Cup and cap products for cohomology and homology groups of ample groupoids

TL;DR

This paper develops the theory of cup and cap products for cohomology and homology of ample groupoids, with applications to operator algebras and explicit computations in tiling spaces.

Contribution

It introduces the cup and cap products for ample groupoids, providing foundational definitions, properties, and an application to automorphisms of groupoid $C^*$-algebras.

Findings

Defined the cup product as a graded ring structure on cohomology

Established the cap product relating homology and cohomology

Computed cup products explicitly for tiling space cohomology

Abstract

This paper explores the cup and cap products within the cohomology and homology groups of ample groupoids, focusing on their applications and fundamental properties. Ample groupoids, which are \'etale groupoids with a totally disconnected unit space, play a crucial role in the study of topological dynamical systems and operator algebras. We introduce the cup product, which defines a bilinear map on cohomology classes, providing a graded ring structure, and the cap product, which defines a bilinear map relating homology and cohomology. The paper aims to make these concepts accessible to a broader mathematical audience, offering clear definitions and detailed explanations. We also demonstrate an application of the cap product in the analysis of automorphisms of groupoid $C^*$-algebras. Specifically, we show how it helps determine the asymptotic innerness of automorphisms. Our results include the first explicit computations of cup products in the cohomology of tiling spaces, which may pave the way for new research in this area.