The largest normalized Laplacian eigenvalue and incidence balancedness of simplicial complexes

TL;DR

This paper characterizes when the largest eigenvalue of the up normalized Laplacian of a simplicial complex equals its theoretical maximum, linking it to the balancedness of associated signed graphs and providing new classes of complexes with this property.

Contribution

It provides a new characterization of the largest eigenvalue condition using signed graph balancedness and extends known results from graphs to simplicial complexes.

Findings

Eigenvalue i+2 occurs iff the complex has a balanced signed incidence graph.

Characterization of the multiplicity of the eigenvalue i+2 in simplicial complexes.

Construction of infinite classes of complexes with eigenvalue i+2 using wedge, Cartesian product, and motif duplication.

Abstract

Let $K$ be a simplicial complex, and let $\Delta_i^{up}(K)$ be the $i$-th up normalized Laplacian of $K$. Horak and Jost showed that the largest eigenvalue of $\Delta_i^{up}(K)$ is at most $i+2$, and characterized the equality case by the orientable or non-orientable circuits. In this paper, by using the balancedness of signed graphs, we show that $\Delta_i^{up}(K)$ has an eigenvalue $i+2$ if and only if $K$ has an $(i+1)$-path connected component $K'$ such that the $i$-th signed incidence graph $B_i(K')$ is balanced, which implies Horak and Jost's characterization. We also characterize the multiplicity of $i+2$ as an eigenvalue of $\Delta_i^{up}(K)$, which generalizes the corresponding result in graph case. Finally we gave some classes of infinitely many simplicial complexes $K$ with $\Delta_i^{up}(K)$ having an eigenvalue $i+2$ by using wedge, Cartesian product and duplication of motifs.