Signed graphs are graphs whose edges get a sign +1 or −1 (the signature). Signed graphs can be studied by means of graph matrices extended to signed graphs in a natural way. Recently, the spectra of signed graphs have attracted much attention from graph spectra specialists. One motivation is that the spectral theory of signed graphs elegantly generalizes the spectral theories of unsigned graphs. On the other hand, unsigned graphs do not disappear completely, since their role can be taken by the special case of balanced signed graphs.Therefore, spectral problems defined and studied for unsigned graphs can be considered in terms of signed graphs, and sometimes such generalization shows nice properties which cannot be appreciated in terms of (unsigned) graphs. Here, we survey some general results on the adjacency spectra of signed graphs, and we consider some spectral problems which are inspired from the spectral theory of (unsigned) graphs.

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Let G be a connected k-regular graph of order n. We find a best upper bound (in terms of k) on the third largest eigenvalue that is sufficient to guarantee that G has a perfect matching when n is even, and a matching of order n − 1 when n is odd. We also examine how other eigenvalues affect the size of matchings in G.

We determine all graphs whose adjacency matrix has at most two eigenvalues (multiplicities included) different from ±1 and decide which of these graphs are determined by their spectrum. This includes the so-called friendship graphs, which consist of a number of edge-disjoint triangles meeting in one vertex. It turns out that the friendship graph is determined by its spectrum, except when the number of triangles equals sixteen.Computing det(Q + I) and det(Q − I) shows that Q has no eigenvalue −1, and Q has an eigenvalue 1 if and only if b = 2. In case b = 2 we can rewrite A aswith k ≥ 2. Thus we obtained the graphs of Case (ii). If ΓX has no edges and at least two vertices, then these two vertices have the same neighbors, contradiction. So |ΓX| = 1 and we find