Spectra of coronae

We introduce a new invariant, the coronal of a graph, and use it to compute the spectrum of the corona G o H of two graphs G and H. In particular, we show that this spectrum is completely determined by the spectra of G and H and the coronal of H. Previous work has computed the spectrum of a corona only in the case that H is regular. We then explicitly compute the coronals for several families of graphs, including regular graphs, complete n-partite graphs, and paths. Finally, we use the corona construction to generate many infinite families of pairs of cospectral graphs. (C) 2011 Elsevier Inc. All rights reserved.