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Cocycle and orbit equivalence superrigidity for coinduced actions
Published online by Cambridge University Press: 03 April 2017
Abstract
We prove a cocycle superrigidity theorem for a large class of coinduced actions. In particular, if $\unicode[STIX]{x1D6EC}$ is a subgroup of a countable group $\unicode[STIX]{x1D6E4}$, we consider a probability measure preserving action $\unicode[STIX]{x1D6EC}\curvearrowright X_{0}$ and let $\unicode[STIX]{x1D6E4}\curvearrowright X$ be the coinduced action. Assume either that $\unicode[STIX]{x1D6E4}$ has property (T) or that $\unicode[STIX]{x1D6EC}$ is amenable and $\unicode[STIX]{x1D6E4}$ is a product of non-amenable groups. Using Popa’s deformation/rigidity theory we prove $\unicode[STIX]{x1D6E4}\curvearrowright X$ is ${\mathcal{U}}_{\text{fin}}$-cocycle superrigid, that is any cocycle for this action to a ${\mathcal{U}}_{\text{fin}}$ (e.g. countable) group ${\mathcal{V}}$ is cohomologous to a homomorphism from $\unicode[STIX]{x1D6E4}$ to ${\mathcal{V}}.$
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