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arXiv · 2502.17190

Type semigroups for twisted groupoids and a dichotomy for groupoid C*-algebras

Bartosz K. Kwaśniewski·Ralf Meyer·Akshara Prasad
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Abstract

We develop a theory of type semigroups for arbitrary twisted, not necessarily Hausdorff \'etale groupoids. The type semigroup is a dynamical version of the Cuntz semigroup. We relate it to traces, ideals, pure infiniteness, and stable finiteness of the reduced and essential C*-algebras. If the reduced C*-algebra of a twisted groupoid is simple and the type semigroup satisfies a weak version of almost unperforation, then the C*-algebra is either stably finite or purely infinite. We apply our theory to Cartan inclusions. We calculate the type semigroup for the possibly non-Hausdorff groupoids associated to self-similar group actions on graphs and deduce a dichotomy for the resulting Exel-Pardo algebras.

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Bartosz K. Kwaśniewski, Ralf Meyer, Akshara Prasad. 2025-02-24. Type semigroups for twisted groupoids and a dichotomy for groupoid C*-algebras. https://arxiv.org/abs/2502.17190

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