The Bárány-Kalai conjecture for certain families of polytopes

Pablo Soberón, Shira Zerbib

Published 2024-04-17Version 1

B\'ar\'any and Kalai conjectured the following generalization of Tverberg's theorem: if $f$ is a linear function from an $m$-dimensional polytope $P$ to $\mathbb{R}^d$ and $m \ge (d + 1)(r - 1)$, then there are $r$ pairwise disjoint faces of $P$ whose images have a point in common. We show that the conjecture holds for cross polytopes, cyclic polytopes, and more generally for $(d+1)$-neighborly polytopes. Moreover, we show that for cross polytopes, the conjecture holds if the map $f$ is assumed to be continuous (but not necessarily linear), and we give a lower bound on the number of sets of $r$ pairwise disjoint faces whose images under $f$ intersect. We also show that the conjecture holds for all polytopes when $d=1$ and $f$ is assumed to be continuous. Finally, when $r$ is prime or large enough with respect to $d$, we prove that there exists a constant $c=c(d,r)$, depending only on $d$ and $r$, such that the conjecture holds (with continuous functions) for the polytope obtained by taking $c$ subdivisions of $P$.