The largest $H$-eigenvalue and spectral radius of Laplacian tensor of non-odd-bipartite generalized power hypergraphs

TL;DR

This paper investigates spectral properties of Laplacian and signless Laplacian tensors of non-odd-bipartite hypergraphs, revealing conditions under which their spectral radii and eigenvalues coincide or differ, especially in generalized power hypergraphs.

Contribution

It characterizes when Laplacian and signless Laplacian spectra coincide in non-odd-bipartite hypergraphs and explores spectral symmetry related to adjacency tensors.

Findings

Spectral radius equality holds iff k is multiple of 4.

For large k with k ≡ 2 mod 4, Laplacian eigenvalues are less than spectral radius.

L(G) and Q(G) spectra coincide only under specific structural conditions.

Abstract

Let $G$ be a simple graph or hypergraph, and let $A(G),L(G),Q(G)$ be the adjacency, Laplacian and signless Laplacian tensors of $G$ respectively. The largest $H$-eigenvalues (resp., the spectral radii) of $L(G),Q(G)$ are denoted respectively by $\lambda_{\max}^L(G), \lambda_{\max}^Q(G)$ (resp., $\rho^L(G), \rho^Q(G)$). For a connected non-bipartite simple graph $G$, $\lambda_{\max}^L(G)=\rho^L(G) < \rho^Q(G)$. But this does not hold for non-odd-bipartite hypergraphs. We will investigate this problem by considering a class of generalized power hypergraphs $G^{k,\frac{k}{2}}$, which are constructed from simple connected graphs $G$ by blowing up each vertex of $G$ into a $\frac{k}{2}$-set and preserving the adjacency of vertices. Suppose that $G$ is non-bipartite, or equivalently $G^{k,\frac{k}{2}}$ is non-odd-bipartite. We get the following spectral properties: (1) $\rho^L(G^{k,{k \over 2}}) =\rho^Q(G^{k,{k \over 2}})$ if and only if $k$ is a multiple of $4$; in this case $\lambda_{\max}^L(G^{k,\frac{k}{2}})<\rho^L(G^{k,{k \over 2}})$. (2) If $k\equiv 2 (\!\!\!\mod 4)$, then for sufficiently large $k$, $\lambda_{\max}^L(G^{k,\frac{k}{2}})<\rho^L(G^{k,{k \over 2}})$. Motivated by the study of hypergraphs $G^{k,\frac{k}{2}}$, for a connected non-odd-bipartite hypergraph $G$, we give a characterization of $L(G)$ and $Q(G)$ having the same spectra or the spectrum of $A(G)$ being symmetric with respect to the origin, that is, $L(G)$ and $Q(G)$, or $A(G)$ and $-A(G)$ are similar via a complex (necessarily non-real) diagonal matrix with modular-$1$ diagonal entries. So we give an answer to a question raised by Shao et al., that is, for a non-odd-bipartite hypergraph $G$, that $L(G)$ and $Q(G)$ have the same spectra can not imply they have the same $H$-spectra.