Integral bases, perfect matchings, and the Petersen graph

Abstract: Let $G=(V,E)$ be a matching-covered graph, denote by $P$ its perfect matching polytope, and by $L$ the integer lattice generated by the integral points in $P$. In this paper, we give polyhedral proofs for two difficult results established by Lov\'{a}sz (1987), and by Carvalho, Lucchesi, and Murty (2002) in a series of three papers. More specifically, we reprove that $L$ has a lattice basis consisting solely of incidence vectors of some perfect matchings of $G$, $2x\in L$ for all $x\in \mathrm{lin}(P)\cap \mathbb{Z}^E$, and if $G$ has no Petersen brick then $L = \mathrm{lin}(P)\cap \mathbb{Z}^E$. This is achieved by studying the facial structure of $P$ and its relationship with the lattice $L$. Along the way, we give a new polyhedral characterization of the Petersen graph.