Popular Differences and the Croot--Lev Half-Threshold Problem

Abstract

Let $A$ be a finite non-empty subset of an abelian group $G$, and let $r_A(d)=|\{(a,a')\in A^2:a-a'=d\}|$. Croot and Lev asked whether the pointwise half-threshold condition $r_A(d)\ge |A|/2$ for every $d\in A-A$ forces $A-A$ to be either a subgroup or a union of three cosets. We resolve this open problem in its sharp general form by identifying the essential obstruction: the statement is false in arbitrary abelian groups, but becomes true after excluding non-zero two-torsion. More precisely, if $G$ is two-torsion-free and the half-threshold condition holds, then either $A-A$ is a finite subgroup of $G$, or there are a finite subgroup $H\le G$ and elements $x,g\in G$ such that \[ A=(x+H)\cup(x+g+H). \] The two-torsion-free hypothesis is essential: for every $r\ge1$ we construct $A\subseteq\F_2^{2r+1}$ with $A-A=\F_2^{2r+1}\setminus\{t\}$ such that every non-zero represented difference has exactly $|A|/2$ representations, giving genuine counterexamples to the Croot--Lev conclusion. The proof of the positive result combines a Kneser quotient reduction with Lev's formulation of Kemperman's critical-pair theory.