Homology of generalized Steinberg varieties and Weyl group invariants

Abstract

Let $G$ be a complex, connected, reductive algebraic group. In this paper we show analogues of the computations by Borho and MacPherson of the invariants and anti-invariants of the cohomology of the Springer fibres of the cone of nilpotent elements, $\mathcal{N}$, of $\mathrm{Lie}(G)$ for the Steinberg variety $Z$ of triples. Using a general specialization argument we show that for a parabolic subgroup $W_P\times W_Q$ of $W\times W$ the space of $W_P\times W_Q$-invariants and the space of $W_P\times W_Q$-anti-invariants of $H_{4n}(Z)$ are isomorphic to the top Borel-Moore homology groups of certain generalized Steinberg varieties introduced in [5]. The rational group algebra of the Weyl group $W$ of $G$ is isomorphic to the opposite of the top Borel-Moore homology $H_{4n}(Z)$ of $Z$, where $2n=\dim \mathcal{N}$. Suppose $W_P\times W_Q$ is a parabolic subgroup of $W\times W$. We show that the space of $W_P\times W_Q$-invariants of $H_{4n}(Z)$ is $e_Q\mathbb{Q}W{e}_P$, where $e_P$ is the idempotent in group algebra of $W_P$ affording the trivial representation of $W_P$ and $e_Q$ is defined similarly. We also show that the space of $W_P\times W_Q$-anti-invariants of $H_{4n}(Z)$ is ${\epsilon }_Q\mathbb{Q}W{\epsilon }_P$, where ${\epsilon }_P$ is the idempotent in group algebra of $W_P$ affording the sign representation of $W_P$ and ${\epsilon }_Q$ is defined similarly.

38 pages, to appear in Trans. Amer. Math. Soc