{"title":"顶点删除后的拉姆齐数","authors":"Yuval Wigderson","doi":"10.1002/jgt.23093","DOIUrl":null,"url":null,"abstract":"<p>Given a graph <span></span><math>\n <semantics>\n <mrow>\n <mi>G</mi>\n </mrow>\n <annotation> $G$</annotation>\n </semantics></math>, its Ramsey number <span></span><math>\n <semantics>\n <mrow>\n <mi>r</mi>\n <mrow>\n <mo>(</mo>\n \n <mi>G</mi>\n \n <mo>)</mo>\n </mrow>\n </mrow>\n <annotation> $r(G)$</annotation>\n </semantics></math> is the minimum <span></span><math>\n <semantics>\n <mrow>\n <mi>N</mi>\n </mrow>\n <annotation> $N$</annotation>\n </semantics></math> so that every two-coloring of <span></span><math>\n <semantics>\n <mrow>\n <mi>E</mi>\n <mrow>\n <mo>(</mo>\n \n <msub>\n <mi>K</mi>\n \n <mi>N</mi>\n </msub>\n \n <mo>)</mo>\n </mrow>\n </mrow>\n <annotation> $E({K}_{N})$</annotation>\n </semantics></math> contains a monochromatic copy of <span></span><math>\n <semantics>\n <mrow>\n <mi>G</mi>\n </mrow>\n <annotation> $G$</annotation>\n </semantics></math>. It was conjectured by Conlon, Fox, and Sudakov that if one deletes a single vertex from <span></span><math>\n <semantics>\n <mrow>\n <mi>G</mi>\n </mrow>\n <annotation> $G$</annotation>\n </semantics></math>, 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 <span></span><math>\n <semantics>\n <mrow>\n <mo>{</mo>\n \n <msub>\n <mi>G</mi>\n \n <mi>n</mi>\n </msub>\n \n <mo>}</mo>\n </mrow>\n <annotation> $\\{{G}_{n}\\}$</annotation>\n </semantics></math> so that in any Ramsey coloring for <span></span><math>\n <semantics>\n <mrow>\n <msub>\n <mi>G</mi>\n \n <mi>n</mi>\n </msub>\n </mrow>\n <annotation> ${G}_{n}$</annotation>\n </semantics></math> (i.e., a coloring of a clique on <span></span><math>\n <semantics>\n <mrow>\n <mi>r</mi>\n <mrow>\n <mo>(</mo>\n \n <msub>\n <mi>G</mi>\n \n <mi>n</mi>\n </msub>\n \n <mo>)</mo>\n </mrow>\n \n <mo>−</mo>\n \n <mn>1</mn>\n </mrow>\n <annotation> $r({G}_{n})-1$</annotation>\n </semantics></math> vertices with no monochromatic copy of <span></span><math>\n <semantics>\n <mrow>\n <msub>\n <mi>G</mi>\n \n <mi>n</mi>\n </msub>\n </mrow>\n <annotation> ${G}_{n}$</annotation>\n </semantics></math>), one of the color classes has density <span></span><math>\n <semantics>\n <mrow>\n <mi>o</mi>\n <mrow>\n <mo>(</mo>\n \n <mn>1</mn>\n \n <mo>)</mo>\n </mrow>\n </mrow>\n <annotation> $o(1)$</annotation>\n </semantics></math>.</p>","PeriodicalId":0,"journal":{"name":"","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2024-03-18","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1002/jgt.23093","citationCount":"0","resultStr":"{\"title\":\"Ramsey numbers upon vertex deletion\",\"authors\":\"Yuval Wigderson\",\"doi\":\"10.1002/jgt.23093\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<p>Given a graph <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>G</mi>\\n </mrow>\\n <annotation> $G$</annotation>\\n </semantics></math>, its Ramsey number <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>r</mi>\\n <mrow>\\n <mo>(</mo>\\n \\n <mi>G</mi>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow>\\n <annotation> $r(G)$</annotation>\\n </semantics></math> is the minimum <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>N</mi>\\n </mrow>\\n <annotation> $N$</annotation>\\n </semantics></math> so that every two-coloring of <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>E</mi>\\n <mrow>\\n <mo>(</mo>\\n \\n <msub>\\n <mi>K</mi>\\n \\n <mi>N</mi>\\n </msub>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow>\\n <annotation> $E({K}_{N})$</annotation>\\n </semantics></math> contains a monochromatic copy of <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>G</mi>\\n </mrow>\\n <annotation> $G$</annotation>\\n </semantics></math>. It was conjectured by Conlon, Fox, and Sudakov that if one deletes a single vertex from <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>G</mi>\\n </mrow>\\n <annotation> $G$</annotation>\\n </semantics></math>, 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 <span></span><math>\\n <semantics>\\n <mrow>\\n <mo>{</mo>\\n \\n <msub>\\n <mi>G</mi>\\n \\n <mi>n</mi>\\n </msub>\\n \\n <mo>}</mo>\\n </mrow>\\n <annotation> $\\\\{{G}_{n}\\\\}$</annotation>\\n </semantics></math> so that in any Ramsey coloring for <span></span><math>\\n <semantics>\\n <mrow>\\n <msub>\\n <mi>G</mi>\\n \\n <mi>n</mi>\\n </msub>\\n </mrow>\\n <annotation> ${G}_{n}$</annotation>\\n </semantics></math> (i.e., a coloring of a clique on <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>r</mi>\\n <mrow>\\n <mo>(</mo>\\n \\n <msub>\\n <mi>G</mi>\\n \\n <mi>n</mi>\\n </msub>\\n \\n <mo>)</mo>\\n </mrow>\\n \\n <mo>−</mo>\\n \\n <mn>1</mn>\\n </mrow>\\n <annotation> $r({G}_{n})-1$</annotation>\\n </semantics></math> vertices with no monochromatic copy of <span></span><math>\\n <semantics>\\n <mrow>\\n <msub>\\n <mi>G</mi>\\n \\n <mi>n</mi>\\n </msub>\\n </mrow>\\n <annotation> ${G}_{n}$</annotation>\\n </semantics></math>), one of the color classes has density <span></span><math>\\n <semantics>\\n <mrow>\\n <mi>o</mi>\\n <mrow>\\n <mo>(</mo>\\n \\n <mn>1</mn>\\n \\n <mo>)</mo>\\n </mrow>\\n </mrow>\\n <annotation> $o(1)$</annotation>\\n </semantics></math>.</p>\",\"PeriodicalId\":0,\"journal\":{\"name\":\"\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":0.0,\"publicationDate\":\"2024-03-18\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"https://onlinelibrary.wiley.com/doi/epdf/10.1002/jgt.23093\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"\",\"FirstCategoryId\":\"100\",\"ListUrlMain\":\"https://onlinelibrary.wiley.com/doi/10.1002/jgt.23093\",\"RegionNum\":0,\"RegionCategory\":null,\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"\",\"JCRName\":\"\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"","FirstCategoryId":"100","ListUrlMain":"https://onlinelibrary.wiley.com/doi/10.1002/jgt.23093","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"","JCRName":"","Score":null,"Total":0}
Given a graph , its Ramsey number is the minimum so that every two-coloring of contains a monochromatic copy of . It was conjectured by Conlon, Fox, and Sudakov that if one deletes a single vertex from , 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 so that in any Ramsey coloring for (i.e., a coloring of a clique on vertices with no monochromatic copy of ), one of the color classes has density .