On Spielman's Laplacian Eigenratio Conjecture and Related Problems
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arXiv
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| Format: | Preprint |
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2026
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| _version_ | 1866915945021177856 |
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| author | Ma, Jie Tang, Quanyu Wang, Yuchang Zheng, Zhiheng |
| author_facet | Ma, Jie Tang, Quanyu Wang, Yuchang Zheng, Zhiheng |
| contents | Let $G$ be an $n$-vertex graph with Laplacian eigenvalues $0=λ_1(G)\le λ_2(G)\le\cdots\le λ_n(G)$. 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 $$ \frac{λ_2(G)}{λ_n(G)} \le \frac{d-2\sqrt{d-1}}{d+2\sqrt{d-1}}+o_n(1), $$ 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 Cioabă 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2604_17907 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | On Spielman's Laplacian Eigenratio Conjecture and Related Problems Ma, Jie Tang, Quanyu Wang, Yuchang Zheng, Zhiheng Combinatorics Let $G$ be an $n$-vertex graph with Laplacian eigenvalues $0=λ_1(G)\le λ_2(G)\le\cdots\le λ_n(G)$. 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 $$ \frac{λ_2(G)}{λ_n(G)} \le \frac{d-2\sqrt{d-1}}{d+2\sqrt{d-1}}+o_n(1), $$ 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 Cioabă 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. |
| title | On Spielman's Laplacian Eigenratio Conjecture and Related Problems |
| topic | Combinatorics |
| url | https://arxiv.org/abs/2604.17907 |