On 2-Level Polytopes Arising in Combinatorial Settings
Manuel Aprile, Alfonso Cevallos, Yuri Faenza · SIAM Journal on Discrete Mathematics · 2018
$2$-level polytopes naturally appear in several areas of pure and applied mathematics, including combinatorial optimization, polyhedral combinatorics, communication complexity, and statistics. In this paper, we present a study of some $2$-level polytopes arising in combinatorial settings. Our first contribution is proving that $f_0(P)f_{d-1}(P)\leq d2^{d+1}$ for a large collection of families of such polytopes $P$. Here $f_0(P)$ (resp., $f_{d-1}(P)$) is the number of vertices (resp., facets) of $P$, and $d$ is its dimension. Whether this holds for all 2-level polytopes was asked in [A. Bohn, Y. Faenza, S. Fiorini, V. Fisikopoulos, M. Macchia, and K. Pashkovich, in Algorithms--ESA 2015, Springer, Berlin, 2015, pp. 191--202], and experimental results from [S. Fiorini, V. Fisikopoulos, and M. Macchia, in Combinatorial Optimization, Springer, Cham, 2016, pp. 285--296] showed it true for $d\leq 7$. The key to most of our proofs is a deeper understanding of the relations among those polytopes and their underlying combinatorial structure. This leads to a number of results that we believe to be of independent interest: a trade-off formula for the number of cliques and stable sets in a graph, a description of stable matching polytopes as affine projections of certain order polytopes, and a linear-size description of the base polytope of matroids that are 2-level in terms of cuts of an associated tree.