Small and not so small equienergetic graphs

Abstract

We show that, contrary to the claims of several authors that there exists no equienergetic graphs on seven vertices, there exists 21 sets of noncospectral, equienergetic graphs on seven vertices. We also give the number and cardinality of sets of equienergetic connected graphs on eight, nine and ten vertices, and equienergetic trees and chemical trees on up to 22 vertices. Finally, we construct two new infinite families of equienergetic graphs.