Higher localization and higher branching laws

TL;DR

This paper extends localization functors to an equivariant derived setting for reductive groups, showing regular holonomic cohomologies and finite-dimensionality of certain homologies, with applications to branching laws and index theorems.

Contribution

It introduces a derived, equivariant localization framework for Harish-Chandra modules, revealing geometric interpretations of Lie algebra homologies and Ext-spaces, and establishing their finiteness and index relations.

Findings

Localizations have regular holonomic cohomologies.

Relative Lie algebra homologies are finite-dimensional.

Euler-Poincaré characteristics relate to local index theorem.

Abstract

For a connected reductive group $G$ and an affine smooth $G$-variety $X$ over the complex numbers, the localization functor takes $\mathfrak{g}$-modules to $D_X$-modules. We extend this construction to an equivariant and derived setting using the formalism of h-complexes due to Beilinson-Ginzburg, and show that the localizations of Harish-Chandra $(\mathfrak{g}, K)$-modules onto $X = H \backslash G$ have regular holonomic cohomologies when $H, K \subset G$ are both spherical reductive subgroups. The relative Lie algebra homologies and $\mathrm{Ext}$-branching spaces for $(\mathfrak{g}, K)$-modules are interpreted geometrically in terms of equivariant derived localizations. As direct consequences, we show that they are finite-dimensional under the same assumptions, and relate Euler-Poincar\'e characteristics to local index theorem; this recovers parts of the recent results of M. Kitagawa. Examples and discussions on the relation to Schwartz homologies are also included.