Laplacian eigenvalues of equivalent cographs

TL;DR

This paper proves a conjecture relating the Laplacian eigenvalues of equivalent cographs, showing they share a minimum number of eigenvalues based on their twin class structures, and that nonisomorphic equivalent cographs are not L-cospectral.

Contribution

It confirms a conjecture on the eigenvalue similarity of equivalent cographs and establishes that nonisomorphic equivalent cographs cannot be L-cospectral.

Findings

Equivalent cographs share at least k + sum(t_i - 1) Laplacian eigenvalues.

No two nonisomorphic equivalent cographs are L-cospectral.

The result links twin class structure to Laplacian spectra.

Abstract

Let G and H be equivalent cographs with their reduction R_G and R_H, and suppose the vertices of R_G and R_H are labeled by the twin numbers t_i of the k twin classes they represent. In this paper, we prove that G and H have at least k + \sum_{i\in I}(t_i-1) Laplacian eigenvalues in common, where I is the indices of the twin classes whose types are identical in G and H. This confirms the conjecture proposed by T. Abrishami \cite{Abris}. We also show that no two nonisomorphic equivalent cographs are L-cospectral.