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The graphs cospectral with the pineapple graph

2019    Topcu, Hatice; Sorgun, Sezer; Haemers, Willem H.

The pineapple graph K-p(q) is obtained by appending q pendant edges to a vertex of a complete graph K-p (p >= 3, q >= 1). We prove that among connected graphs, the pineapple graph is determined by its adjacency spectrum. Moreover, we determine all disconnected graphs which are cospectral with a pineapple graph. Thus we find for which values of p and q the pineapple graph K-p(q) is determined by its adjacency spectrum. The main tool is a recent classification of all graphs with all but three eigenvalues equal to 0 or -1 by the third author. (C) 2018 Elsevier B.V. All rights reserved.