Cartan Subalgebras in C*-Algebras of Haus dorff étale Groupoids

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Integral Equations and Operator Theory, ISSN: 0378-620X, Vol: 85, Issue: 1, Page: 109-126

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Brown, Jonathan H; Nagy, Gabriel; Reznikoff, Sarah; Sims, Aidan; Williams, Dana P
Springer Nature
Mathematics; tale; cartan; groupoids; subalgebras; c; algebras; hausdorff; Engineering; Science and Technology Studies
article description
The reduced C*-algebra of the interior of the isotropy in any Hausdorff étale groupoid G embeds as a C*-subalgebra M of the reduced C*-algebra of G. We prove that the set of pure states of M with unique extension is dense, and deduce that any representation of the reduced C*-algebra of G that is injective on M is faithful. We prove that there is a conditional expectation from the reduced C*-algebra of G onto M if and only if the interior of the isotropy in G is closed. Using this, we prove that when the interior of the isotropy is abelian and closed, M is a Cartan subalgebra. We prove that for a large class of groupoids G with abelian isotropy—including all Deaconu–Renault groupoids associated to discrete abelian groups—M is a maximal abelian subalgebra. In the specific case of k-graph groupoids, we deduce that M is always maximal abelian, but show by example that it is not always Cartan.