On the eigenvalues of complete bipartite signed graphs

Let Γ = (G, σ) be a signed graph, where σ is the sign function on the edges of G. The adjacency matrix of Γ is defined canonically. Let (Kp, q, σ), p ≤ q, be a complete bipartite signed graph with bipartition (Up, Vq), where Up = {u1, u2, …, up} and Vq = {v1, v2, …, vq}. Let (Kp, q, σ)[Ur ∪ Vs], r ≤ p and s ≤ q, be an induced signed subgraph on minimum vertices r + s, which contains all negative edges of the signed graph (Kp, q, σ). In this paper, we show that the nullity of the signed graph (Kp, q, σ) is at least p + q − 2k − 2, where k = min (r, s). The spectrum of a complete bipartite signed graph whose negative edges induce either a disjoint complete bipartite subgraphs or a path is determined. Finally, we obtain the spectrum of a complete bipartite signed graph whose negative edges (positive edges) induce a regular subgraph H. It turns out that there is a relationship between the eigenvalues of this complete bipartite signed graph and the non-negative eigenvalues of H.