Sophus Lie: A Real Giant in Mathematics by Lizhen Ji* Abstract. This article presents a brief introduction other. If we treat discrete or finite groups as spe- to the life and work of Sophus Lie, in particular cial (or degenerate, zero-dimensional) Lie groups, his early fruitful interaction and later conflict with then almost every subject in mathematics uses Lie Felix Klein in connection with the Erlangen program, groups. As H. Poincaré told Lie [25] in October 1882, his productive writing collaboration with Friedrich “all of mathematics is a matter of groups.” It is Engel, and the publication and editing of his collected clear that the importance of groups comes from works. their actions. For a list of topics of group actions, see [17]. Lie theory was the creation of Sophus Lie, and Lie Contents is most famous for it. But Lie’s work is broader than this. What else did Lie achieve besides his work in the 1 Introduction ..................... 66 Lie theory? This might not be so well known. The dif- 2 Some General Comments on Lie and His Impact 67 ferential geometer S. S. Chern wrote in 1992 that “Lie 3 A Glimpse of Lie’s Early Academic Life .... 68 was a great mathematician even without Lie groups” 4 A Mature Lie and His Collaboration with Engel 69 [7]. What did and can Chern mean? We will attempt 5 Lie’s Breakdown and a Final Major Result . 71 to give a summary of some major contributions of 6 An Overview of Lie’s Major Works ....... 72 Lie in §6. 7 Three Fundamental Theorems of Lie in the One purpose of this article is to give a glimpse Lie Theory ...................... 73 of Lie’s mathematical life by recording several things 8 Relation with Klein I: The Fruitful Coopera- which I have read about Lie and his work. Therefore, tion .......................... 74 it is short and emphasizes only a few things about his 9 Relation with Klein II: Conflicts and the Fa- mathematics and life. For a fairly detailed account of mous Preface .................... 74 his life (but not his mathematics), see the full length 10 Relations with Others ............... 77 biography [27]. 11 Collected Works of Lie: Editing, Commentar- We also provide some details about the unfor- ies and Publication ................. 78 tunate conflict between Lie and Klein and the fa- mous quote from Lie’s preface to the third volume 1. Introduction of his books on transformation groups, which is usu- ally only quoted without explaining the context. The There are very few working mathematicians and fruitful collaboration between Engel and Lie and the physicists who have not heard of Lie groups or Lie publication of Lie’s collected works are also men- algebras and made use of them in some way or an- tioned. We hope that this article will be interesting and * Department of Mathematics, University of Michigan, Ann Arbor, Mich., U.S.A. instructive to the reader and might serve as a brief E-mail: [email protected] introduction to the work and life of Lie. 66 NOTICES OF THE ICCM VOLUME 3,NUMBER 1 2. Some General Comments on Lie and will learn to appreciate this visionary’s mind better than the present generation, who can only appre- His Impact ciate the mathematicians’ sharp intellect. The all- It is known that Lie’s main work is concerned encompassing scope of this man’s vision, which, above all, demands recognition, is nearly com- with understanding how continuous transformation pletely lost. But, the coming generation [...] will un- groups provide an organizing principle for different derstand the importance of the theory of transfor- areas of mathematics, including geometry, mechan- mation groups and ensure the scientific status that this magnificent work deserves. ics, and partial differential equations. But it might not be well known that Lie’s collected works consist of What Lie studied are infinitesimal Lie groups, or 7 large volumes of the total number of pages about essentially Lie algebras. Given what H. Weyl and E. 5600. (We should keep in mind that a substantial por- Cartan contributed to the global theory of Lie groups tion of these pages are commentaries on his papers starting around the middle of 1920s and hence made written by the editors. In spite of this, Lie’s output Lie groups one of the most basic and essential ob- was still enormous.) Probably it is also helpful to keep jects in modern (or contemporary) mathematics, one in mind that Lie started to do mathematics at the age must marvel at the above visionary evaluation of Lie’s of 26 and passed away at 57. Besides many papers, he work by Study. For a fairly detailed overview of the wrote multiple books, which total over several thou- historical development of Lie groups with particular sands of pages as well. According to Lie, only a part emphasis on the works of Lie, Killing, E. Cartan and of his ideas had been put down into written form. In Weyl, see the book [14]. an autobiographic note [9, p. 1], Lie wrote: Two months after Lie died, a biography of him ap- My life is actually quite incomprehensible to me. peared in the American Mathematical Monthly [12]. It As a young man, I had no idea that I was blessed was written by George Bruce Halsted, an active math- with originality. Then, as a 26-year-old, I suddenly ematics educator and a mathematician at the Univer- realized that I could create. I read a little and began sity of Texas at Austin, who taught famous mathe- to produce. In the years 1869–1874, I had a lot of ideas which, in the course of time, I have developed maticians like R. L. Moore and L. E. Dickson. Reading only very imperfectly. it more than one hundred years later, his strong state- In particular, it was group theory and its great im- ment might sound a bit surprising but is more justi- portance for the differential equations which inter- ested me. But publication in this area went woefully fied than before, “[...] the greatest mathematician in slow. I could not structure it properly, and I was the world, Sophus Lie, died [...] His work is cut short; always afraid of making mistakes. Not the small his influence, his fame, will broaden, will tower from inessential mistakes… No, it was the deep-rooted errors I feared. I am glad that my group theory in day to day.” its present state does not contain any fundamental Probably a more accurate evaluation of Lie was errors. given by Engel in a memorial speech on Lie [9, p. 24] Lie was a highly original and technically pow- in 1899: erful mathematician. The recognition of the idea of If the capacity for discovery is the true measure Lie groups (or transformation groups) took time. In of a mathematician’s greatness, then Sophus Lie must be ranked among the foremost mathemati- 1870s, he wrote in a letter [26, p. XVIII]: cians of all time. Only extremely few have opened If I only knew how to get the mathematicians inter- up so many vast areas for mathematical research ested in transformation groups and their applica- and created such rich and wide-ranging methods tions to differential equations. I am certain, abso- as he [...] In addition to a capacity for discovery, lutely certain in my case, that these theories in the we expect a mathematician to posses a penetrat- future will be recognized as fundamental. I want to ing mind, and Lie was really an exceptionally gifted form such an impression now, since for one thing, mathematician [...] His efforts were based on tack- I could then achieve ten times as much. ling problems which are important, but solvable, and it often happened that he was able to solve In 1890, Lie was confident and wrote that he problems which had withstood the efforts of other strongly believed that his work would stand through eminent mathematicians. all times, and in the years to come, it would be more In this sense, Lie was a giant in mathematics for and more appreciated by the mathematical world. his deep and original contribution to mathematics, Eduard Study was a privatdozent (lecturer) in and is famous not for other reasons. (One can eas- Leipzig when Lie held the chair in geometry there. In ily think of several mathematicians, without nam- 1924, the mature Eduard Study summarized Lie as ing them, who are famous for various things be- follows [26, p. 24]: sides mathematics). Incidentally, he was also a giant Sophus Lie had the shortcomings of an autodidact, in the physical sense. There are some vivid descrip- but he was also one of the most brilliant mathe- tions of Lie by people such as E. Cartan [1, p. 7], En- maticians who ever lived. He possessed something gel [27, p. 312], and his physics colleague Ostwald at which is not found very often and which is now becoming even rarer, and he possessed it in abun- Leipzig [27, p. 396]. See also [27, p. 3]. For some in- dance: creative imagination. Coming generations teresting discussions on the relations between giants JULY 2015 NOTICES OF THE ICCM 67 and scientists, see [11, pp. 163–164, p. 184] and [22, well as others who had made important contributions pp. 9–13]. to algebraic and analytic geometry. Lie plowed through many volumes of the leading 3. A Glimpse of Lie’s Early Academic mathematical journals from Paris and Berlin, and in the Science Students Association he gave several lec- Life tures during the spring of 1869 on what he called his Lie was born on December 17, 1842.