Antidiagonal and square averages for amenable groups

Abstract

We prove that, for commuting unitary representations $U,V$ of a countable amenable group $G$, the averages $$ D_F=\frac1{|F|}\sum_{g\in F}U_g^*V_g $$ have a common selfadjoint weak operator limit along all left and right \Folner{} sequences, with strong* convergence along two-sided sequences. This answers Bergelson's question on commuting actions. For any unitary representation $π$, we also prove that $A_F=|F|^{-1}\sum_{g\in F}π(g^2)$ converges strongly along right \Folner{} sequences to a selfadjoint central operator supported on the finite type~I part of $π(G)''$; the adjoints $A_F^*$ converge strongly along left \Folner{} sequences. This settles the convergence question for square averages posed by Bergelson and Rodríguez-Martín. For measure-preserving actions of finitely generated amenable or countable virtually FC groups, we obtain $$ \lim_n\frac1{|F_n|}\sum_{g\in F_n}μ(A\cap T_{g^2}A)>0 \qquad\text{whenever }μ(A)>0. $$ For each $\varepsilon>0$, the set $\{g:μ(A\cap T_{g^2}A)>μ(A)^2-\varepsilon\}$ meets every Bohr neighborhood of the identity in a set of positive lower density, uniformly over \Folner{} sequences. These intersections are also left and right IP$^*$ for virtually FC groups. We construct a unitary representation $π$ of a locally finite nilpotent group and a weakly mixing vector $f$ for which $\{g:\text{Re}\langleπ(g^2)f,f\rangle>-\varepsilon\}$ has upper Banach density zero for small $\varepsilon>0$. We also give a weakly mixing action of a two-generated solvable group for which the IP$^*$ conclusion fails. The recurrence results yield product sets $BB$ inside difference sets, with $B$ of positive upper density.