{"title":"用禁止子图最大化给定大小的图的谱半径","authors":"Yanting Zhang , Ligong Wang","doi":"10.1016/j.laa.2025.07.032","DOIUrl":null,"url":null,"abstract":"<div><div>Let <span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>7</mn></mrow></msub></math></span> denote the 7-vertex <em>fan graph</em> consisting of a 6-vertex path plus a vertex adjacent to each vertex of the path. Let <span><math><msub><mrow><mi>K</mi></mrow><mrow><mn>3</mn></mrow></msub><mo>∨</mo><mfrac><mrow><mi>m</mi><mo>−</mo><mn>3</mn></mrow><mrow><mn>3</mn></mrow></mfrac><msub><mrow><mi>K</mi></mrow><mrow><mn>1</mn></mrow></msub></math></span> be the graph obtained by joining each vertex of a triangle <span><math><msub><mrow><mi>K</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span> to <span><math><mfrac><mrow><mi>m</mi><mo>−</mo><mn>3</mn></mrow><mrow><mn>3</mn></mrow></mfrac></math></span> isolated vertices. In this paper, we show that if <em>G</em> is an <span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>7</mn></mrow></msub></math></span>-free graph with size <span><math><mi>m</mi><mo>≥</mo><mn>33</mn></math></span>, then the spectral radius <span><math><mi>ρ</mi><mo>(</mo><mi>G</mi><mo>)</mo><mo>≤</mo><mn>1</mn><mo>+</mo><msqrt><mrow><mi>m</mi><mo>−</mo><mn>2</mn></mrow></msqrt></math></span>, equality holds if and only if <span><math><mi>G</mi><mo>≅</mo><msub><mrow><mi>K</mi></mrow><mrow><mn>3</mn></mrow></msub><mo>∨</mo><mfrac><mrow><mi>m</mi><mo>−</mo><mn>3</mn></mrow><mrow><mn>3</mn></mrow></mfrac><msub><mrow><mi>K</mi></mrow><mrow><mn>1</mn></mrow></msub></math></span> (possibly, with some isolated vertices).</div></div>","PeriodicalId":18043,"journal":{"name":"Linear Algebra and its Applications","volume":"727 ","pages":"Pages 10-23"},"PeriodicalIF":1.1000,"publicationDate":"2025-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Maximizing the spectral radius of graphs of given size with a forbidden subgraph\",\"authors\":\"Yanting Zhang , Ligong Wang\",\"doi\":\"10.1016/j.laa.2025.07.032\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><div>Let <span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>7</mn></mrow></msub></math></span> denote the 7-vertex <em>fan graph</em> consisting of a 6-vertex path plus a vertex adjacent to each vertex of the path. Let <span><math><msub><mrow><mi>K</mi></mrow><mrow><mn>3</mn></mrow></msub><mo>∨</mo><mfrac><mrow><mi>m</mi><mo>−</mo><mn>3</mn></mrow><mrow><mn>3</mn></mrow></mfrac><msub><mrow><mi>K</mi></mrow><mrow><mn>1</mn></mrow></msub></math></span> be the graph obtained by joining each vertex of a triangle <span><math><msub><mrow><mi>K</mi></mrow><mrow><mn>3</mn></mrow></msub></math></span> to <span><math><mfrac><mrow><mi>m</mi><mo>−</mo><mn>3</mn></mrow><mrow><mn>3</mn></mrow></mfrac></math></span> isolated vertices. In this paper, we show that if <em>G</em> is an <span><math><msub><mrow><mi>H</mi></mrow><mrow><mn>7</mn></mrow></msub></math></span>-free graph with size <span><math><mi>m</mi><mo>≥</mo><mn>33</mn></math></span>, then the spectral radius <span><math><mi>ρ</mi><mo>(</mo><mi>G</mi><mo>)</mo><mo>≤</mo><mn>1</mn><mo>+</mo><msqrt><mrow><mi>m</mi><mo>−</mo><mn>2</mn></mrow></msqrt></math></span>, equality holds if and only if <span><math><mi>G</mi><mo>≅</mo><msub><mrow><mi>K</mi></mrow><mrow><mn>3</mn></mrow></msub><mo>∨</mo><mfrac><mrow><mi>m</mi><mo>−</mo><mn>3</mn></mrow><mrow><mn>3</mn></mrow></mfrac><msub><mrow><mi>K</mi></mrow><mrow><mn>1</mn></mrow></msub></math></span> (possibly, with some isolated vertices).</div></div>\",\"PeriodicalId\":18043,\"journal\":{\"name\":\"Linear Algebra and its Applications\",\"volume\":\"727 \",\"pages\":\"Pages 10-23\"},\"PeriodicalIF\":1.1000,\"publicationDate\":\"2025-08-05\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Linear Algebra and its Applications\",\"FirstCategoryId\":\"100\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S0024379525003301\",\"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":"Linear Algebra and its Applications","FirstCategoryId":"100","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S0024379525003301","RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"MATHEMATICS","Score":null,"Total":0}
Maximizing the spectral radius of graphs of given size with a forbidden subgraph
Let denote the 7-vertex fan graph consisting of a 6-vertex path plus a vertex adjacent to each vertex of the path. Let be the graph obtained by joining each vertex of a triangle to isolated vertices. In this paper, we show that if G is an -free graph with size , then the spectral radius , equality holds if and only if (possibly, with some isolated vertices).
期刊介绍:
Linear Algebra and its Applications publishes articles that contribute new information or new insights to matrix theory and finite dimensional linear algebra in their algebraic, arithmetic, combinatorial, geometric, or numerical aspects. It also publishes articles that give significant applications of matrix theory or linear algebra to other branches of mathematics and to other sciences. Articles that provide new information or perspectives on the historical development of matrix theory and linear algebra are also welcome. Expository articles which can serve as an introduction to a subject for workers in related areas and which bring one to the frontiers of research are encouraged. Reviews of books are published occasionally as are conference reports that provide an historical record of major meetings on matrix theory and linear algebra.