A generalization of Fiedler's lemma and the spectra of H-join of graphs

Abstract

A new generalization of Fiedler's lemma is obtained by introducing the concept of the main function of a matrix. As applications, the universal spectra of the H-join of any graphs (possibly non-regular) and the adjacency spectra of the H-generalized join constrained by (arbitrary) vertex subsets are obtained. The adjacency spectra of the generalized corona of graphs is deduced from the spectra of the H-join of graphs. Also, the construction of infinitely many pairs of non-regular universal cospectral graphs is provided.