Square-Difference-Free Sets beyond the Three-Quarter Barrier

Abstract

Let $D(N)$ denote the largest cardinality of a subset of $\{1,\dots ,N\}$ containing no nonzero square difference. While a construction certifying $D(N)\ge (1-o(1))N^{1/2}$ is almost trivial, Erdős conjectured that this bound is sharp up to polylogarithmic factors. This was disproved by Sárközy and later again by Ruzsa, who found an elegant construction showing that $D(N)\ge c\cdot N^{0.733077\dots }$, with an absolute constant $c>0$. His approach was subsequently refined, leading to the previously best known lower bound with exponent $0.7334117\dots$ due to Beigel-Gasarch and, independently, Lewko. However, in the original paper Ruzsa observed that $3/4$ seems to be the natural barrier of his approach. In this paper we develop a new construction leading to the lower bound \[ \liminf_{N\to\infty}\frac{\log D(N)}{\log N} \geq α_*:= 0.7527964558\ldots; \] thus crossing the natural exponent-$3/4$ barrier of Ruzsa's method. The value $0.7527964558\dots$ arises from a simple optimisation problem and appears to be the limit of the new approach.

7 pages