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

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arxiv 2502.17190 v3 pith:ODDGY4LX submitted 2025-02-24 math.OA

classification math.OA
keywords typesemigroupalgebrasgroupoidstwistedalgebradichotomygroupoid
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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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  1. A dichotomy for inverse-semigroup crossed products via dynamical Cuntz semigroups

    math.OA 2026-01 conditional novelty 6.0 of 10

    Minimal aperiodic inverse-semigroup crossed products, when reduced equals essential and the dynamical Cuntz semigroup has plain paradoxes, are either stably finite or purely infinite; which one is detected by existenc...

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