Cohomology support loci of local systems

TL;DR

This paper studies the support loci of Sabbah's specialization complex, showing it is always a hypersurface and providing a formula for it, with applications to the (semi-)simplicity of certain sheaves and D-modules.

Contribution

It proves that the support locus is always a hypersurface and offers a log resolution formula, advancing understanding of its geometric structure and applications.

Findings

Support locus is always a hypersurface.

Provides a formula for the support locus using log resolutions.

Offers a combinatorial criterion for hyperplane arrangements.

Abstract

The support S of Sabbah's specialization complex is a simultaneous generalization of the set of eigenvalues of the monodromy on Deligne's nearby cycles complex, of the support of the Alexander modules of an algebraic knot, and of certain cohomology support loci. Moreover, it equals conjecturally the image under the exponential map of the zero locus of the Bernstein-Sato ideal. Sabbah showed that S is contained in a union of translated subtori of codimension one in a complex affine torus. Budur-Wang showed recently that S is a union of torsion-translated subtori. We show here that S is always a hypersurface, and that it admits a formula in terms of log resolutions. As an application, we give a criterion in terms of log resolutions for the (semi-)simplicity as perverse sheaves, or as regular holonomic D-modules, of the direct images of rank one local systems under an open embedding. For hyperplane arrangements, this criterion is combinatorial.