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An eigenvalue of the adjacency matrix of a graph is said to be main if the all-1 vector is non-orthogonal to the associated eigenspace. This paper explores some new aspects of the study of main eigenvalues of graphs, investigating specifically cones over strongly regular graphs and graphs for which the least eigenvalue is non-main. In this case, we characterize paths and trees with diameter-3 satisfying the property. We may note that the importance of least eigenvalues of graphs for the equilibria of social and economic networks was recently uncovered in literature.

There exist pentadiagonal matrices which are diagonally similar to symmetric matrices. In this work we describe explicitly the diagonal matrix that gives this transformation for certain pentadiagonal matrices. We also consider particular classes of pentadiagonal matrices and obtain recursive formulas for the characteristic polynomial and explicit formulas for their eigenvalues.

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Let G be a graph of order n, A(G) its adjacency matrix and D(G) the diagonal matrix of degrees of G. In 2017, for every α in [0, 1], Nikiforov defined the matrix Aα(G) = αD(G) + (1 − α)A(G). In this paper, we investigate the Aα-spectrum of graphs obtained from the duplicate and corona operations. As an application of our results, we provide conditions for the construction of some pairs of non isomorphic Aα-cospectral graphs.