Multiple ergodic averages for three polynomials and applications

Abstract

We find the smallest characteristic factor and a limit formula for the multiple ergodic averages associated to any family of three polynomials and polynomial families of the form $\{l_1p,l_2p,...,l_kp\}$. We then derive several multiple recurrence results and combinatorial implications, including an answer to a question of Brown, Graham, and Landman, and a generalization of the Polynomial Szemerédi Theorem of Bergelson and Leibman for families of three polynomials with not necessarily zero constant term. We also simplify and generalize a recent result of Bergelson, Host, and Kra, showing that for all $ε>0$ and every subset of the integers $Λ$ the set $$ \big\{n\in\N\colon d^*\big(Λ\cap (Λ+p_1(n))\cap (Λ+p_2(n))\cap (Λ+ p_3(n))\big)>(d^*(Λ))^4-ε\big\} $$ has bounded gaps for "most" choices of integer polynomials $p_1,p_2,p_3$.