The strong Macdonald conjecture and Hodge theory on the Loop Grassmannian

Abstract

We prove the strong Macdonald conjecture of Hanlon and Feigin for reductive groups G. In a geometric reformulation, we show that the Dolbeault cohomology $H^q(X;\Omega^p)$ of the loop Grassmannian X is freely generated by de Rham's forms on the disk coupled to algebra generators of $H*(BG)$. Equating Euler characteristics of the two gives an identity, independently known to Macdonald [M], which generalises Ramanujan's_1\psi_1 sum. Simply laced root systems at level 1 are related to a `strong'_4\psi_4 sum. Failure of Hodge decomposition implies the singularity of X, and of the algebraic loop groups.