Sets of cospectral graphs with least eigenvalue at least -2 and some related results


D. Cvetković, M. Lepović


In this paper we study the phenomenon of cospectrality in generalized line graphs and in exceptional graphs. The paper contains a table of sets of cospectral graphs with least eigenvalue at least $-2$ and at most 8 vertices together with some comments and theoretical explanations of the phenomena suggested by the table. In particular, we prove that the multiplicity of the number $0$ in the spectrum of a generalized line graph $L(G)$ is at least the number of petals of the corresponding root graph $G$.