A structural Szemer\'edi-Trotter Theorem for Cartesian Products

Abstract

We study configurations of $n$ points and $n$ lines that form $\Theta(n^{4/3})$ incidences, when the point set is a Cartesian product. We prove structural properties of such configurations, such that there exist many families of parallel lines or many families of concurrent lines. We show that the line slopes have multiplicative structure or that many sets of $y$-intercepts have additive structure. We introduce the first infinite family of configurations with $\Theta(n^{4/3})$ incidences. We also derive a new variant of a different structural point-line result of Elekes. Our techniques are based on the concept of line energy. Recently, Rudnev and Shkredov introduced this energy and showed how it is connected to point-line incidences. We also prove that their bound is tight up to sub-polynomial factors.