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

Abstract

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