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 -liking digraph if every vertices have exactly common out-neighbors. In 1975, Plesn\'{i}k [Graphs with a homogeneity, 1975. {\it Glasnik Mathematicki} 10:9-23] proved that any -liking digraph is the complete digraph on vertices for each . Choi {\it et al}. [A digraph version of the Friendship Theorem, 2025. {\it Discrete mathematics}, 348(1), 114238] showed that a -liking digraph is a fancy wheel digraph or a -diregular digraph for some positive integer . 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.
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}
}