顶点删除后的拉姆齐数

Pub Date : 2024-03-18 DOI:10.1002/jgt.23093
Yuval Wigderson
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引用次数: 0

摘要

康伦、福克斯和苏达科夫猜想,如果从一个图中删除一个顶点,拉姆齐数最多只能以一个常数因子变化。我们推翻了这一猜想,展示了一个无限图族,从每个图中删除一个顶点都会使拉姆齐数减少一个超常数因子。这一结果的一个结果如下。存在这样一个图族:在任何拉姆齐着色(即没有单色副本的顶点上的一个小群的着色)中,其中一个颜色类的密度为 .
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Ramsey numbers upon vertex deletion

Given a graph G $G$ , its Ramsey number r ( G ) $r(G)$ is the minimum N $N$ so that every two-coloring of E ( K N ) $E({K}_{N})$ contains a monochromatic copy of G $G$ . It was conjectured by Conlon, Fox, and Sudakov that if one deletes a single vertex from G $G$ , the Ramsey number can change by at most a constant factor. We disprove this conjecture, exhibiting an infinite family of graphs such that deleting a single vertex from each decreases the Ramsey number by a super-constant factor. One consequence of this result is the following. There exists a family of graphs { G n } $\{{G}_{n}\}$ so that in any Ramsey coloring for G n ${G}_{n}$ (i.e., a coloring of a clique on r ( G n ) 1 $r({G}_{n})-1$ vertices with no monochromatic copy of G n ${G}_{n}$ ), one of the color classes has density o ( 1 ) $o(1)$ .

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