Torsion and abelianization in equivariant cohomology

Tara Holm, Reyer Sjamaar

Published 2006-07-03, updated 2009-06-09Version 4

Let $X$ be a topological space upon which a compact connected Lie group $G$ acts. It is well-known that the equivariant cohomology $H_G^*(X;\Q)$ is isomorphic to the subalgebra of Weyl group invariants of the equivariant cohomology $H_T^*(X;\Q)$, where $T$ is a maximal torus of $G$. This relationship breaks down for coefficient rings $\k$ other than $\Q$. Instead, we prove that under a mild condition on $\k$ the algebra $H_G^*(X,\k)$ is isomorphic to the subalgebra of $H_T^*(X,\k)$ annihilated by the divided difference operators.