On lattices, distinct distances, and the Elekes-Sharir framework

Abstract

In this note we consider distinct distances determined by points in an integer lattice. We first consider Erdos's lower bound for the square lattice, recast in the setup of the so-called Elekes-Sharir framework \cite{ES11,GK11}, and show that, without a major change, this framework \emph{cannot} lead to Erdos's conjectured lower bound. This shows that the upper bound of Guth and Katz \cite{GK11} for the related 3-dimensional line-intersection problem is tight for this instance. The gap between this bound and the actual bound of Erdos arises from an application of the Cauchy-Schwarz inequality (which is an integral part of the Elekes-Sharir framework). Our analysis relies on two number-theoretic results by Ramanujan. We also consider distinct distances in rectangular lattices of the form $\{(i,j) \mid 0\le i\le n^{1-\alpha},\ 0\le j\le n^{\alpha}\}$, for some $0<\alpha<1/2$, and show that the number of distinct distances in such a lattice is $\Theta(n)$. In a sense, our proof "bypasses" a deep conjecture in number theory, posed by Cilleruelo and Granville \cite{CG07}. A positive resolution of this conjecture would also have implied our bound.