Weak mixing and non-recurrence along finitely generated multiplicative semigroups of $\mathbb N$

Abstract

Let $\mathcal P$ be a finite nonempty set of primes, and let $S_{\mathcal P}$ be the multiplicative semigroup that they generate. We prove that there are an invertible weakly mixing measure-preserving transformation $T$ and a set $A$ of positive measure such that $A\cap T^{-r}A=\varnothing$ for every $r\in S_{\mathcal P}$. This gives a negative answer to Bergelson's 1996 question concerning recurrence along $\{2^m3^n:m,n\geq1\}$, and applies to every finitely generated multiplicative semigroup of positive integers. More generally, we give a sufficient condition for weakly mixing non-recurrence based on divisibility properties and separation by finitely many rotations of the circle. This criterion also applies to an infinitely generated multiplicative semigroup not contained in any $S_{\mathcal P}$ with $\mathcal P$ finite.