Automorphisms of even unimodular lattices and equivariant Witt groups

Abstract

We characterize the irreducible polynomials that occur as a characteristic polynomial of an automorphism of an even unimodular lattice of given signature, generalizing a theorem of Gross and McMullen. As part of the proof, we give a general criterion in terms of Witt groups for a bilinear form equipped with an action of a group G over a discretely valued field to contain a unimodular G-stable lattice.

arXiv comment(v4: minor corrections in Propositions 7.2 and 9.1 ). To appear in JEMS