{"title":"与第5号掩体相交的家族","authors":"Peter Frankl , Jian Wang","doi":"10.1016/j.disc.2025.114546","DOIUrl":null,"url":null,"abstract":"<div><div>A family <span><math><mi>F</mi><mo>⊂</mo><mrow><mo>(</mo><mtable><mtr><mtd><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></mtd></mtr><mtr><mtd><mi>k</mi></mtd></mtr></mtable><mo>)</mo></mrow></math></span> is called intersecting if any two members of it have non-empty intersection. The covering number of <span><math><mi>F</mi></math></span> is defined as the minimum integer <em>p</em> such that there exists <span><math><mi>T</mi><mo>⊂</mo><mo>{</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mo>…</mo><mo>,</mo><mi>n</mi><mo>}</mo></math></span> satisfying <span><math><mo>|</mo><mi>T</mi><mo>|</mo><mo>=</mo><mi>p</mi></math></span> and <span><math><mi>T</mi><mo>∩</mo><mi>F</mi><mo>≠</mo><mo>∅</mo></math></span> for all <span><math><mi>F</mi><mo>∈</mo><mi>F</mi></math></span>. Define <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>p</mi><mo>)</mo></math></span> as the maximum size of an intersecting family <span><math><mi>F</mi><mo>⊂</mo><mrow><mo>(</mo><mtable><mtr><mtd><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></mtd></mtr><mtr><mtd><mi>k</mi></mtd></mtr></mtable><mo>)</mo></mrow></math></span> with covering number at least <em>p</em>. The value of <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>p</mi><mo>)</mo></math></span> is only known for <span><math><mi>p</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mn>3</mn><mo>,</mo><mn>4</mn></math></span>. About thirty years ago, <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mn>5</mn><mo>)</mo></math></span> was determined asymptotically by the first author, Ota and Tokushige. In the present paper, we determine <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mn>5</mn><mo>)</mo></math></span> for <span><math><mi>k</mi><mo>≥</mo><mn>69</mn></math></span> and <span><math><mi>n</mi><mo>≥</mo><mn>5</mn><msup><mrow><mi>k</mi></mrow><mrow><mn>6</mn></mrow></msup></math></span>.</div></div>","PeriodicalId":50572,"journal":{"name":"Discrete Mathematics","volume":"348 9","pages":"Article 114546"},"PeriodicalIF":0.7000,"publicationDate":"2025-04-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Intersecting families with covering number five\",\"authors\":\"Peter Frankl , Jian Wang\",\"doi\":\"10.1016/j.disc.2025.114546\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><div>A family <span><math><mi>F</mi><mo>⊂</mo><mrow><mo>(</mo><mtable><mtr><mtd><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></mtd></mtr><mtr><mtd><mi>k</mi></mtd></mtr></mtable><mo>)</mo></mrow></math></span> is called intersecting if any two members of it have non-empty intersection. The covering number of <span><math><mi>F</mi></math></span> is defined as the minimum integer <em>p</em> such that there exists <span><math><mi>T</mi><mo>⊂</mo><mo>{</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mo>…</mo><mo>,</mo><mi>n</mi><mo>}</mo></math></span> satisfying <span><math><mo>|</mo><mi>T</mi><mo>|</mo><mo>=</mo><mi>p</mi></math></span> and <span><math><mi>T</mi><mo>∩</mo><mi>F</mi><mo>≠</mo><mo>∅</mo></math></span> for all <span><math><mi>F</mi><mo>∈</mo><mi>F</mi></math></span>. Define <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>p</mi><mo>)</mo></math></span> as the maximum size of an intersecting family <span><math><mi>F</mi><mo>⊂</mo><mrow><mo>(</mo><mtable><mtr><mtd><mrow><mo>[</mo><mi>n</mi><mo>]</mo></mrow></mtd></mtr><mtr><mtd><mi>k</mi></mtd></mtr></mtable><mo>)</mo></mrow></math></span> with covering number at least <em>p</em>. The value of <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mi>p</mi><mo>)</mo></math></span> is only known for <span><math><mi>p</mi><mo>=</mo><mn>1</mn><mo>,</mo><mn>2</mn><mo>,</mo><mn>3</mn><mo>,</mo><mn>4</mn></math></span>. About thirty years ago, <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mn>5</mn><mo>)</mo></math></span> was determined asymptotically by the first author, Ota and Tokushige. In the present paper, we determine <span><math><mi>m</mi><mo>(</mo><mi>n</mi><mo>,</mo><mi>k</mi><mo>,</mo><mn>5</mn><mo>)</mo></math></span> for <span><math><mi>k</mi><mo>≥</mo><mn>69</mn></math></span> and <span><math><mi>n</mi><mo>≥</mo><mn>5</mn><msup><mrow><mi>k</mi></mrow><mrow><mn>6</mn></mrow></msup></math></span>.</div></div>\",\"PeriodicalId\":50572,\"journal\":{\"name\":\"Discrete Mathematics\",\"volume\":\"348 9\",\"pages\":\"Article 114546\"},\"PeriodicalIF\":0.7000,\"publicationDate\":\"2025-04-24\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Discrete Mathematics\",\"FirstCategoryId\":\"100\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S0012365X25001542\",\"RegionNum\":3,\"RegionCategory\":\"数学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q2\",\"JCRName\":\"MATHEMATICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Discrete Mathematics","FirstCategoryId":"100","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0012365X25001542","RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"MATHEMATICS","Score":null,"Total":0}
A family is called intersecting if any two members of it have non-empty intersection. The covering number of is defined as the minimum integer p such that there exists satisfying and for all . Define as the maximum size of an intersecting family with covering number at least p. The value of is only known for . About thirty years ago, was determined asymptotically by the first author, Ota and Tokushige. In the present paper, we determine for and .
期刊介绍:
Discrete Mathematics provides a common forum for significant research in many areas of discrete mathematics and combinatorics. Among the fields covered by Discrete Mathematics are graph and hypergraph theory, enumeration, coding theory, block designs, the combinatorics of partially ordered sets, extremal set theory, matroid theory, algebraic combinatorics, discrete geometry, matrices, and discrete probability theory.
Items in the journal include research articles (Contributions or Notes, depending on length) and survey/expository articles (Perspectives). Efforts are made to process the submission of Notes (short articles) quickly. The Perspectives section features expository articles accessible to a broad audience that cast new light or present unifying points of view on well-known or insufficiently-known topics.