A combinatorial bound on the number of distinct eigenvalues of a graph

TL;DR

This paper establishes a combinatorial lower bound on the number of distinct eigenvalues of a graph based on unique shortest paths and explores the properties of the minor-monotone floor of this bound.

Contribution

It introduces the concept of the minor-monotone floor of the eigenvalue bound and provides initial results on its properties.

Findings

Bound on $q(G)$ using unique shortest paths

Introduction of the minor-monotone floor of $n-k$

Results on the properties of the minor-monotone floor

Abstract

The smallest possible number of distinct eigenvalues of a graph $G$, denoted by $q(G)$, has a combinatorial bound in terms of unique shortest paths in the graph. In particular, $q(G)$ is bounded below by $k$, where $k$ is the number of vertices of a unique shortest path joining any pair of vertices in $G$. Thus, if $n$ is the number of vertices of $G$, then $n-q(G)$ is bounded above by the size of the complement (with respect to the vertex set of $G$) of the vertex set of the longest unique shortest path joining any pair of vertices of $G$. The purpose of this paper is to commence the study of the minor-monotone floor of $n-k$, which is the minimum of $n-k$ among all graphs of which $G$ is a minor. Accordingly, we prove some results about this minor-monotone floor.