Braces and Pfaffian Orientations

Abstract

Robertson, Seymour, Thomas and simultaneously McCuaig answered several equivalent questions. Specifically, when can some of the 1's of a 0-1 square matrix, A, be changed to -1's so that the permanent of A equals the determinant of the modified matrix? When is a hypergraph with n vertices and n hyperedges minimally nonbipartite? When does a bipartite graph have a Pfaffian orientation? Given a digraph, does it have an even directed circuit? When is a square matrix sign non-singular? We provide a much shorter proof using elementary methods for their theorem. This is joint work with Robin Thomas.