Every action of a non-amenable group is the factor of a small action (1311.0738v2)

Abstract: It is well known that if $G$ is a countable amenable group and $G \curvearrowright (Y, \nu)$ factors onto $G \curvearrowright (X, \mu)$, then the entropy of the first action must be greater than or equal to the entropy of the second action. In particular, if $G \curvearrowright (X, \mu)$ has infinite entropy, then the action $G \curvearrowright (Y, \nu)$ does not admit any finite generating partition. On the other hand, we prove that if $G$ is a countable non-amenable group then there exists a finite integer $n$ with the following property: for every probability-measure-preserving action $G \curvearrowright (X, \mu)$ there is a $G$-invariant probability measure $\nu$ on $n^G$ such that $G \curvearrowright (n^G, \nu)$ factors onto $G \curvearrowright (X, \mu)$. For many non-amenable groups, $n$ can be chosen to be $4$ or smaller. We also obtain a similar result with respect to continuous actions on compact spaces and continuous factor maps.