## Mathematics Faculty Articles

# Minimum Number of Distinct Eigenvalues of Graphs

## Document Type

Article

## Publication Date

9-1-2013

## Publication Title

Electronic Journal of Linear Algebra

## Keywords

Symmetric matrix, Eigenvalue, Join of graphs, Diameter, Trees, Bipartite graph, Maximum multiplicity

## ISSN

1081-3810

## Volume

26

## First Page

673

## Last Page

691

## Abstract

The minimum number of distinct eigenvalues, taken over all real symmetric matrices compatible with a given graph G, is denoted by q(G). Using other parameters related to G, bounds for q(G) are proven and then applied to deduce further properties of q(G). It is shown that there is a great number of graphs G for which q(G)=2. For some families of graphs, such as the join of a graph with itself, complete bipartite graphs, and cycles, this minimum value is obtained. Moreover, examples of graphs Gare provided to show that adding and deleting edges or vertices can dramatically change the value ofq(G). Finally, the set of graphs Gwith q(G) near the number of vertices is shown to be a subset of known families of graphs with small maximum multiplicity

## NSUWorks Citation

Ahmadi, Bahman; Alinaghipour, Fatehmeh; Cavers, Michael S.; Fallat, Shaun; Meagher, Karen; and Nasserasr, Shahla, "Minimum Number of Distinct Eigenvalues of Graphs" (2013). *Mathematics Faculty Articles*. 75.

https://nsuworks.nova.edu/math_facarticles/75

## DOI

10.13001/1081-3810.1679

COinS

## Comments

AMS subject classifications. 05C50, 15A18