Discrete sumsets with one large summand

TL;DR

This paper investigates the structure of sumsets in discrete abelian groups when at least one summand has positive upper Banach density, establishing new correspondences and improving previous results on their combinatorial properties.

Contribution

It introduces a novel connection between sumsets in discrete groups and level sets in compact groups, extending results to groups of any size without using measure-preserving dynamics.

Findings

Sumsets with a dense summand are piecewise syndetic or Bohr.

Established a correspondence between sumsets and convolution level sets.

Results apply to all cardinalities of discrete abelian groups.

Abstract

If $A$ and $B$ are subsets of an abelian group, their sumset is $A+B:=\{a+b:a\in A, b\in B\}$. We study sumsets in discrete abelian groups, where at least one summand has positive upper Banach density. Renling Jin proved that if $A$ and $B$ are sets of integers having positive upper Banach density, then $A+B$ is piecewise syndetic. Bergelson, Furstenberg, and Weiss improved the conclusion to "$A+B$ is piecewise Bohr." Beiglb\"ock, Bergelson, and Fish showed this to be qualitatively optimal, in the sense that if $C\subseteq \mathbb Z$ is piecewise Bohr, then there are $A, B\subseteq \mathbb Z$ having positive upper Banach density such that $A+B\subseteq C$. We improve these results by establishing a strong correspondence between sumsets in discrete abelian groups, level sets of convolutions in compact abelian groups, and sumsets in compact abelian groups. Our proofs avoid measure preserving dynamics and nonstandard analysis, and our results apply to discrete abelian groups of any cardinality.