Bohr recurrence and density of non-lacunary semigroups of $\mathbb{N}$ (2406.01353v3)

Abstract: A subset $R$ of integers is a set of Bohr recurrence if every rotation on $\mathbb{T}^d$ returns arbitrarily close to zero under some non-zero multiple of $R$. We show that the set ${k!\, 2^m3^n\colon k,m,n\in \mathbb{N}}$ is a set of Bohr recurrence. This is a particular case of a more general statement about images of such sets under any integer polynomial with zero constant term. We also show that if $P$ is a real polynomial with at least one non-constant irrational coefficient, then the set ${P(2^m3^n)\colon m,n\in \mathbb{N}}$ is dense in $\mathbb{T}$, thus providing a joint generalization of two well-known results, one of Furstenberg and one of Weyl.