Lie algebra cohomology and Macdonald’s conjectures
Master’s Thesis of Maarten Solleveld, under supervision of Prof. Dr. E. M. Opdam
University of Amsterdam Faculty of Sciences Korteweg-De Vries Institute for Mathematics Plantage Muidergracht 24 1018 TV Amsterdam
September 2002
Contents
Introduction 3
1 Compact Lie groups and reductive Lie algebras 5 1.1 Representations ...... 5 1.2 Compact Lie groups and reductive Lie algebras ...... 9 1.3 Structure theory ...... 12 1.4 The Weyl group and invariant polynomials ...... 19 1.5 Harmonic polynomials ...... 24
2 Cohomology theory for Lie algebras 30 2.1 De Rham cohomology and Lie group actions ...... 31 2.2 The definition of the cohomology of a Lie algebra ...... 33 2.3 Relative Lie algebra cohomology ...... 39 2.4 Lie algebra cohomology with coefficients ...... 43 2.5 Isomorphism theorems ...... 50
3 The cohomology of compact Lie groups 55 3.1 G/T ...... 55 3.2 G and g ...... 60
4 Some specific Lie algebras and their cohomology 64 4.1 g[z, s]...... 65 4.2 g[z]/(zk)...... 76
5 Macdonald’s conjectures 82 5.1 The original setting ...... 82 5.2 Affine Lie algebras ...... 86
Bibliography 90
1 2 Introduction
In 1982 Macdonald [21] published his famous article Some conjectures for root sys- tems. The questions posed there inspired a lot of new research, and by know most of them are answered. This thesis deals with one of the most persisting conjectures:
Let R be a root system with exponents m1, . . . , ml and take k ∈ N∪{∞}. The constant term (depending only on q) of
k l Y Y Y k(m + 1) (1 − qieα)(1 − qi−1e−α) is i . k α∈R+ i=1 i=1 q
n Here is the q-binomial coefficient of n and r. It is known that one can prove r q this if the following conjecture holds:
Let g be a finite-dimensional complex semisimple Lie algebra with expo- k nents m1, . . . , ml. The cohomology of g ⊗C C[z]/(z ) is a free exterior algebra with kl generators. For each 1 ≤ i ≤ l, there are k generators of cohomology degree 2mi + 1, and the z-weight of these generators are the negatives of 0, mik + 1, mik + 2, . . . , mik + k − 1.
This conjecture occupies a central place in my thesis. Although I did not succeed in finding a complete proof, I strived to explain what appears to be the most promising way to look at it. Generally, in this thesis I sought to compute the cohomology rings of several classes of Lie algebras, all intimately connected to finite-dimensional semisimple Lie algebras. I aimed to make this accessible for anyone with a general knowledge Lie groups and Lie algebras, describing other background at a more elementary level. We start with a collection of well-known results for compact Lie groups, reductive Lie algebras and their invariants. In the second chapter all necessary cohomology theory for Lie algebras is developed. Here we prove one new result:
3 Let g be a Lie algebra over a field a characteristic 0, h a subalgebra and V a g-module, all finite-dimensional. Suppose that g and V are completely reducible, both as g- and as h-modules. (So in particular g is reductive.) Then H∗(g, h; V ) ∼= H∗(g, h) ⊗ V g.
With this machinery we compute the cohomology of a compact Lie group (or equiv- alently of a reductive Lie algebra) in a more algebraic way than usual. 2 In chapter 4 we introduce a Lie algebra g[z, s] = g ⊗C C[z, s], where s = 0 and g is complex, finite-dimensional and reductive. We try to compute the cohomology of this Lie algebra, which unfortunately just falls short. Nevertheless we come across a generalization of Chevalley’s restriction, which is probably new: Let g be a finite-dimensional complex reductive Lie algebra with adjoint group G, h a Cartan subalgebra and W the Weyl group. Then the restriction map Sg∗ ⊗ V(sg)∗ → Sh∗ ⊗ V(sg)∗ induces an isomorphism