English

Obstructions to lifting cocycles on groupoids and the associated $C^*$-algebras

Operator Algebras 2017-06-19 v2

Abstract

Given a short exact sequence of locally compact abelian groups 0ABC00 \to A \to B \to C \to 0 and a continuous CC-valued 11-cocycle ϕ\phi on a locally compact Hausdorff groupoid Γ\Gamma we construct a twist of Γ\Gamma by AA that is trivial if and only if ϕ\phi lifts. The cocycle determines a strongly continuous action of C^\widehat{C} into AutC(Γ)\operatorname{Aut} C^*(\Gamma) and we prove that the CC^*-algebra of the twist is isomorphic to the induced algebra of this action if Γ\Gamma is amenable. We apply our results to a groupoid determined by a locally finite cover of a space XX and a cocycle provided by a \v{C}ech 1-cocycle with coefficients in the sheaf of germs of continuous T\mathbb{T}-valued functions. We prove that the CC^*-algebra of the resulting twist is continuous trace and we compute its Dixmier-Douady invariant.

Keywords

Cite

@article{arxiv.1612.07257,
  title  = {Obstructions to lifting cocycles on groupoids and the associated $C^*$-algebras},
  author = {Marius Ionescu and Alex Kumjian},
  journal= {arXiv preprint arXiv:1612.07257},
  year   = {2017}
}

Comments

We made some changes indicated by the referee. The paper is accepted for publication in the M\"unster Journal of Mathematics