On product of difference sets for sets of positive density

Abstract:

In this paper we prove that given two sets $E_1,E_2 \subset \mathbb {Z}$ of positive density, there exists $k \geq 1$ which is bounded by a number depending only on the densities of $E_1$ and $E_2$ such that $k\mathbb {Z} \subset (E_1-E_1)\cdot (E_2-E_2)$. As a corollary of the main theorem we deduce that if $\alpha ,\beta > 0$, then there exist $N_0$ and $d_0$ which depend only on $\alpha$ and $\beta$ such that for every $N \geq N_0$ and $E_1,E_2 \subset \mathbb {Z}_N$ with $|E_1| \geq \alpha N, |E_2| \geq \beta N$ there exists $d \leq d_0$ a divisor of $N$ satisfying $d \mathbb {Z}_N \subset (E_1-E_1)\cdot (E_2-E_2)$.