Cohomology of hyperfinite Borel actions

Abstract

We study cocycles of countable groups $\Gamma$ of Borel automorphisms of a standard Borel space $(X,\mathcal{B})$ taking values in a locally compact second countable group $G$. We prove that for a hyperfinite group $\Gamma$ the subgroup of coboundaries is dense in the group of cocycles. We describe all Borel cocycles of the $2$-odometer and show that any such cocycle is cohomologous to a cocycle with values in a countable dense subgroup $H$ of $G$. We also provide a Borel version of Gottschalk–Hedlund theorem.