Transversals to colorful intersecting convex sets

Abstract

Let $K$ be a compact convex set in $\mathbb{R}^2$ and let $\mathcal{F}_1, \mathcal{F}_2, \mathcal{F}_3$ be finite families of translates of $K$ such that $A \cap B \neq \emptyset$ for every $A \in \mathcal{F}_i$ and $B \in \mathcal{F}_j$ with $i \neq j$. A conjecture by Dolnikov is that, under these conditions, there is always some $j \in \lbrace 1,2,3 \rbrace$ such that $\mathcal{F}_j$ can be pierced by $3$ points. In this paper we prove a stronger version of this conjecture when $K$ is a body of constant width or when it is close in Banach-Mazur distance to a disk. We also show that the conjecture is true with $8$ piercing points instead of $3$. Along the way we prove more general statements both in the plane and in higher dimensions. A related result was given by Mart\'inez-Sandoval, Rold\'an-Pensado and Rubin. They showed that if $\mathcal{F}_1, \dots, \mathcal{F}_d$ are finite families of convex sets in $\mathbb{R}^d$ such that for every choice of sets $C_1 \in \mathcal{F}_1, \dots, C_d \in \mathcal{F}_d$ the intersection $\bigcap_{i=1}^{d} C_i$ is non-empty, then either there exists $j \in \lbrace 1,2, \dots, n \rbrace$ such that $\mathcal{F}_j$ can be pierced by few points or $\bigcup_{i=1}^{n} \mathcal{F}_i$ can be crossed by few lines. We give optimal values for the number of piercing points and crossing lines needed when $d=2$ and also consider the problem restricted to special families of convex sets.