English

Digraphs in which every $t$ vertices have exactly $\lambda$ common out-neighbors

Combinatorics 2025-07-18 v2

Abstract

We say that a digraph is a (t,λ)(t,\lambda)-liking digraph if every tt vertices have exactly λ\lambda common out-neighbors. In 1975, Plesn\'{i}k [Graphs with a homogeneity, 1975. {\it Glasnik Mathematicki} 10:9-23] proved that any (t,1)(t,1)-liking digraph is the complete digraph on t+1t+1 vertices for each t3t\geq 3. Choi {\it et al}. [A digraph version of the Friendship Theorem, 2025. {\it Discrete mathematics}, 348(1), 114238] showed that a (2,1)(2,1)-liking digraph is a fancy wheel digraph or a kk-diregular digraph for some positive integer kk. In this paper, we extend these results by completely characterizing the (t,λ)(t,\lambda)-liking digraphs with tλ+2t \geq \lambda+2 and giving some equivalent conditions for a (t,λ)(t,\lambda)-liking digraph being a complete digraph on t+λt+\lambda vertices.

Keywords

Cite

@article{arxiv.2405.02662,
  title  = {Digraphs in which every $t$ vertices have exactly $\lambda$ common out-neighbors},
  author = {Myungho Choi and Hojin Chu and Suh-Ryung Kim},
  journal= {arXiv preprint arXiv:2405.02662},
  year   = {2025}
}
R2 v1 2026-06-28T16:16:39.023Z