Non-conventional ergodic averages for several commuting actions of an amenable group (1309.4315v4)

Abstract: Let $(X,\mu)$ be a probability space, $G$ a countable amenable group and $(F_n)n$ a left F\o lner sequence in $G$. This paper analyzes the non-conventional ergodic averages [\frac{1}{|F_n|}\sum{g \in F_n}\prod_{i=1}^d (f_i\circ T_1^g\cdots T_i^g)] associated to a commuting tuple of $\mu$-preserving actions $T_1$, ..., $T_d:G\curvearrowright X$ and $f_1$, ..., $f_d \in L^\infty(\mu)$. We prove that these averages always converge in $|\cdot|_2$, and that they witness a multiple recurrence phenomenon when $f_1 = \ldots = f_d = 1_A$ for a non-negligible set $A\subseteq X$. This proves a conjecture of Bergelson, McCutcheon and Zhang. The proof relies on an adaptation from earlier works of the machinery of sated extensions.