On Spielman's Laplacian Eigenratio Conjecture and Related Problems

Let $G$ be an $n$-vertex graph with Laplacian eigenvalues $0=\lambda_1(G)\le \lambda_2(G)\le\cdots\le \lambda_n(G)$. Motivated by the Alon-Boppana bound and the Ramanujan phenomenon for regular graphs, Spielman conjectured that, for every graph $G$ with fixed average degree $d\ge 1$, its Laplacian eigenratio satisfies $$\frac{\lambda_2(G)}{\lambda_n(G)} \le \frac{d-2\sqrt{d-1}}{d+2\sqrt{d-1}}+o_n(1),$$ where $o_n(1)\to 0$ as $n\to\infty$. The main purpose of this paper is to investigate this conjecture. We show that the situation is mixed. On the negative side, the conjecture fails for infinitely many average degrees $d>2$, via constructions based on bipartite Ramanujan graphs. On the positive side, it holds in two important settings: we verify it for all average degrees $d\le 2$, and we prove it for all regular graphs. In fact, for regular graphs we obtain stronger bounds comparing higher Laplacian eigenvalues. As a consequence, we show that for every fixed $d\ge 3$ and every $\varepsilon>0$, every sufficiently large $d$-regular Ramanujan graph has linearly many adjacency eigenvalues below $-2\sqrt{d-1}+\varepsilon$, thereby strengthening earlier results of Li and Cioab\u{a} by giving an unconditional result of this form. We also settle two related conjectures: one of You and Liu concerning the maximum Laplacian eigenratio of trees, and one of Gu concerning the Hamiltonicity of graphs with large Laplacian eigenratio.