Mixing and double recurrence in probability groups

Abstract: We define a class of groups equipped with an invariant probability measure, which includes all compact groups and is closed under taking ultraproducts with the induced Loeb measure; in fact, this class also contains the ultraproducts all locally compact unimodular amenable groups. We call the members of this class probability groups and develop the basics of the theory of their measure-preserving actions on probability spaces, including a natural notion of mixing. A short proof reveals that for probability groups mixing implies double recurrence, which generalizes a theorem of Bergelson and Tao proved for ultraproducts of finite groups. Moreover, a quantitative version of our proof gives that $\varepsilon$-approximate mixing implies $3\sqrt{\varepsilon}$-approximate double recurrence. Examples of approximately mixing probability groups are quasirandom groups introduced by Gowers, so the last theorem generalizes and sharpens the corresponding results for quasirandom groups of Bergelson and Tao, as well as of Austin. Lastly, we point out that the fact that the ultraproduct of locally compact unimodular amenable groups is a probability group provides a general alternative to Furstenberg correspondence principle.