Equivariant K-theory and Resolution II: Non-Abelian actions

TL;DR

This paper extends equivariant K-theory and cohomology models to non-Abelian group actions on manifolds, providing explicit resolutions, twisted bundles, and a natural Chern character linking K-theory to delocalized equivariant cohomology.

Contribution

It develops a non-Abelian equivariant K-theory framework using torsion-twisted bundles and iterated deRham models, generalizing previous Abelian cases.

Findings

Provides a resolution model for non-Abelian group actions.

Establishes a natural Chern character from K-theory to delocalized cohomology.

Shows the Atiyah-Hirzebruch isomorphism in this setting.

Abstract

The smooth action of a compact Lie group on a compact manifold can be resolved to an iterated space, as made explicit by Pierre Albin and the second author. On the resolution the lifted action has fixed isotropy type corresponding to the open stratum and also in an iterated sense, with connecting equivariant fibrations over the boundary hypersurfaces covering the resolutions of the other strata. This structure descends to a resolution of the quotient as a stratified space. For an Abelian group action the equivariant K-theory can then be described in terms of bundles over the bases `dressed' by the representations of the isotropy types with morphisms covering the connecting maps. A similar model is given here covering the non-Abelian case. Now the reduced objects are torsion-twisted bundles over finite covers of the bases, corresponding to the projective action of the normalizers on the representations of the isotropy groups, again with morphisms over all the boundaries. This leads to a closely related iterated deRham model for equivariant cohomology and, now with values in forms twisted by flat bundles of representation rings over the bases, for delocalized equivariant cohomology. We show, as envisioned by Baum, Brylinksi and MacPherson, that the usual equivariant Chern character, mapping to equivariant cohomology, factors through a natural Chern character from equivariant K-theory to delocalized equivariant cohomology with the latter giving an Atiyah-Hirzebruch isomorphism.