$2$-large sets are sets of nil-Bohr recurrence

Abstract

A set $S$ of positive integers is $2$-large if every two-coloring contains arbitrarily long monochromatic arithmetic progressions with common difference in $S$. We prove that every such set meets every nil-Bohr neighborhood of $0$. This proves a conjecture of the first author and gives a sharp answer to a question of Host, Kra, and Maass. We also show that, for every fixed nil-Bohr neighborhood $B$ of $0$ and every $\epsilon >0$, restricting the common difference to $B$ increases the extremal number for $k$-term arithmetic progressions by at most a factor of $1+\epsilon$, provided $k$ is sufficiently large.

arXiv comment17 pages and 1 figure