Inverse Systems of Groupoids, with Applications to Groupoid $C^*$-algebras

TL;DR

This paper introduces a framework for inverse systems of groupoids with Haar systems, demonstrating how to approximate complex groupoids and extend key theorems in the context of groupoid $C^*$-algebras.

Contribution

It defines Haar system preserving morphisms, constructs inverse systems of groupoids, and extends Renault's Maximal Equivalence Theorem to $\sigma$-compact groupoids.

Findings

Inverse systems of groupoids with Haar-preserving maps have well-defined limits.

Approximation of $\sigma$-compact groupoids by second countable groupoids is possible.

Extension of Renault's theorem to $\sigma$-compact groupoids achieved.

Abstract

We define what it means for a proper continuous morphism between groupoids to be Haar system preserving, and show that such a morphism induces (via pullback) a *-morphism between the corresponding convolution algebras. We proceed to provide a plethora of examples of Haar system preserving morphisms and discuss connections to noncommutative CW-complexes and interval algebras. We prove that an inverse system of groupoids with Haar system preserving bonding maps has a limit, and that we get a corresponding direct system of groupoid $C^*$-algebras. An explicit construction of an inverse system of groupoids is used to approximate a $\sigma$-compact groupoid $G$ by second countable groupoids; if $G$ is equipped with a Haar system and 2-cocycle then so are the approximation groupoids, and the maps in the inverse system are Haar system preserving. As an application of this construction, we show how to easily extend the Maximal Equivalence Theorem of Jean Renault to $\sigma$-compact groupoids.