Piercing translates and homothets of a convex body

TL;DR

This paper extends classical geometric bounds on the transversal number of intersecting translates and homothets of convex bodies, providing new bounds, inequalities, and efficient algorithms across all dimensions.

Contribution

It proves that the ratio of transversal to packing numbers is bounded for homothets, introduces improved bounds for various convex bodies, and develops constructive methods for approximation algorithms.

Findings

Bounded the ratio of transversal to packing numbers for homothets in any dimension.

Derived new inequalities linking covering and packing densities of convex bodies.

Developed efficient algorithms for piercing sets of translates or homothets.

Abstract

According to a classical result of Gr\"unbaum, the transversal number $\tau(\F)$ of any family $\F$ of pairwise-intersecting translates or homothets of a convex body $C$ in $\RR^d$ is bounded by a function of $d$. Denote by $\alpha(C)$ (resp. $\beta(C)$) the supremum of the ratio of the transversal number $\tau(\F)$ to the packing number $\nu(\F)$ over all families $\F$ of translates (resp. homothets) of a convex body $C$ in $\RR^d$. Kim et al. recently showed that $\alpha(C)$ is bounded by a function of $d$ for any convex body $C$ in $\RR^d$, and gave the first bounds on $\alpha(C)$ for convex bodies $C$ in $\RR^d$ and on $\beta(C)$ for convex bodies $C$ in the plane. Here we show that $\beta(C)$ is also bounded by a function of $d$ for any convex body $C$ in $\RR^d$, and present new or improved bounds on both $\alpha(C)$ and $\beta(C)$ for various convex bodies $C$ in $\RR^d$ for all dimensions $d$. Our techniques explore interesting inequalities linking the covering and packing densities of a convex body. Our methods for obtaining upper bounds are constructive and lead to efficient constant-factor approximation algorithms for finding a minimum-cardinality point set that pierces a set of translates or homothets of a convex body.