Bounds for the largest eigenvalue and sum of Laplacian eigenvalues of signed graphs

Abstract

In this paper, we consider the bounds for the largest eigenvalue and the sum of the $k$ largest Laplacian eigenvalues of signed graphs. Firstly, we give an upper bound on the largest eigenvalue of the adjacency matrix of a signed graph and characterize the extremal graphs that attain this bound. Secondly, we prove that a non-bipartite signed graph $Γ$ of order $n$ and size $m$ contains a balanced triangle if $λ_{1}(Γ)\ge \sqrt{m-1}$, $λ_{1}(Γ) \ge |λ_{n}(Γ)|$ and $Γ\not \sim (C_{5}\cup (n-5)K_{1},+)$, where $λ_{1}(Γ)$ is the largest eigenvalue of the adjacency matrix of $Γ$. Thirdly, we confirm a conjecture proposed in [Linear Multilinear Algebra 51 (1) (2003) 21--30] that: if $Γ$ is a connected signed graph, then $$ \sum_{i=1}^{k}μ_{i}(Γ) >\sum_{i=1}^{k}d_{i}(Γ)~~(1\le k\le n-1), $$ where $μ_{1}(Γ)\geμ_{2}(Γ)\ge\cdots \ge μ_{n}(Γ)$ are Laplacian eigenvalues of $Γ$, and $d_{1}(Γ)\ge d_{2}(Γ)\ge \dots \ge d_{n}(Γ)$ are vertex degrees of $Γ$. Finally, we give a lower bound for the sum of the $k$ largest Laplacian eigenvalues of a connected signed graph.