Bohr sets in sumsets III: expanding difference sets and almost Bohr sets

Abstract: Let $G$ be a discrete abelian group. Følner showed that if $A \subseteq G$ has positive upper Banach density, then $A - A$ contains an almost Bohr set -- a set of the form $B \setminus E$ where $B$ is a Bohr set and $E$ has zero Banach density. We study the sets $S \subseteq G$ for which $A - A + S$ contains a Bohr set for every $A \subseteq G$ of positive upper Banach density. For $G = \mathbb{Z}$, we show that the sets ${n^2: n \in \mathbb{N}}$, ${p - 1: p \text{ prime}}$, and ${ \lfloor n^c \rfloor: n \in \mathbb{N} }$ with $c > 0$, have this property. We also study those sets $S$ such that $A + S$ contains a Bohr set for every almost Bohr set $A$. As applications, we prove: (i) If $φ_1, φ_2: G \to G$ are (not necessarily commuting) homomorphisms with finite indices $[G: φ_i(G)]$, and $C \subseteq G$ is a central set, then $φ_1(C) - φ_1(C) + φ_2(C)$ contains a Bohr set. This answers one of our questions in [35] and generalizes results in [44, 48]; (ii) Every set of pointwise recurrence in $\mathbb{Z}$ is a set of nice recurrence and a van der Corput set, extending known properties of sets of pointwise recurrence studied in [26, 27, 40].