Orbifold Euler characteristics of non-orbifold groupoids

TL;DR

This paper introduces generalized orbifold Euler characteristics for a broad class of topological groupoids, proving their invariance and relating them to classical invariants, thus extending the theory to non-orbifold groupoids.

Contribution

It defines new Euler characteristic invariants for topological groupoids, proves their Morita invariance, and connects them to existing orbifold invariants, broadening the scope of Euler characteristic theory.

Findings

The $ ext{Gamma}$-Euler and $ ext{Gamma}$-inertia Euler characteristics coincide.

These invariants generalize higher-order orbifold Euler characteristics.

The invariants are Morita invariant and satisfy classical properties.

Abstract

For a finitely presented discrete group $\Gamma$, we introduce two generalizations of the orbifold Euler characteristic and $\Gamma$-orbifold Euler characteristic to a class of proper topological groupoids large enough to include all cocompact proper Lie groupoids. The $\Gamma$-Euler characteristic is defined as an integral with respect to the Euler characteristic over the orbit space of the groupoid, and the $\Gamma$-inertia Euler characteristic is the usual Euler characteristic of the $\Gamma$-inertia space associated to the groupoid. A key ingredient is the application of o-minimal structures to study orbit spaces of topological groupoids. Our main result is that the $\Gamma$-Euler characteristic and $\Gamma$-inertia Euler characteristic coincide and generalize the higher-order orbifold Euler characteristics of Gusein-Zade, Luengo, and Melle-Hern\'{a}ndez from the case of a translation groupoid by a compact Lie group and $\Gamma = \mathbb{Z}^\ell$. By realizing the $\Gamma$-Euler characteristic as the usual Euler characteristic of a topological space, we demonstrate that it is Morita invariant in the category of topological groupoids and satisfies familiar properties of the classical Euler characteristic. We give an additional formulation of the $\Gamma$-Euler characteristic for a cocompact proper Lie groupoid in terms of a finite covering by orbispace charts. In the case that the groupoid is an abelian extension of a translation groupoid by a bundle of groups, we relate the $\Gamma$-Euler characteristics to those of the translation groupoid and bundle of groups.