A Positive Answer to Bárány's Question on Face Numbers of Polytopes

Abstract

Despite a full characterization of the face vectors of simple and simplicial polytopes, the face numbers of general polytopes are poorly understood. Around 1997, Bárány asked whether for all convex $d$-polytopes $P$ and all $0\le k\le d-1$, $f_k(P)\ge \min \{f_0(P),f_{d-1}(P)\}$. We answer Bárány's question in the affirmative and prove a stronger statement: for all convex $d$-polytopes $P$ and all $0\le k\le d-1$, \[ \frac{f_k(P)}{f_0(P)} \geq \frac{1}{2}\biggl[{\lceil \frac{d}{2} \rceil \choose k} + {\lfloor \frac{d}{2} \rfloor \choose k}\biggr], \qquad \frac{f_k(P)}{f_{d-1}(P)} \geq \frac{1}{2}\biggl[{\lceil \frac{d}{2} \rceil \choose d-k-1} + {\lfloor \frac{d}{2} \rfloor \choose d-k-1}\biggr]. \] In the former, equality holds precisely when $k=0$ or when $k=1$ and $P$ is simple. In the latter, equality holds precisely when $k=d-1$ or when $k=d-2$ and $P$ is simplicial.

arXiv comment8 pages, 1 figure; fixed incorrect argument in Proposition 3.1, added acknowledgement