Improved Helly numbers of product sets

Abstract

A finite family $\mathcal{F}$ of convex sets is $k$-intersecting in $S\subseteq {\mathbb{R}}^d$ if the intersection of every subset of $k$ convex sets in $\mathcal{F}$ contains a point in $S$. The Helly number of $S$ is the minimum $k$, if it exists, such that every $k$-intersecting family contains a point of $S$ in its intersection. In this paper, we improve bounds on the Helly number of product sets of the form $A^d$ for various sets $A\subseteq \mathbb{R}$, including the ``exponential grid'' $A=\{\hat{I}{\pm }^n:n\in \mathbb{N}\}$ and sets $A\subseteq \mathbb{Z}$ defined by congruence relations.

11 pages, 2 figures. The results in this version (v2) are a subset of those in v1 after splitting v1 into two papers