On the multiplicity of α as an A_α (Γ)-eigenvalue of signed graphs with pendant vertices

Abstract

A signed graph is a pair Γ = (G; ), where x = (V (G);E(G)) is a graph and E(G) -> {+1;−1} is the sign function on the edges of G. For any > [0; 1] we consider the matrix Aα(Γ) = αD(G) + (1 −α )A(Γ); where D(G) is the diagonal matrix of the vertex degrees of G, and A(Γ) is the adjacency matrix of Γ. Let mAα(Γ) be the multiplicity of α as an A(Γ)-eigenvalue, and let G have p(G) pendant vertices, q(G) quasi-pendant vertices, and no components isomorphic to K2. It is proved that mA(Γ)() = p(G) − q(G) whenever all internal vertices are quasi-pendant. If this is not the case, it turns out that mA(Γ)() = p(G) − q(G) +mN(Γ)(); where mN(Γ)() denotes the multiplicity of as an eigenvalue of the matrix N(Γ) obtained from A(Γ) taking the entries corresponding to the internal vertices which are not quasipendant. Such results allow to state a formula for the multiplicity of 1 as an eigenvalue of the Laplacian matrix L(Γ) = D(G) − A(Γ). Furthermore, it is detected a class G of signed graphs whose nullity – i.e. the multiplicity of 0 as an A(Γ)-eigenvalue – does not depend on the chosen signature. The class G contains, among others, all signed trees and all signed lollipop graphs. It also turns out that for signed graphs belonging to a subclass G ` G the multiplicity of 1 as Laplacian eigenvalue does not depend on the chosen signatures. Such subclass contains trees and circular caterpillars