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Hermann Weyl and Representation Theory GENERAL ARTICLE Hermann Weyl and Representation Theory B Sury He invented the character formula, the unitarian trick, and decoded uniform distribution – pretty quick; told us to hear the volume of a drum but, about the shape, he said it was mum. His favourite contribution? Take your pick! A mathematcian with a physicist’s guile, Sury was at the Tata Institute and a global viewpoint – his signature style; of Fundamental Research in be it number theory, eigenvalues, Lie groups, Mumbai during 1981–1999 by his grace, readily jumped through hoops; after which he moved to the such was the magic of Hermann Weyl! Indian Statistical Institute in Bengaluru. He is the National Weyl was a universal mathematician whose fundamental con- co-ordinator for the tributions to mathematics encompassed all areas, and pro- Mathematical Olympiad Programme in India. vided a unification seldom seen. His work on the theory of Lie groups was motivated by his life-long interest in quan- tum mechanics and relativity. When Weyl entered Lie theory, it mostly focussed on the infinitesimal, and he strove to bring in a global perspective. Time and again, Weyl’s ideas arising in one context have been adapted and applied to wholly new contexts. In 1925–26, Weyl wrote four epochal papers in rep- resentation theory of Lie groups which solved fundamental problems, and also gave birth to the subject of harmonic anal- ysis of semisimple Lie groups. In these papers, Weyl proved complete reducibility theorems and introduced many tech- niques which have become the standard way to study rep- resentations of Lie groups and their various generalizations in the last seven decades. Weyl’s work covers several parts of Keywords mathematics, as well as parts of physics. In this article, we Lie groups, gauge theory, epigen- discuss mainly his contributions to the representation theory values, unitarian trick, character formula, Peter–Weyl theorem. of Lie groups via the four papers mentioned above. RESONANCE | December 2016 1073 GENERAL ARTICLE 1. Weyl’s Philosophy of Mathematics. “The gods have imposed upon my writing the yoke of a foreign language that was not sung at my cradle” – these famous words appear in the preface to the celebrated book on classical groups by Hermann Weyl. Weyl was a poet, a visionary scientist, and a universal mathematician – all bundled into one. His contributions to mathematics encompassed all areas, and provided a unification seldom seen in the work of mathematicians. Weyl strove to bring in a global perspective to the theory of Lie groups which, at that time, focussed on the infinitesimal. The spectral properties of differential operators like the Laplacian – especially their asymp- totics, was a refrain that kept surfacing in his work. This aspect has grown in riches with Mark Kac and others who discovered far-reaching connections between isopectrality and isometricity. After his time, these ideas have led to the so-called elliptic oper- ators proof of the Atiyah–Patodi–Singer theorem. An asymptotic result on the count of eigenvalues proved by him for a bounded domain in 3D-space is known as Weyl’s law and has been gener- alized to diverse contexts like automorphic forms. For instance, such an asymptotic formula proved for Riemann surfaces by Atle Selberg is of fundamental importance in showing the existence of abundantly many Maass forms for congruence subgroups in SL(2, R). 1Joseph Samuel, Of Connec- His enduring interest in general relativity prompted him to pro- tions and Fields - I, Resonance, pose a method to explain electromagnetism and Maxwell’s equa- Vol.10, No.4, pp.10–21, 2005. tions via Einstein’s theory. This failed as the physics did not sup- port it – as Einstein said, if Weyl was right, the size of a particle would depend on its past history which would contradict experi- Apart from their ments showing that all hydrogen atoms had identical properties. importance in particle However, Weyl’s ideas were reinterpreted by Kaluza using quan- physics, these tum mechanics, and this led to gauge theory1,aflourishing sub- non-abelian gauge theories have led to ject today. Apart from their importance in particle physics, these important mathematical non-abelian gauge theories have led to important mathematical work on four manifolds work on four manifolds by Simon Donaldson and others. by Simon Donaldson and others. As mentioned above, Weyl was the first to take a giant step in 1074 RESONANCE | December 2016 GENERAL ARTICLE relating the spectral properties of the Laplacian of Riemannian manifolds22with their geometry and topology. In 1911, he con- Harish Seshadri and Kaushal sidered a conjecture due to Lorentz and Sommerfeld, and showed Verma, The Embedding that the dimension and volume of a bounded domain M in the Theorems of Whitney and Nash, Resonance, Vol.21, Euclidean space of dimension 2 or 3 is determined by its Dirich- No.9, pp.815–826, 2016. let or Neumann spectrum. More precisely, Weyl proved that the number of eigenvalues not exceeding λ is asymptotically of the Vol(Bn)Vol(M) λn/2 form (2π)n , where Bn is the unit ball in Euclidean n- space. Nowadays, this is called Weyl’s Law. The law has now been generalized to closed Riemannian manifolds, and apart from the spectrum determining the volume and dimension, it also de- termines the total scalar curvature. In 1925–26, Weyl wrote four path-breaking papers in representa- tion theory which apart from solving fundamental problems, also gave birth to the subject of harmonic analysis of semisimple Lie groups. In these papers, Weyl proved complete reducibility the- orems and introduced many techniques like ‘character formulae’ which have become the standard way to study representations of Lie groups and their generalizations. In 1926, along with F Peter, Weyl proved what he had announced in a 1925 paper, now known as the Peter–Weyl theorem. Weyl’s abiding interest in differential equations played a role in the proof of the Peter–Weyl theorem. There is a masterful exposition on Weyl’s work on Lie theory by A Borel, and a volume on Weyl’s legacy edited by Katrin Trent. The reader is referred to these for detailed and insightful descrip- tions of Weyl’s work in diverse areas; for representation theory, I have followed the excellent essays by Thomas Hawkins men- tioned at the end of this article. Weyl, as we mentioned, was a universalist who strongly believed 3Rajendra Bhatia, David in the unity of mathematical methodology. In this, he was a Hilbert, Resonance,Vol.4, true disciple of Hilbert3. Both believed in, and followed what No.8, pp.3–5, 1999. is sometimes known as the Riemannian doctrine whose essence is that proofs should be driven solely by ideas and not by cal- Weyl believed in the culations. This was exemplified by Weyl’s work time and again; doctrine that proofs for instance, ideas from Riemann surfaces and covering spaces should be driven solely by ideas and not by were brought to bear on the algebraic problem of complete re- calculations. RESONANCE | December 2016 1075 GENERAL ARTICLE In his book on group ducibility. In his book on group theory and quantum mechanics, theory and quantum Weyl considered for the first time, quantum systems in which the mechanics, Weyl configuration spaces are replaced by abelian groups in duality – considered for the first 4 time, quantum systems including finite abelian groups! Later, Andre Weil took over the in which the study of these Weyl systems over locally compact abelian groups configuration spaces are in duality in a path-breaking work. This led to the study of the so- replaced by abelian called Weil (or oscillator) representation, and choice of the ‘adele groups in duality – including finite abelian group’ as the abelian group enabled reinterpretation of Siegel’s groups! work on quadratic forms over number fields. 4Kapil H Paranjape, Andre Weyl developed general ideas leading to the synthesis of parts Weil, Resonance, Vol.4, No.5, of mathematics, and had a gift for bringing to focus notions and pp.3–5, 1999. naming them! He was the first to define vector spaces; the first to use the adjective ‘symplectic’ for the groups now known under that name; the first to coin the phrase ‘maximal torus’, and the first to use the phrase ‘Lie algebra’. He had a flair for analogy. At one point, when defining group action on characters by (gχ)(x) = χ(g−1 x), he likened the process to shoes and socks – the latter are worn first but the former are removed first! 2. Weyl’s Unitarian Trick The top practitioners of Lie theory at that time usually dealt with infinitesimal aspects. They made little distinction between a (global) Lie group and its Lie algebra. Weyl, being a globalist, was the first to break new ground, not only exposing the subtleties while making a distinction, but also pointing out the way to accomplish the desired goals globally. Weyl’s unitarian principle is essen- tially an outcome of the principle that a holomorphic function He had a flair for which is zero on the real line must be the zero function. analogy. At one point, , × when defining group Consider the ‘special linear’ group SL(n C)ofn n complex action on characters by matrices with unit determinant. Weyl was interested in describ- − (gχ)(x) = χ(g 1 x), he ing the representation theory of this group completely. The group likened the process to SU(n)ofn × n unitary matrices of unit determinant is a subgroup shoes and socks – the latter are worn first but of the special linear group. This is a compact group which makes the former are removed it possible to adapt methods from finite group theory by replac- first! ing addition by integration with respect to a translation-invariant 1076 RESONANCE | December 2016 GENERAL ARTICLE measure/volume.
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