necessary preliminary to establishing the connection with the theory of linear substitutions. Let us consider the dis- continuous ternary group of linear substitutions, u' = u -f- 0*iti>i -f- where a, ft, y, & are integers whose determinant a$py=i, while tn lt m 2 are any integers whatever. The fundamental functions of Weierstrass's theory, Ctfj, are nothing but the complete system of invariants of that group. It appears, moreover, that g v g z are also the ordinary (Cay- leyan) invariants of the binary biquadratic form f^(x^, x z ), on which depends the integral of the first kind This significant feature that the transcendental invariants turn out to be at the same time invariants of the algebraic irration- ality corresponding to the transcendental theory will hold in all higher cases. As a next step in the theory of elliptic functions we have to mention the introduction by Glebsch of the systematic con- sideration of algebraic curves of deficiency i. He considered in particular the plane curve of the third order (C s ) and the HYPERELLIPTIC AND ABELIAN FUNCTIONS. 77 first species of quartic curves (C^) in space, and showed how convenient it is for the derivation of numerous geometrical propositions to regard the elliptic integrals as taken along these curves. The theory of elliptic functions is thus broadened by bringing to bear upon it the ideas of modern projective geometry. By combining and generalizing these considerations, I was led to the formulation of a very general programme which may be stated as follows (see Vorlesungen iiber die Theorie der ellip- tischen Modulfunctionen, Vol. II.). Beginning with the discontinuous group mentioned before u = u oV = awj -f- /2w 2 , <2 = ywi + Su) 2 , our first task is to construct all its sub-groups. Among these the simplest and most useful are those that I have called congruence sub-groups ; they are obtained by putting mi = o, m 2 = o,^\ = i, /3 = o, [-(mod. ri). y = o, 8=1, J The second problem is to construct the invariants of all these groups and the relations between them. Leaving out of consideration all sub-groups except these congruence sub- groups, we have still attained a very considerable enlargement of the theory of elliptic functions. According to the value assigned to the number n, I distinguish different stages (Stufen) of the problem. It will be noticed that Weierstrass's theory corresponds to the first stage (=i), while Jacobi's answers, generally speaking, to the second ( = 2); the higher stages have not been considered before in a systematic way. Thirdly, for the purpose of geometrical illustration, I apply Clebsch's idea of the algebraic curve. I begin by introducing 78 LECTURE X. the ordinary square root of the binary form which requires the axis of x to be covered twice ; i.e. we have to use a T 2 in an 5 r I next proceed to the general cubic curve of the plane (C 3 in an 5 2 ), to the quartic curve in space of three dimensions (C in an S 3 ), and generally to the elliptic curve C n+l in an S n . These are what I call the normal elliptic curves ; they serve best to illustrate any algebraic relations between elliptic functions. I may notice, by the way, that the treatment here proposed is strictly followed in the Elliptische Modulfunctionen, except that there the quantity u is of course assumed to be zero, since this is precisely what characterizes the modular functions. I hope some time to be able to treat the whole theory of elliptic functions (i.e. with u different from zero) according to this programme. The successful extension of this programme to the theory of hyperelliptic and Abelian functions is the best proof of its being a real step in advance. I have therefore devoted my efforts for many years to this extension ; and in laying before you an account of what has been accomplished in this rather special field, I hope to attract your attention to various lines of research along which new work can be spent to advantage. As regards the hyperelliptic functions, we may premise as a general definition that they are functions of two variables u lt 2 , with four periods (while the elliptic functions have one vari- able , and two periods). Without attempting to give an historical account of the development of the theory of hyper- elliptic functions, I turn at once to the researches that mark a progress along the lines specified above, beginning with the geometric application of these functions to surfaces in a space of any number of dimensions. Here we have first the investigation by Rohn of Kummer's surface, the well-known surface of the fourth order, with 16 HYPERELLIPTIC AND ABELIAN FUNCTIONS. 79 conical points. I have myself given a report on this work in the Math. Annalen, Vol. 27 (1886).* If every mathematician is struck by the beauty and simplicity of the relations developed in the corresponding cases of the elliptic functions (the C B in the plane, etc.). the remarkable configurations inscribed and circumscribed to the Kummer surface that have here been developed by Rohn and myself, should not fail to elicit interest. Further, I have to mention an extensive memoir by Reichardt, published in 1886, in the Acta Leopoldina, where the connec- tion between hyperelliptic functions and Kummer's surface is summarized in a convenient and comprehensive form, as an introduction to this branch. The starting-point of the investi- gation is taken in the theory of line-complexes of the second degree. Quite recently the French mathematicians have turned their attention to the general question of the representation of sur- faces by means of hyperelliptic functions, and a long memoir by Humbert on this subject will be found in the last volume of the Journal de Mathhnatiqnes.^ I turn now to the abstract theory of hyperelliptic functions. It is well known that Gopel and Rosenhain established that theory in 1847 in a manner closely corresponding to the Jaco- bian theory of elliptic functions, the integrals /dx C , "2= | * J xdx taking the place of the single elliptic integral u. Here, then, the question arises : what is the relation of the hyperelliptic functions to the invariants of the binary form of the sixth order / 6 (*i, x 2 ] ? In the investigation of this question by myself and * Ueber Configurationen, welche der Kummer 1 schen Flache zugleich eingeschrieben und umgesehrieben sine/, pp. 106-142. t Theorie generate des surfaces hyperelliptiques, annee 1893, pp. 29-170. 8o LECTURE X. Burkhardt, published in Vol. 27 (1886)* and Vol. 32 (1888) f of the Math. Annalen, we found that the decompositions of the form f 6 into two factors of lower order, / 6 = ^Jr 5 = ^3^3, had to be considered. These being, of course, irrational decom- positions, the corresponding invariants are irrational ; and a study of the theory of such invariants became necessary. But another new step had to be taken. The hyperelliptic integrals involve the form / 6 under the square root, The corresponding Riemann surface has, therefore, two leaves connected at six points ; and the problem arises of considering binary forms of x^ x^ on such a Riemann surface, just as ordi- narily functions of x alone are considered thereon. It can be shown that there exists a particular kind of forms called prime- forms, strictly analogous to the determinant x-^y^x^-^ in the ordinary complex plane. The primeform on the two-leaved Riemann surface, like this determinant in the ordinary theory, has the property of vanishing only when the points (x lt x^ and (y\> y^> co-incide (on the same leaf). Moreover, the primeform does not become infinite anywhere. The analogy to the deter- minant x^y^x^y-^ fails only in so far as the primeform is no longer an algebraic but a transcendental form. Still, all alge- braic forms on the surface can be decomposed into prime factors. Moreover, these primeforms give the natural means for the construction of the ^-functions. As an intermediate step we have here functions called by me o--functions in analogy to the -o--functions of Weierstrass's elliptic theory. In the papers referred to (Math. Annalen, Vols. 27, 32) all these con- siderations are, of course, given for the general case of hyper- elliptic functions, the irrationality being V/^+a^, x<), where f 2p+2 is a binary form of the order 2/4-2. * Ueber hyperelliptische Sigmafunetionen, pp. 431-464. t PP- 35 '-380 and 381-442. HYPERELLIPTIC AND ABELIAN FUNCTIONS. 8 1 Having thus established the connection between the ordinary theory of hyperelliptic functions of / = 2 and the invariants of the binary sextic, I undertook the systematic development of what I have called, in the case of elliptic functions, the Stufen- theorie. The lectures I gave on this subject in 1887-88 have been developed very fully by Burkhardt in the Math. Annalen, Vol. 35 (1890).* As regards the first stage, which, owing to the connection with the theory of rational invariants and covariants, requires very complicated calculations, the Italian mathematician, Pascal, has made much progress (Annali di matematica). In this connection I must refer to the paper by Bolzaf in Math. Annalen, Vol. 30 (1887), where the question is discussed in how far it is possible to represent the rational invariants of the sextic by means of the zero values of the ^-functions. For higher stages, in particular stage three, Burkhardt has given very valuable developments in the Math. Annalen, Vol. 36 (1890), p. 371 ; Vol. 38 (1891), p. 161 ; Vol. 41 (1893), p. 313. He considers, however, only the hyperelliptic modular functions (j and u z being assumed to be zero). The final aim, which Burkhardt seems to have attained, although a large amount of numerical calculation remains to be filled in, consists here in establishing the so-called multiplier-equation for transforma- tions of the third order. The equation is of the 4Oth degree ; and Burkhardt has given the general law for the formation of the coefficients. I invite you to compare his treatment with that of Krause in his book Die Transformation der hyperelliptischen Func- tionen erster Ordnnng, Leipzig, Teubner, 1886. His investiga- * Grundzilge einer allgemeinen Systematik der hyperelliptischen Functionen I. Ordnung, pp. 198-296. t Darstellung der rationalen ganzen Invariantcn der Binarform scchsten Grades durch die Nullwerthe der zugehorigen 9- Functionen, pp. 478-495. 82 LECTURE X. tions, based on the general relations between ^-functions, may go farther ; but they are carried out from the purely formal point of view, without reference to the theories of invariants, of groups, or other allied topics. So much as regards hyperelliptic functions. I now proceed to report briefly on the corresponding advances made in the theory of Abelian functions. I give merely a list of papers ; they may be classed under three heads : (1) A preliminary question relates to the invariant represen- tation of the integral of the third kind on algebraic curves of higher deficiency. Pick * has considered this problem for plane curves having no singular points. On the other hand, White, in his dissertation,! briefly reported in Math. Annalen, Vol. 36 (1890), p. 597, and printed in full in the Acta Leopoldina, has treated such curves in space as are the complete intersection of two surfaces and have no singular point. We may here also notice the researches of Pick and Osgoodij: on the so-called binomial integrals. (2) An exposition of the general theory of forms on Rie- mann surfaces of any kind, in particular a definition of the primeform belonging to each surface, was given by myself in Vol. 36 (1890) of the Math. Annalen.^ I may add that during the last year this subject was taken up anew and farther developed by Dr. Ritter ; see Gbttinger Nachrichten for 1893, and Math. Annalen, Vol. 44. Dr. Ritter considers the algebraic forms as special cases of more general forms, the multiplicative forms, and thus takes a real step in advance. * Zur Theorie der Abefschen Functionen, Math. Annalen, Vol. 29 (1887), pp. 259-271. t AbeFsche Integrate auf singularit'dtenfreien, einfach iiberdeckten, vollstdndigen Schnittcurven eines beliebig ausgedehnten Raumes, Halle, 1891, pp. 43-128. J Osgood, Zur Theorie der zum algebraischen Gebilde y m = R(x) gehorigen Abel'schen Functionen, Gottingen, 1890, 8vo, 61 pp. Zur Theorie der AbeFschen Functionen, pp. 1-83. HYPERELLIPTIC AND ABELIAN FUNCTIONS. 83 (3) Finally, the particular case / = 3 has been studied on the basis of our programme in various directions. The normal curve for this case is well known to be the plane quartic C whose geometric properties have been investigated by Hesse and others. I found (Math. Annalen, Vol. 36) that these geometrical results, though obtained from an entirely different point of view, corresponded exactly to the needs of the Abelian problem, and actually enabled me to define clearly the 64 ^-functions with the aid of the T 4 . Here, as elsewhere, there seems to reign a certain pre-established harmony in the develop- ment of mathematics, what is required in one line of research being supplied by another line, so that there appears to be a logical necessity in this, independent of our individual disposition. In this case, also, I have introduced o. It is here, of course, essen- tial to adopt the system of von Staudt and not that of Steiner, since the latter defines the anharrnonic ratio by means of distances of points, and not by pure projective constructions. (3) Finally, we have the point of view of Riemann and Helm- holtz. Riemann starts with the idea of the element of distance ds, which he assumes to be expressible in the form ds = Helmholtz, in trying to find a reason for this assumption, con- siders the motions of a rigid body in space, and derives from these the necessity of giving to ds the form indicated. On the other hand, Riemann introduces the fundamental notion of the measure of curvature of space. The idea of a measure of curvature for the case of two variables, i.e. for a surface in a three-dimensional space, is due to Gauss, who showed that this is an intrinsic characteristic of the surface quite independent of the higher space in which the surface happens to be situated. This point has given rise to a misunderstanding on the part of many non-Euclidean writers. When Riemann attributes to his space of three dimensions a measure of curvature k, he only wants to say that there exists an invariant of the "form" 2d^&x&*; he does not mean to imply that the three-dimensional space necessarily exists as a curved space in a space of four dimensions. Similarly, the illustration of a space of constant positive measure of curvature by the familiar example of the sphere is somewhat misleading. Owing to the fact that on the sphere the geodesic lines (great circles) issuing from any point all meet again in another definite RESEARCHES IN NON-EUCLIDEAN GEOMETRY. 8/ point, antipodal, so to speak, to the original point, the existence of such an antipodal point has sometimes been regarded as a necessary consequence of the assumption of a constant positive curvature. The projective theory of non-Euclidean space shows immediately that the existence of an antipodal point, though compatible with the nature of an elliptic space, is not necessary, but that two geodesic lines in such a space may intersect in one point if at all.* I call attention to these details in order to show that there is some advantage in adopting the second of the three points of view characterized above, although the third is at least equally important. Indeed, our ideas of space come to us through the senses of vision and motion, the "optical properties" of space forming one source, while the "mechanical properties" form another ; the former corresponds in a general way to the pro- jective properties, the latter to those discussed by Helmholtz. As mentioned before, from the point of view of projective geometry, von Staudt's system should be adopted as the basis. It might be argued that von Staudt practically assumes the axiom of parallels (in postulating a one-to-one correspondence between a pencil of lines and a row of points). But I have shown in the Math. Annalen^ how this apparent difficulty can be overcome by restricting all constructions of von Staudt to a limited portion of space. I now proceed to give an account of the most recent re- searches in non-Euclidean geometry made by Lie and myself. Lie published a brief paper on the subject in the Berichte of the Saxon Academy (1886), and a more extensive exposition of his views in the same BericJite for 1890 and 1891. These * This theory has also been developed by Newcomb, in the Journal fur reine und angewandte Mathematik, Vol. 83 (1877), pp. 293-299. t Ueber die sogenannte Nicht-Euklidische Geometric, Math. Annalen, Vol. 6 (1873), pp. 112-145. 88 LECTURE XI. papers contain an application of Lie's theory of continuous groups to the problem formulated by Helmholtz. I have the more pleasure in placing before you the results of Lie's investi- gations as they are not taken into due account in my paper on the foundations of projective geometry in Vol. 37 of the Math. Annalen (1890),* nor in my (lithographed) lectures on non-Euclidean geometry delivered at Gottingen in 1889-90; the last two papers of Lie appeared too late to be considered, while the first had somehow escaped my memory.' I must begin by stating the problem of Helmholtz in modern terminology. The motions of three-dimensional space are oo 6 , and form a group, say G G . This group is known to have an invariant for any two points /, /', viz. the distance H (p, p'} of these points. But the form of this invariant (and generally the form of the group) in terms of the co-ordinates x v x^ x z , y\> y^i y% f the P m ts is not known a priori. The question arises whether the group of motions is fully characterized by these two properties so that none but the Euclidean and the two non-Euclidean systems of geometry are possible. For illustration Helmholtz made use of the analogous case in two dimensions. Here we have a group of oo 3 motions ; the distance is again an invariant ; and yet it is possible to construct a group not belonging to any one of our three systems, as follows. Let 2- be a complex variable ; the substitution characterizing the group of Euclidean geometry can be written in the well- known form z' = z -\-m-\-in= (cos < + i sin )z -\-m-\- in. Now modifying this expression by introducing a complex number in the exponent, z' = + i sin )z + m -f- in, * Zur Nicht- Euklidischen Geometrie, pp. 544-572. RESEARCHES IN NON-EUCLIDEAN GEOMETRY. 8 9 we obtain a group of transformations by which a point (in the simple case m=o, n = o) would not move about the origin in a circle, but in a logarithmic spiral ; and yet this is a group G B with three variable parameters m, n, , having an invariant for every two points, just like the original group. Helmholtz concludes, therefore, that a new condition, that of monodromy, must be added to determine our group completely. I now proceed to the work of Lie. First as to the results : Lie has confirmed those of Helmholtz with the single exception that in space of three dimensions the axiom of monodromy is not needed, but that the groups to be considered are fully determined by the other axioms. As regards the proofs, how- ever, Lie has shown that the considerations of Helmholtz must be supplemented. The matter is this. In keeping one point of space fixed, our G Q will be reduced to a G 3 . Now Helmholtz inquires how the differentials of the lines issuing from the fixed point are transformed by this G s . For this purpose he writes down the formulae dxi = a^dx-i + a l2 dx.> + a l3 dx s , and considers the coefficients a n , # 12 , a 33 as depending on three variable parameters. But Lie remarks that this is not sufficiently general. The linear equations given above repre- sent only the first terms of power series, and the possibility must be considered that the three parameters of the group may not all be involved in the linear terms. In order to treat all possible cases, the general developments of Lie's theory of groups must be applied, and this is just what Lie does. Let me now say a few words on my own recent researches in non-Euclidean geometry which will be found in a paper pub- lished in the Math. Annalen, Vol. 37 (1890), p. 544. Their cp LECTURE XI. result is that our ideas as to non-Euclidean space are still very incomplete. Indeed, all the researches of Riemann, Helmholtz, Lie, consider only a portion of space surrounding the origin ; they establish the existence of analytic laws in the vicinity of that point. Now this space can of course be continued, and the question is to see what kind of connection of space may result from this continuation. It is found that there are dif- ferent possibilities, each of the three geometries giving rise to a series of subdivisions. To understand better what is meant by these varieties of connection, let us compare the geometry on a sphere with that in the sheaf of lines formed by the diameters of the sphere. Considering each diameter as an infinite line or ray passing through the centre (not a half-ray issuing from the centre), to each line of the sheaf there will correspond two points on the sphere, viz. the two points of intersection of the line with the sphere. We have, therefore, a one-to-two correspondence between the lines of the sheaf and the points of the sphere. Let us now take a small area on the sphere ; it is clear that the distance of two points contained in this area is equal to the angle of the corresponding lines of the sheaf. Thus the geometry of points on the sphere and the geometry of lines in the sheaf are identical as far as small regions are concerned, both corresponding to the assumption of a constant positive measure of curvature. A difference appears, however, as soon as we consider the whole closed sphere on the one hand and the com- plete sheaf on the other. Let us take, for instance, two geodesic lines of the sphere, i.e. two great circles, which evidently inter- sect in two (diametral) points. The corresponding pencils of the sheaf have only one straight line in common. A second example for this distinction occurs in comparing the geometry of the Euclidean plane with the geometry on a closed cylindrical surface. The latter can be developed in the RESEARCHES IN NON-EUCLIDEAN GEOMETRY. 91 usual way into a strip of the plane bounded by two parallel lines, as will appear from Fig. 20, the arrows indicating that the opposite points of the edges are coincident on the cylin- drical surface. We notice at once the difference : while in the plane all geodesic lines are infinite, on the cylinder there is Fig. 20. one geodesic line that is of finite length, and while in the plane two geodesic lines always intersect in one point, if at all, on the cylinder there may be oo points of intersection. This second example was generalized by Clifford in an address before the Bradford meeting of the British Associa- Fig. 21. tion (1873). In accordance with Clifford's general idea, we may define a closed surface by taking a parallelogram out of an ordinary plane and making the opposite edges correspond point to point as indicated in Fig. 21. It is not to be understood that the opposite edges should be brought to 92 LECTURE XI. coincidence by bending the parallelogram (which evidently would be impossible without stretching) ; but only the logical convention is made that the opposite points should be con- sidered as identical. Here, then, we have a closed mani- foldness of the connectivity of an anchor-ring, and every one will see the great differences that exist here in comparison with the Euclidean plane in everything concerning the lengths and the intersections of geodesic lines, etc. It is interesting to consider the G 3 of Euclidean motions on this surface. There is no longer any possibility of moving the surface on itself in oo 3 ways, the closed surface being consid- ered in its totality. But there is no difficulty in moving any small area over the closed surface in oo 3 ways. We have thus found, in addition to the Euclidean plane, two other forms of surfaces : the strip between parallels and Clifford's parallelogram. Similarly we have by the side of ordinary Euclidean space three other types with the Euclid- ean element of arc ; one of these results from considering a parallelepiped. Here I introduce the axiomatic element. There is no way of proving that the whole of space can be moved in itself in oo 6 ways ; all we know is that small portions of space can be moved in space in oo 6 ways. Hence there exists the possibility that our actual space, the measure of curvature being taken as zero, may correspond to any one of the four cases. Carrying out the same considerations for the spaces of con- stant positive measure of curvature, we are led back to the two cases of elliptic and spherical geometry mentioned before. If, however, the measure of curvature be assumed as a negative constant, we obtain an infinite number of cases, corresponding exactly to the configurations considered by Poincar6 and myself in the theory of automorphic functions. This I shall not stop to develop here. RESEARCHES IN NON-EUCLIDEAN GEOMETRY. 93 I may add that Killing has verified this whole theory.* It is evident that from this point of view many assertions con- cerning space made by previous writers are no longer correct (e.g. that infinity of space is a consequence of zero curvature), so that we are forced to the opinion that our geometrical demonstrations have no absolute objective truth, but are true only for the present state of our knowledge. These demon- strations are always confined within the range of the space- conceptions that are familiar to us ; and we can never tell whether an enlarged conception may not lead to further possibilities that would have to be taken into account. From this point of view we are led in geometry to a certain modesty, such as is always in place in the physical sciences. * Ueber die Clifford-Klein 'schen Raumformcn, Math. Annalen, Vol. 39 (1891), pp. 257-278. LECTURE XII.: THE STUDY OF MATHEMATICS AT GOTTINGEN. (September 9, 1893.) IN this last lecture I should like to make some general remarks on the way in which the study of mathematics is organized at the university of Gottingen, with particular refer- ence to what may be of interest to American students. At the same time I desire to give you an opportunity to ask any ques- tions that may occur to you as to the broader subject of mathe- matical study at German universities in general. I shall be glad to answer such inquiries to the extent of my ability. It is perhaps inexact to speak of an organization of the mathematical teaching at Gottingen ; you know that Lern- und Lehr-Freiheit prevail at a German university, so that the organ- ization I have in mind consists merely in a voluntary agreement among the mathematical professors and instructors. We dis- tinguish at Gottingen between a general and a higher course in mathematics. The general course is intended for that large majority of our students whose intention it is to devote them- selves to the teaching of mathematics and physics in the higher schools (Gymnasien, Realgymnasien, Realschuleri), while the higher course is designed specially for those whose final aim is original investigation. As regards the former class of students, it is my opinion that in Germany (here in America, I presume, the conditions are very different) the abstractly theoretical instruction given to 94 THE STUDY OF MATHEMATICS AT GOTTINGEN. 95 them has been carried too far. It is no doubt true that what the university should give the student above all other things is the scientific ideal. For this reason even these students should push their mathematical studies far beyond the elemen- tary branches they may have to teach in the future. But the ideal set before them should not be chosen so far distant, and so out of connection with their more immediate wants, as to make it difficult or impossible for them to perceive the bear- ing that this ideal has on their future work in practical life. In other words, the ideal should be such as to fill the future teacher with enthusiasm for his life-work, not such as to make him look upon this work with contempt as an unworthy drudgery. For this reason we insist that our students of this class, in addition to their lectures on pure mathematics, should pursue a thorough course in physics, this subject forming an integral part of the curriculum of the higher schools. Astronomy is also recommended as showing an important application of mathematics ; and I believe that the technical branches, such as applied mechanics, resistance of materials, etc., would form a valuable aid in showing the practical bearing of mathematical science. Geometrical drawing and descriptive geometry form also a portion of the course. Special exercises in the solution of problems, in lecturing, etc., are arranged in connection with the mathematical lectures, so as to bring the students into personal contact with the instructors. I wish, however, to speak here more particularly on the higher courses, as these are of more special interest to Ameri- can students. Here specialization is of course necessary. Each professor and decent delivers certain lectures specially designed for advanced students, in particular for those studying for the doctor's degree. Owing to the wide extent of modern mathematics, it would be out of the question to cover the whole 96 LECTURE XII. field. These lectures are therefore not regularly repeated every year ; they depend largely on the special line of research that happens at the time to engage the attention of the professor. In addition to the lectures we have the higher seminaries, whose principal object is to guide the student in original investigation and give him an opportunity for individual work. As regards my own higher lectures, I have pursued a certain plan in selecting the subjects for different years, my general aim being to gain, in the course of time, a complete view of the whole field of modern mathematics, with particular regard to the intuitional or (in the highest sense of the term) geometrical standpoint. This general tendency you will, I trust, also find expressed in this colloquium, in which I have tried to present, within certain limits, a general programme of my individual work. To carry out this plan in Gottingen, and to bring it to the notice of my students, I have, for many years, adopted the method of having my higher lectures carefully written out, and, in recent years, of having them lithographed, so as to make them more readily accessible. These former lectures are at the disposal of my hearers for consultation at the mathematical reading-room of the university ; those that are lithographed can be acquired by anybody, and I am much pleased to find them so well known here in America. As another important point, I wish to say that I have always regarded my students not merely as hearers or pupils, but as collaborators. I want them to take an active part in my own researches ; and they are therefore particularly welcome if they bring with them special knowledge and new ideas, whether these be original with them, or derived from some other source, from the teachings of other mathematicians. Such men will spend their time at Gottingen most profitably to themselves. I have had the pleasure of seeing many Americans among my students, and gladly bear testimony to their great enthusi- THE STUDY OF MATHEMATICS AT GOTTINGEN. 97 asm and energy. Indeed, I do not hesitate to say that, for some years, my higher lectures were mainly sustained by stu- dents whose home is in this country. But I deem it my duty to refer here to some difficulties that have occasionally arisen in connection with the coming of American students to Gottin- gen. Perhaps a frank statement on my part, at this opportunity, will contribute to remove these difficulties in part. What I wish to speak of is this. It frequently happens at Gottingen, and probably at other German universities as well, that American students desire to take the higher courses when their prepara- tion is entirely inadequate for such work. A student having nothing but an elementary knowledge of the differential and integral calculus, usually coupled with hardly a moderate famil- iarity with the German language, makes a decided mistake in attempting to attend my advanced lectures. If he comes to Got- tingen with such a preparation (or, rather, the lack of it), he may, of course, enter the more elementary courses offered at our university ; but this is generally not the object of his coming. Would he not do better to spend first a year or two in one of the larger American universities ? Here he would find more readily the transition to specialized studies, and might, at the same time, arrive at a clearer judgment of his own mathematical ability ; this would save him from the severe disappointment that might result from his going to Germany. I trust that these remarks will not be misunderstood. My presence here among you is proof enough of the value I attach to the coming of American students to Gottingen. It is in the interest of those wishing to go there that I speak ; and for this reason I should be glad to have the widest publicity given to what I have said on this point. Another difficulty lies in the fact that my higher lectures have frequently an encyclopedic character, conformably to the general tendency of my programme. This is not always just 98 LECTURE XII. what is most needful to the American student, whose work is naturally directed to gaining the doctor's degree. He will need, in addition to what he may derive from my lectures, the concentration on a particular subject ; and this he will often find best with other instructors, at Gottingen or elsewhere. I wish to state distinctly that I do not regard it as at all desira- ble that all students should confine their mathematical studies to my courses or even to Gottingen. On the contrary, it seems to me far preferable that the majority of the students should attach themselves to other mathematicians for certain special lines of work. My lectures may then serve to form the wider background on which these special studies are pro- jected. It is in this way, I believe, that my lectures will prove of the greatest benefit. In concluding I wish to thank you for your kind attention, and to give expression to the pleasure I have found in meeting here at Evanston, so near to Chicago, the great metropolis of this commonwealth, a number of enthusiastic devotees of my chosen science. THE DEVELOPMENT OF MATHEMATICS AT THE GERMAN UNIVERSITIES.* BY F. KLEIN. THE eighteenth century laid the firm foundation for the development of mathematics in all directions. The universi- ties as such, however, did not take a prominent part in this work ; the academies must here be considered of prime impor- tance. Nor can any fixed limits of nationality be recognized. At the beginning of the period there appears in Germany no less a man than Leibniz; then follow, among the kindred Swiss, the dynasty of the Bernoullis and the incomparable Elder. But the activity of these men, even in its outward manifestation, was not confined within narrow geographical bounds ; to encompass it we must include the Netherlands, and in particular Russia, with Germany and Switzerland. On the other hand, under Frederick the Great, the most eminent French mathematicians, Lagrange, d'Alembert, Maupertuis, formed side by side with Euler and Lambert the glory of the Berlin Academy. The impulse toward a complete change in these conditions came from the French Revolution. The influence of this great historical event on the devel- opment of science has manifested itself in two directions. On the one hand it has effected a wider separation of nations * Translation, with a few slight modifications by the author, of the section Mathe- matik in the work Die deutschen Universitaten, Berlin, A. Asher & Co., 1893, prepared by Professor Lexis for the World's Columbian Exposition at Chicago. 99 100 THE DEVELOPMENT OF MATHEMATICS with a distinct development of characteristic national quali- ties. Scientific ideas preserve, of course, their universality ; indeed, international intercourse between scientific men has become particularly important for the progress of science ; but the cultivation and development of scientific thought now progress on national bases. The other effect of the French Revolution is in the direction of educational methods. The decisive event is the foundation of the Ecole polytechnique at Paris in 1794. That scientific research and active instruction can be directly combined, that lectures alone are not suffi- cient, and must be supplemented by direct personal intercourse between the lecturer and his students, that above all it is of prime importance to arouse the student's own activity, these are the great principles that owe to this source their recogni- tion and acceptance. The example of Paris has been the more effective in this direction as it became customary to publish in systematic form the lectures delivered at this institution ; thus arose a series of admirable text-books which remain even now the foundation of mathematical study everywhere in Germany. Nevertheless, the principal idea kept in view by the founders of the Polytechnic School has never taken proper root in the German universities. This is the combination of the technical with the higher mathematical training. It is true that, prima- rily, this has been a distinct advantage for the unrestricted development of theoretical investigation. Our professors, find- ing themselves limited to a small number of students who, as future teachers and investigators, would naturally take great interest in matters of pure theory, were able to follow the bent of their individual predilections with much greater freedom than would have been possible otherwise. But we anticipate our historical account. First of all we must characterize the position that Gauss holds in the science of this age. Gauss stands in the very front of the new develop- AT THE GERMAN UNIVERSITIES. IO I ment : first, by the time of his activity, his publications reach- ing back to the year 1799, and extending throughout the entire first half of the nineteenth century ; then again, by the wealth of new ideas and discoveries that he has brought forward in almost every branch of pure and applied mathematics, and which still preserve their fruitfulness ; finally, by his methods, for Gauss was the first to restore that rigour of demonstration which we admire in the ancients, and which had been forced unduly into the background by the exclusive interest of the preceding period in new developments. And yet I prefer to rank Gauss with the great investigators of the eighteenth century, with Euler, Lagrange, etc. H^Jhelongs to them by the universality of his work, in which no trace' s yet appears of that specialization which has become the characteristic of our times. He belongs to them by his exclusively acaden?^ interest, by the absence of the modern teaching activity just char/C^erized. We shall have a picture of the development of mathematics if we imagine a chain of lofty mountains as representative of the men of the eighteenth century, terminating in a mighty outlying summit, Gauss, and then a broader, hilly country of lower elevation ; but teeming with new elements of life. More immediately con- nected with Gauss we find in the following period only the astronomers and geodesists under the dominating influence of Bessel ; while in theoretical mathematics, as it begins hence- forth to be independently cultivated in our universities, a new epoch begins with the second quarter of the present century, marked by the illustrious names oijacobi and Dirichlet. Jacobi came originally from Berlin and returned there for the closing years of his life (died 1851). But it is the period from 1826 to 1843, when he worked at Konigsberg with Bessel and Franz Neumann, that must be regarded as the culmination of his activity. There he published in 1829 his Fundamenta nova theories functionum ellipticarum, in which he gave, in 102 THE DEVELOPMENT OF MATHEMATICS analytic form, a systematic exposition .of his own discoveries and those of Abel in this field. Then followed a prolonged resi- dence in Paris, and finally that remarkable activity as a teacher, which still remains without a parallel in stimulating power as well as in direct results in the field of pure mathematics. An idea of this work can be derived from the lectures on dynamics, edited by Clebsch in 1866, and from the complete list of his Konigsberg lectures as compiled by Kronecker in the seventh volume of the Gesammelte Werke. The new feature is that Jacobi lectured exclusively on those problems on which he was working himself, and made it his sole object to introduce his students into his own circle of ideas. With this end in view he founded, for instance, the first mathematical seminary. And so great was his enthusiasm that often he not only gave the most important new results of his researches in these lectures, but did not even take the time to publish them elsewhere. Dirichlet worked first in Breslau, then for a long period (1831-1855) in Berlin, and finally for four years in Gottingen. Following Gauss, but at the same time in close connection with the contemporary French scholars, he chose mathemati- cal physics and the theory of numbers as the central points of his scientific activity. It is to be noticed that his interest is directed less towards comprehensive developments than towards simplicity of conception and questions of principle ; these are also the considerations on which he insists particularly in his lectures. These lectures are characterized by perfect lucidity and a certain refined objectivity ; they are at the same time particularly accessible to the beginner and suggestive in a high degree to the more advanced reader. It may be sufficient to refer here to his lectures on the theory of numbers, edited by Dedekind ; they still form the standard text-book on this subject. With Gauss, Jacobi, Dirichlet, we have named the men who have determined the direction of the subsequent development. AT THE GERMAN UNIVERSITIES. IO 3 We shall now continue our account in a different manner, arranging it according to the universities that have been most prominent from a mathematical standpoint. For henceforth, besides the special achievements of individual workers, the principle of co-operation, with its dependence on local condi- tions, comes to have more and more influence on the advance- ment of our science. Setting the upper limit of our account about the year 1870, we may name the universities of Konigs- berg, Berlin, Gottingen, and Heidelberg. Of Jacobi's activity at Konigsberg enough has already been said. It may now be added that even after his departure the university remained a centre of mathematical instruction. Richelot and Hesse knew how to maintain the high tradition of Jacobi, the former on the analytical, the latter on the geomet- rical side. At the same time Franz Neumann s lectures on mathematical physics began to attract more and more atten- tion. A stately procession of mathematicians has come from Konigsberg ; there is scarcely a university in Germany to which Konigsberg has not sent a professor. Of Berlin, too, we have already anticipated something in our account. The years from 1845 to 1851, during which Jacobi and Dirichlet worked together, form the culminating period of the first Berlin school. Besides these men the most promi- nent figure is that of Steiner (connected with the university from 1835 to 1864), the founder of the German synthetic geometry. An altogether original character, he was a highly effective teacher, owing to the one-sidedness with which he developed his geometrical conceptions. As an event of no mean importance, we must here record the foundation (in 1826) of Crelle s Journal fur reine und angewandte Mathematik. This, for decades the only German mathematical periodical, contained in its pages the fundamental memoirs of nearly all the emi- nent representatives of the rapidly growing science in Germany. 104 THE DEVELOPMENT OF MATHEMATICS Among foreign contributions the very first volumes presented Abel's pioneer researches. Crelle himself conducted this peri- odical for thirty years; then followed Borchardt, 1856-1880; now the Journal has reached its uoth volume. We must also mention the formation (in 1844) of the Berliner physi- kalische Gesellschaft. Men like Helmholtz, Kirclihoff, and Clansius have grown up here ; and while these men cannot be assigned to mathematics in the narrower sense, their work has been productive of important results for our science in various ways. During the same period, Encke exercised, as director of the Berlin astronomical observatory (1825-1862), a far-reaching influence by elaborating the methods of astro- nomical calculation on the lines first laid down by Gauss. We leave Berlin at this point, reserving for the present the account of the more recent development of mathematics at this university. The discussion of the Gottingen school will here find its appropriate place. The permanent foundation on which the mathematical importance of Gottingen rests is necessarily the Gauss tradition. This found, indeed, its direct continuation on the physical side when Wilhelm Weber returned from Leipsic to Gottingen (1849) and for the first time established systematic exercises in those methods of exact electro-magnetic measurement that owed their origin to Gauss and himself. On the mathematical side several eminent names follow in rapid succession. After Gauss's death, Dirichlet was called as his successor and transferred his great activity as a teacher to Gottingen, for only too brief a period (1855-59). By his side grew up Riemann (1854-66), to be followed later by Clebsch (1868-72). Riemann takes root in Gauss and Dirichlet ; on the other hand he fully assimilated Cauchy's ideas as to the use of complex variables. Thus arose his profound creations in the AT THE GERMAN UNIVERSITIES. 105 theory of functions which ever since have proved a rich and permanent source of the most suggestive material. Clebsch sustains, so to speak, a complementary relation to Riemann. Coming originally from Konigsberg, and occupied with mathe- matical physics, he had found during the period of his work at Giessen (1863-68) the particular direction which he after- wards followed so successfully at Gottingen. Well acquainted with the work of Jacobi and with modern geometry, he intro- duced into these fields the results of the algebraic researches of the English mathematicians Cayley and Sylvester, and on the double foundation thus constructed, proceeded to build up new approaches to the problems of the entire theory of functions, and in particular to Riemann's own developments. But with this the significance of Clebsch for the development of our science is not completely characterized. A man of vivid imagi- nation who readily entered into the ideas of others, he influ- enced his students far beyond the limits of direct instruction ; of an active and enterprising character, he founded, together with C. Neumann in Leipsic, a new periodical, the Mathe- matische Annalen, which has since been regularly continued, and is just concluding its 41 st volume. We recall further those memorable years of Heidelberg, from 1855 to perhaps 1870. Here were delivered Hesse's elegant and widely read lectures on analytic geometry. Here Kirch- hoff produced his lectures on mathematical physics. Here, above all, Helmholtz completed his great papers on mathe- matical physics, which in their turn served as basis for Kirch- hoff's elegant later researches. It remains now to speak of the second Berlin school, beginning also about the middle of the century, but still operating upon the present age. Kummer, Kronecker, Weierstrass, have been its leaders, the first two, as students of Dirichlet, pre-eminently engaged in developing the theory of numbers, while the last, 106 THE DEVELOPMENT OF MATHEMATICS leaning more on Jacobi and Cauchy, became, together with Riemann, the creator of the modern theory of functions. Kummer's lectures can here merely be named in passing ; with their clear arrangement and exposition they have always proved especially useful to the majority of students, without being particularly notable for their specific contents. Quite different is the case of Kronecker and Weierstrass, whose lectures became in the course of time more and more the expression of their scientific individuality. To a certain ex- tent both have thrust intuitional methods into the back- ground and, on the other hand, have in a measure avoided the long formal developments of our science, applying them- selves with so much the keener criticism to the fundamental analytical ideas. In this direction Kronecker has gone even farther than Weierstrass in trying to banish altogether the idea of the irrational number, and to reduce all developments to relations between integers alone. The tendencies thus characterized have exerted a wide-felt influence, and give a distinctive character to a large part of our present mathe- matical investigations. We have thus sketched in general outlines the state reached by our science about the year 1870. It is impossible to carry our account beyond this date in a similar form. For the devel- opments that now arise are not yet finished ; the persons whom we should have to name are still in the midst of their creative activity. All we can do is to add a few remarks of a more general nature on the present aspect of mathematical science in Germany. Before doing this, however, we must supple- ment the preceding account in two directions. Let it above all be emphasized that even within the limits here chosen, we have by no means exhausted the subject. It is, indeed, characteristic of the German universities that their life is not wholly centralized, that wherever a leader appears, AT THE GERMAN UNIVERSITIES. 107 he will find a sphere of activity. We may name here, from an earlier period, the acute analyst J. Fr. Pfaff, who worked in Helmstadt and Halle from 1788 to 1825, and, at one time, had Gauss among his students. Pfaff was the first representative of the combinatory school, which, for a time, played a great r61e in different German universities, but was finally pushed aside in the manifold development of the advancing science. We must further mention the three great geometers, Mbbius in Leipsic, Pliicker in Bonn, von Staudt in, Erlangen. Mobius was, at the same time, an astronomer, and conducted the Leipsic observa- tory from 1816 till 1868. Pliicker, again, devoted only the first half of his productive period (1826-46) to mathematics, turning his attention later to experimental physics (where his researches are well known), and only returning to geometrical investigation towards the close of his life (1864-68). The accidental circum- stance that each of these three men worked as teacher only in a narrow circle has kept the development of modern geometry unduly in the background in our sketch. Passing beyond university circles, we may be allowed to add the name of Grassmann, of Stettin, who, in his Ausdehnungslehre (1844 an ^ 1862), conceived a system embracing the results of modern geometrical speculation, and, from a very different field, that of Hansen, of Gotha, the celebrated representative of theoretical astronomy. We must also mention, in a few words, the development of technical education. About the middle of the century, it became the custom to call mathematicians of scientific eminence to the polytechnic schools. Foremost in this respect stands Zurich, which, in spite of the political boundaries, may here be counted as our own ; indeed, quite a number of professors have taught in the Zurich polytechnic school who are to-day ornaments of the German universities. Thus the ideal of the Paris school, the combination of mathematical with technical education, 108 THE DEVELOPMENT OF MATHEMATICS became again more prominent. A considerable influence in this direction was exercised by Redtenbacher s lectures on the theory of machines which attracted to Carlsruhe an ever-increas- ing number of enthusiastic students. Descriptive geometry and kinematics were scientifically elaborated. Culmann of Zurich, in creating graphical statics, introduced the principles of modern geometry, in the happiest manner, into mechanics. In connec- tion with the scientific advance thus outlined, numerous new polytechnic schools were founded in Germany about 1870 and during the following years, and some of the older schools were reorganized. At Munich and Dresden, in particular, in accord- ance with the example of Zurich, special departments for the training of teachers and professors were established. The polytechnic schools have thus attained great importance for mathematical education as well as for the advancement of the science. We must forbear to pursue more closely the many interesting questions that present themselves in this connection. If we survey the entire field of development described above, this, at any rate, appears as the obvious conclusion, in Germany as elsewhere, that the number of those who have an earnest interest in mathematics has increased very rapidly and that, as a consequence, the amount of mathematical production has grown to enormous proportions. In this respect an imperative need was supplied when Ohrtmann and Mutter established in Berlin (1869) an annual bibliographical review, Die Fortschritte der Mathematik, of which the 2ist volume has just appeared. In conclusion a few words should here be said concerning the modern development of university instruction. The principal effort has been to reduce the difficulty of mathematical study by improving the seminary arrangements and equipments. Not only have special seminary libraries been formed, but study rooms have been set aside in which these libraries are immediately accessible to the students. Collections of AT THE GERMAN UNIVERSITIES. 109 mathematical models and courses in drawing are calculated to disarm, in part at least, the hostility directed against the excessive abstractness of the university instruction. And while the students find everywhere inducements to specialized study, as is indeed necessary if our science is to flourish, yet the tendency has at the same time gained ground to emphasize more and more the mutual interdependence of the different special branches. Here the individual can accomplish but little ; it seems necessary that many co-operate for the same purpose. Such considerations have led in recent years to the formation of a German mathematical association (Deutsche Mathematiker-Vereinigung). The first annual report just issued (which contains a detailed report on the development of the theory of invariants) and a comprehensive catalogue of mathe- matical models and apparatus published at the same time indi- cate the direction that is here to be followed. With the present means of publication and the continually increasing number of new memoirs, it has become almost impossible to survey comprehensively the different branches of mathematics. Hence it is the object of the association to collect, systema- tize, maintain communication, in order that the work and progress of the science may not be hampered by material difficulties. Progress itself, however, remains in mathe- matics even more than in other sciences always the right and the achievement of the individual. GOTTINGEN, January, 1893. University of California SOUTHERN REGIONAL LIBRARY FACILITY 305 De Neve Drive - Parking Lot 17 Box 951388 LOS ANGELES, CALIFORNIA 90095-1388 Return this material to the library from which it was borrowed. QL, DUE 5WWIP SEL/EMS THE LIBRARY UNIVERSITY OF CALIFORNIA LOS ANGELES Engineering & Mathematical Sciences STACK QA 7 K67 1