ON 2-LEVEL POLYTOPES ARISING IN COMBINATORIAL SETTINGS

摘要

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) = d(2)(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 = 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.