On Full Brouwer's Laplacian Conjecture

Abstract

Brouwer's Laplacian conjecture asserts that for any graph $G$ with $n$ vertices and $m$ edges, the sum of the $k$ largest Laplacian eigenvalues satisfies $s_k(G) \le m + \binom{k+1}{2}$ for $k=1, \ldots, n$. The conjecture has been verified for numerous graph classes and for several values of $k$. Recently, Kothari and Tudose (2026) proved the conjecture. In this paper, we prove that equality holds for some $1\le k\le n-1$ if and only if $G$ is a threshold graph with clique number $k+1$, which confirms the full Brouwer conjecture formulated by Li and Guo (2022).