Partially ordering the class of invertible trees

Abstract

A tree T is invertible if and only if T has a perfect matching. Godsil considers an invertible tree T and finds that the inverse of the adjacency matrix of T has entries in {0, 1, -1} and is the signed adjacency matrix of a graph which contains T. In this paper, we give a new proof of this theorem, which gives rise to a partial ordering relation on the class of all invertible trees on 2n vertices. In particular, we show that given an invertible tree T whose inverse graph has strictly more edges, we can remove an edge from T and add another edge to obtain an invertible tree T' whose median eigenvalue is strictly greater. This extends naturally to a partial ordering. We characterize the maximal and minimal elements of this poset and explore the implications about the median eigenvalues of invertible trees.