Ademir Hujdurović , István Kovács , Klavdija Kutnar , Dragan Marušič
{"title":"具有小循环点稳定子的传递群的交集密度","authors":"Ademir Hujdurović , István Kovács , Klavdija Kutnar , Dragan Marušič","doi":"10.1016/j.ejc.2024.104079","DOIUrl":null,"url":null,"abstract":"<div><div>For a permutation group <span><math><mi>G</mi></math></span> acting on a set <span><math><mi>V</mi></math></span>, a subset <span><math><mi>F</mi></math></span> of <span><math><mi>G</mi></math></span> is said to be an <em>intersecting set</em> if for every pair of elements <span><math><mrow><mi>g</mi><mo>,</mo><mi>h</mi><mo>∈</mo><mi>F</mi></mrow></math></span> there exists <span><math><mrow><mi>v</mi><mo>∈</mo><mi>V</mi></mrow></math></span> such that <span><math><mrow><mi>g</mi><mrow><mo>(</mo><mi>v</mi><mo>)</mo></mrow><mo>=</mo><mi>h</mi><mrow><mo>(</mo><mi>v</mi><mo>)</mo></mrow></mrow></math></span>. The <em>intersection density</em> <span><math><mrow><mi>ρ</mi><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow></mrow></math></span> of a transitive permutation group <span><math><mi>G</mi></math></span> is the maximum value of the quotient <span><math><mrow><mrow><mo>|</mo><mi>F</mi><mo>|</mo></mrow><mo>/</mo><mrow><mo>|</mo><msub><mrow><mi>G</mi></mrow><mrow><mi>v</mi></mrow></msub><mo>|</mo></mrow></mrow></math></span> where <span><math><msub><mrow><mi>G</mi></mrow><mrow><mi>v</mi></mrow></msub></math></span> is a stabilizer of a point <span><math><mrow><mi>v</mi><mo>∈</mo><mi>V</mi></mrow></math></span> and <span><math><mi>F</mi></math></span> runs over all intersecting sets in <span><math><mi>G</mi></math></span>. If <span><math><msub><mrow><mi>G</mi></mrow><mrow><mi>v</mi></mrow></msub></math></span> is a largest intersecting set in <span><math><mi>G</mi></math></span> then <span><math><mi>G</mi></math></span> is said to have the <em>Erdős-Ko-Rado (EKR)-property</em>. This paper is devoted to the study of transitive permutation groups, with point stabilizers of prime order with a special emphasis given to orders 2 and 3, which do not have the EKR-property. Among others, constructions of an infinite family of transitive permutation groups having point stabilizer of order 3 with intersection density <span><math><mrow><mn>4</mn><mo>/</mo><mn>3</mn></mrow></math></span> and of infinite families of transitive permutation groups having point stabilizer of order 3 with arbitrarily large intersection density are given.</div></div>","PeriodicalId":50490,"journal":{"name":"European Journal of Combinatorics","volume":null,"pages":null},"PeriodicalIF":1.0000,"publicationDate":"2024-10-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Intersection density of transitive groups with small cyclic point stabilizers\",\"authors\":\"Ademir Hujdurović , István Kovács , Klavdija Kutnar , Dragan Marušič\",\"doi\":\"10.1016/j.ejc.2024.104079\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><div>For a permutation group <span><math><mi>G</mi></math></span> acting on a set <span><math><mi>V</mi></math></span>, a subset <span><math><mi>F</mi></math></span> of <span><math><mi>G</mi></math></span> is said to be an <em>intersecting set</em> if for every pair of elements <span><math><mrow><mi>g</mi><mo>,</mo><mi>h</mi><mo>∈</mo><mi>F</mi></mrow></math></span> there exists <span><math><mrow><mi>v</mi><mo>∈</mo><mi>V</mi></mrow></math></span> such that <span><math><mrow><mi>g</mi><mrow><mo>(</mo><mi>v</mi><mo>)</mo></mrow><mo>=</mo><mi>h</mi><mrow><mo>(</mo><mi>v</mi><mo>)</mo></mrow></mrow></math></span>. The <em>intersection density</em> <span><math><mrow><mi>ρ</mi><mrow><mo>(</mo><mi>G</mi><mo>)</mo></mrow></mrow></math></span> of a transitive permutation group <span><math><mi>G</mi></math></span> is the maximum value of the quotient <span><math><mrow><mrow><mo>|</mo><mi>F</mi><mo>|</mo></mrow><mo>/</mo><mrow><mo>|</mo><msub><mrow><mi>G</mi></mrow><mrow><mi>v</mi></mrow></msub><mo>|</mo></mrow></mrow></math></span> where <span><math><msub><mrow><mi>G</mi></mrow><mrow><mi>v</mi></mrow></msub></math></span> is a stabilizer of a point <span><math><mrow><mi>v</mi><mo>∈</mo><mi>V</mi></mrow></math></span> and <span><math><mi>F</mi></math></span> runs over all intersecting sets in <span><math><mi>G</mi></math></span>. If <span><math><msub><mrow><mi>G</mi></mrow><mrow><mi>v</mi></mrow></msub></math></span> is a largest intersecting set in <span><math><mi>G</mi></math></span> then <span><math><mi>G</mi></math></span> is said to have the <em>Erdős-Ko-Rado (EKR)-property</em>. This paper is devoted to the study of transitive permutation groups, with point stabilizers of prime order with a special emphasis given to orders 2 and 3, which do not have the EKR-property. Among others, constructions of an infinite family of transitive permutation groups having point stabilizer of order 3 with intersection density <span><math><mrow><mn>4</mn><mo>/</mo><mn>3</mn></mrow></math></span> and of infinite families of transitive permutation groups having point stabilizer of order 3 with arbitrarily large intersection density are given.</div></div>\",\"PeriodicalId\":50490,\"journal\":{\"name\":\"European Journal of Combinatorics\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":1.0000,\"publicationDate\":\"2024-10-07\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"European Journal of Combinatorics\",\"FirstCategoryId\":\"100\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S0195669824001641\",\"RegionNum\":3,\"RegionCategory\":\"数学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q1\",\"JCRName\":\"MATHEMATICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"European Journal of Combinatorics","FirstCategoryId":"100","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0195669824001641","RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"MATHEMATICS","Score":null,"Total":0}
引用次数: 0
摘要
对于作用于集合 V 的置换群 G,如果每一对元素 g、h∈F 都存在 v∈V,使得 g(v)=h(v) ,则称 G 的子集 F 为交集。跨正交置换群 G 的交集密度 ρ(G) 是商 |F|/|Gv| 的最大值,其中 Gv 是点 v∈V 的稳定子,而 F 遍历 G 中的所有交集。如果 Gv 是 G 中的最大交集,则称 G 具有厄尔多斯-科-拉多(EKR)属性。本文致力于研究具有素阶点稳定器的传递置换群,特别强调不具有 EKR 属性的 2 阶和 3 阶。除其他外,本文还给出了具有交集密度为 4/3 的 3 阶点稳定器的传递置换群无穷族的构造,以及具有任意大交集密度的 3 阶点稳定器的传递置换群无穷族的构造。
Intersection density of transitive groups with small cyclic point stabilizers
For a permutation group acting on a set , a subset of is said to be an intersecting set if for every pair of elements there exists such that . The intersection density of a transitive permutation group is the maximum value of the quotient where is a stabilizer of a point and runs over all intersecting sets in . If is a largest intersecting set in then is said to have the Erdős-Ko-Rado (EKR)-property. This paper is devoted to the study of transitive permutation groups, with point stabilizers of prime order with a special emphasis given to orders 2 and 3, which do not have the EKR-property. Among others, constructions of an infinite family of transitive permutation groups having point stabilizer of order 3 with intersection density and of infinite families of transitive permutation groups having point stabilizer of order 3 with arbitrarily large intersection density are given.
期刊介绍:
The European Journal of Combinatorics is a high standard, international, bimonthly journal of pure mathematics, specializing in theories arising from combinatorial problems. The journal is primarily open to papers dealing with mathematical structures within combinatorics and/or establishing direct links between combinatorics and other branches of mathematics and the theories of computing. The journal includes full-length research papers on important topics.