Degree Majorization and Laplacian Eigenvalue Sums for Simplicial Complexes

Abstract

Let $K$ be an $r$-dimensional simplicial complex. We prove that the spectrum of its $(r-1)$-dimensional up-Laplacian is majorized by the conjugate degree sequence of its $(r-1)$-dimensional faces: \[ {\mathbfλ}_{r-1}(K) \preccurlyeq {\mathbf d}_{r-1}^\top(K). \] We also establish a Brouwer-type inequality: for every integer $\ell \ge 1$, \[ \sum_{i = 1}^{\ell}λ_{r-1,i}(K) \leq \frac{r + 1}{2}f_r(K) + \frac{f_{r - 2}(K)}{r} \binom{\ell + 1}{2}, \] where $\hat{I}{»}_{r-1,i}(K)$ denotes the $i$-th largest eigenvalue in the spectrum ${\hat{\mathbf{I}}»}_{r-1}(K)$, and $f_t(K)$ denotes the number of $t$-dimensional faces of $K$. These results provide higher-dimensional analogs of the Grone-Merris-Bai theorem and the Brouwer-Kothari-Tudose theorem and recover the corresponding graph results when $r=1$. We show that the Duval-Reiner conjecture on the majorization by the conjugate degree sequence of vertices fails in every dimension $r\ge 2$. More precisely, for every $n\ge r+5$, we construct a pure $r$-dimensional complex on $n$ vertices that violates the conjectured inequality at the fifth partial sum.