Inertias of Laplacian matrices of weighted signed graphs

TL;DR

This paper investigates the inertias of Laplacian matrices of weighted signed graphs, characterizing cases with unique inertias, showing generic simplicity of eigenvalues under perturbations, and establishing bounds related to graph parameters.

Contribution

It provides a comprehensive analysis of Laplacian inertias in signed graphs, including characterizations, perturbation results, and bounds based on graph parameters.

Findings

Characterized signed graphs with unique Laplacian inertia.

Proved generic simplicity of eigenvalues under small weight perturbations.

Established sharp upper bounds on possible inertias based on graph parameters.

Abstract

We study the sets of inertias achieved by Laplacian matrices of weighted signed graphs. First we characterize signed graphs with a unique Laplacian inertia. Then we show that there is a sufficiently small perturbation of the nonzero weights on the edges of any connected weighted signed graph so that all eigenvalues of its Laplacian matrix are simple. Next, we give upper bounds on the number of possible Laplacian inertias for signed graphs with a fixed flexibility $\tau$ (a combinatorial parameter of signed graphs), and show that these bounds are sharp for an infinite family of signed graphs. Finally, we provide upper bounds for the number of possible Laplacian inertias of signed graphs in terms of the number of vertices.