Szemerédi'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 $$\underset{M-N\to \infty }{\mathrm{liminf}}{\mathbb{E}}_{n\in [N]-[M]}m(A\cap T^{-k_n}A\cap \cdots \cap T^{-\ell k_n}A)>0.$$ The averages above are uniform in the index $n$, and so the set of indices $n$ for which $m(A\cap T^{-k_n}A\cap \cdots \cap T^{-\ell k_n}A)>0$ is syndetic. By the Furstenberg correspondence principle, it follows that for every set of integers $A$ of positive upper Banach density there is a syndetic set $B$ of indices such that, for each $n\in B$, the set of $a$ with $a,a+k_n,\dots ,a+\ell k_n\in A$ again has positive upper Banach density. A Cantor set contains an IP-set, so the existence of such progressions, and the positivity of the density above for a single index, already follow from the IP Szemerédi theorem of Furstenberg and Katznelson, and from the work of Kra and Shalom. The point of this paper is instead the frequency with which the step sizes $k_n$ may be found, measured in the natural enumeration of the Cantor set itself.

arXiv comment30 pages. A partial formalization is at https://github.com/lacey-gatech/CantorSzemeredi The formalization does not include a Furstenberg Dichotomy result, nor a proof of the ergodic decomposition