{"title":"有向图中每t个顶点恰好有λ个共同外邻点","authors":"Myungho Choi , Hojin Chu , Suh-Ryung Kim","doi":"10.1016/j.disc.2025.114580","DOIUrl":null,"url":null,"abstract":"<div><div>We say that a digraph is a <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span>-liking digraph if every <em>t</em> vertices have exactly <em>λ</em> common out-neighbors. In 1975, Plesník (1975) <span><span>[14]</span></span> proved that any <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mn>1</mn><mo>)</mo></math></span>-liking digraph is the complete digraph on <span><math><mi>t</mi><mo>+</mo><mn>1</mn></math></span> vertices for each <span><math><mi>t</mi><mo>≥</mo><mn>3</mn></math></span>. Choi et al. (2025) <span><span>[5]</span></span> showed that a <span><math><mo>(</mo><mn>2</mn><mo>,</mo><mn>1</mn><mo>)</mo></math></span>-liking digraph is a fancy wheel digraph or a <em>k</em>-diregular digraph for some positive integer <em>k</em>. In this paper, we extend these results by completely characterizing the <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span>-liking digraphs with <span><math><mi>t</mi><mo>≥</mo><mi>λ</mi><mo>+</mo><mn>2</mn></math></span> and giving some equivalent conditions for a <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span>-liking digraph being a complete digraph on <span><math><mi>t</mi><mo>+</mo><mi>λ</mi></math></span> vertices.</div></div>","PeriodicalId":50572,"journal":{"name":"Discrete Mathematics","volume":"348 10","pages":"Article 114580"},"PeriodicalIF":0.7000,"publicationDate":"2025-05-21","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Digraphs in which every t vertices have exactly λ common out-neighbors\",\"authors\":\"Myungho Choi , Hojin Chu , Suh-Ryung Kim\",\"doi\":\"10.1016/j.disc.2025.114580\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><div>We say that a digraph is a <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span>-liking digraph if every <em>t</em> vertices have exactly <em>λ</em> common out-neighbors. In 1975, Plesník (1975) <span><span>[14]</span></span> proved that any <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mn>1</mn><mo>)</mo></math></span>-liking digraph is the complete digraph on <span><math><mi>t</mi><mo>+</mo><mn>1</mn></math></span> vertices for each <span><math><mi>t</mi><mo>≥</mo><mn>3</mn></math></span>. Choi et al. (2025) <span><span>[5]</span></span> showed that a <span><math><mo>(</mo><mn>2</mn><mo>,</mo><mn>1</mn><mo>)</mo></math></span>-liking digraph is a fancy wheel digraph or a <em>k</em>-diregular digraph for some positive integer <em>k</em>. In this paper, we extend these results by completely characterizing the <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span>-liking digraphs with <span><math><mi>t</mi><mo>≥</mo><mi>λ</mi><mo>+</mo><mn>2</mn></math></span> and giving some equivalent conditions for a <span><math><mo>(</mo><mi>t</mi><mo>,</mo><mi>λ</mi><mo>)</mo></math></span>-liking digraph being a complete digraph on <span><math><mi>t</mi><mo>+</mo><mi>λ</mi></math></span> vertices.</div></div>\",\"PeriodicalId\":50572,\"journal\":{\"name\":\"Discrete Mathematics\",\"volume\":\"348 10\",\"pages\":\"Article 114580\"},\"PeriodicalIF\":0.7000,\"publicationDate\":\"2025-05-21\",\"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/S0012365X25001888\",\"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/S0012365X25001888","RegionNum":3,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"MATHEMATICS","Score":null,"Total":0}
引用次数: 0
摘要
我们说一个有向图是一个(t,λ)类有向图如果每t个顶点恰好有λ共同外邻点。1975年Plesník(1975)[14]证明了任意(t,1)类有向图都是t+1个顶点上的完全有向图,且t≥3。Choi et al.(2025)[5]证明了(2,1)- like有向图是花式轮有向图或对某正整数k的k-不规则有向图。本文通过完全刻画t≥λ+2的(t,λ)- like有向图,并给出了(t,λ)- like有向图是t+λ顶点上的完全有向图的一些等价条件,扩展了这些结果。
Digraphs in which every t vertices have exactly λ common out-neighbors
We say that a digraph is a -liking digraph if every t vertices have exactly λ common out-neighbors. In 1975, Plesník (1975) [14] proved that any -liking digraph is the complete digraph on vertices for each . Choi et al. (2025) [5] showed that a -liking digraph is a fancy wheel digraph or a k-diregular digraph for some positive integer k. In this paper, we extend these results by completely characterizing the -liking digraphs with and giving some equivalent conditions for a -liking digraph being a complete digraph on vertices.
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.