Proof of a conjectured spectral upper bound on the chromatic number of a graph
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arXiv
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| Natura: | Preprint |
| Pubblicazione: |
2025
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| _version_ | 1866911735352393728 |
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| author | Tang, Quanyu Elphick, Clive |
| author_facet | Tang, Quanyu Elphick, Clive |
| contents | Let $G$ be a simple graph on $n$ vertices and $m$ edges with chromatic number $χ$, and let $λ_n$ denote the least adjacency eigenvalue. Solving a conjecture of Fan, Yu and Wang~[Electron. J. Combin., 2012], we prove that when $3\le χ\le n-1$, the chromatic number satisfies the following upper bound: $$ χ\le \left(\frac{n}{2}+1+λ_n\right) + \sqrt{\left(\frac{n}{2}+1+λ_n\right)^{2}-4(λ_n+1)\left(λ_n+\frac{n}{2}\right)}, $$ with equality if and only if $G \cong \left(K_{\fracχ{2}}\cup\tfrac{n-χ}{2}K_1\right) \vee \left(K_{\fracχ{2}}\cup\tfrac{n-χ}{2}K_1\right)$, where both $n$ and $χ$ are even. This extends the validity of Fan--Yu--Wang's bound from the range $3\le χ\le \frac{n}{2}$ to the full range $3\le χ\le n-1$.
We also compare this bound with the well-known bound due to Wilf that $χ\le 1 + λ_1$, where $λ_1$ denotes the largest eigenvalue. In particular we show that while Wilf's bound is an upper bound for some parameters larger than $χ$, this bound using $λ_n$ is not an upper bound for these parameters. We conclude with a similar conjectured upper bound for $χ(G)$, which uses $m$ in place of $n$. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2511_07712 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Proof of a conjectured spectral upper bound on the chromatic number of a graph Tang, Quanyu Elphick, Clive Combinatorics Primary 05C50 Let $G$ be a simple graph on $n$ vertices and $m$ edges with chromatic number $χ$, and let $λ_n$ denote the least adjacency eigenvalue. Solving a conjecture of Fan, Yu and Wang~[Electron. J. Combin., 2012], we prove that when $3\le χ\le n-1$, the chromatic number satisfies the following upper bound: $$ χ\le \left(\frac{n}{2}+1+λ_n\right) + \sqrt{\left(\frac{n}{2}+1+λ_n\right)^{2}-4(λ_n+1)\left(λ_n+\frac{n}{2}\right)}, $$ with equality if and only if $G \cong \left(K_{\fracχ{2}}\cup\tfrac{n-χ}{2}K_1\right) \vee \left(K_{\fracχ{2}}\cup\tfrac{n-χ}{2}K_1\right)$, where both $n$ and $χ$ are even. This extends the validity of Fan--Yu--Wang's bound from the range $3\le χ\le \frac{n}{2}$ to the full range $3\le χ\le n-1$. We also compare this bound with the well-known bound due to Wilf that $χ\le 1 + λ_1$, where $λ_1$ denotes the largest eigenvalue. In particular we show that while Wilf's bound is an upper bound for some parameters larger than $χ$, this bound using $λ_n$ is not an upper bound for these parameters. We conclude with a similar conjectured upper bound for $χ(G)$, which uses $m$ in place of $n$. |
| title | Proof of a conjectured spectral upper bound on the chromatic number of a graph |
| topic | Combinatorics Primary 05C50 |
| url | https://arxiv.org/abs/2511.07712 |