A Proof of the Tratnik-Ye Medianity Conjecture of Resonance Graphs on Surfaces

Abstract

We prove the Tratnik--Ye conjecture that every connected component of a resonance graph of perfect matchings on a closed surface is median whenever the allowed even faces form a proper subset of all faces. The embedding need not be cellular or strong, and the graph need not be bipartite. We also show that the quotient associated with each face is a tree and that these quotients give an isometric embedding of each component into a Cartesian product of trees. This yields a criterion for the facial parity embedding into a hypercube to be isometric.