Index perturbation of signed graphs

Abstract

Let $Γ= (G, σ)$ be a signed graph and $v$ a non-isolated vertex of $Γ$. Let $Γ-v$ denote the graph obtained by deleting the vertex $v$ together with all signed edges incident to it from $Γ$, and $d_Γ(v)$ the degree of $v$ in $Γ$. In this paper, we prove that the largest eigenvalue $λ_1(Γ)$ of $Γ$ satisfies \[ λ_1(Γ) \le \sqrt{λ_1^2(Γ- v) + 2d_Γ(v) - 1}, \] and we also present a refined version of this bound. Moreover, we characterize the extremal signed graphs achieving equality when $Γ$ is connected and $d_Γ(v)\ge 2$, which are switching equivalent to the balanced complete signed graph.