Szemer\'edi's Theorem Along Cantor Sets of Integers

Abstract

Let $\mathcal C= \{k_1<k_2 < \cdots\}$ be Cantor set of integers, that is a set of integers with restricted digits modulo a base $b$, and suppose $0$ is one of the restricted digits. We show that $$ \liminf_N \Expectation_{n\in [N]} m(A\cap T^{-k_n} A \cap \cdots \cap T^{-\ell k_n} A )>0. $$ This is an extension of the IP Ergodic Theorem of Furstenberg and Katznelson, and a partial extension of recent work of Kra and Shalom. In particular, this implies that for any subset of integers $A$ of positive upper Banach density, there is a set $B$ of integers $n$ of positive lower Banach density such that $A$ contains an $\ell+1$ term progression, with step size $k_n$, where $n\in B$. This is a complement to recent results of Kra and Shalom, for IP Sets of integers, and Burgin, concerning Sarkozy's Theorem for Primes with restricted digits.