## Advanced Studies in Pure Mathematics

### Splitting in orbit equivalence, treeable groups, and the Haagerup property

Yoshikata Kida

#### Abstract

Let $G$ be a discrete countable group and $C$ its central subgroup with $G/C$ treeable. We show that for any treeable action of $G/C$ on a standard probability space $X$, the groupoid $G\ltimes X$ is isomorphic to the direct product of $C$ and $(G/C)\ltimes X$, through cohomology of groupoids. We apply this to show that any group in the minimal class of groups containing treeable groups and closed under taking direct products, commensurable groups and central extensions has the Haagerup property.

#### Article information

Dates
Revised: 23 April 2015
First available in Project Euclid: 4 October 2018