Paper detail

On the intrinsic volumes of intersections of congruent balls

Let ${\mathbb E}^d$ denote the $d$-dimensional Euclidean space. The $r$-ball body generated by a given set in ${\mathbb E}^d$ is the intersection of balls of radius $r$ centered at the points of the given set. In this paper we prove the following Blaschke-Santaló-type inequalities for $r$-ball bodies: for all $1\leq k\leq d$ and for any set of given volume in ${\mathbb E}^d$ the $k$-th intrinsic volume of the $r$-ball body generated by the set becomes maximal if the set is a ball. As an application we investigate the Gromov-Klee-Wagon problem for congruent balls in ${\mathbb E}^d$, which is a question on proving or disproving that if the centers of a family of $N$ congruent balls in ${\mathbb E}^d$ are contracted, then the volume of the intersection does not decrease. In particular, we investigate this problem for uniform contractions, which are contractions where all the pairwise distances in the first set of centers are larger than all the pairwise distances in the second set of centers, that is, when the pairwise distances of the two sets are separated by some positive real number. The author and M. Naszódi [Discrete Comput. Geom. 60/4 (2018), 967-980] proved that the intrinsic volumes of the intersection of $N$ congruent balls in ${\mathbb E}^d$, $d>1$ increase under any uniform contraction of the center points when $ N\geq \left(1+\sqrt{2}\right)^d$. We give a short proof of this result using the Blaschke-Santaló-type inequalities of $r$-ball bodies and improve it for $d\geq 42$.