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

TL;DR

This paper characterizes digraphs where every set of t vertices shares exactly λ common out-neighbors, extending previous results for specific cases and providing conditions for such digraphs to be complete.

Contribution

It extends the classification of (t,λ)-liking digraphs for t ≥ λ+2 and offers conditions under which these digraphs are complete.

Findings

Complete characterization of (t,λ)-liking digraphs for t ≥ λ+2.

Conditions for (t,λ)-liking digraphs to be complete digraphs.

Generalization of previous results for specific (t,λ) cases.

Abstract

We say that a digraph is a $(t,\lambda)$-liking digraph if every $t$ vertices have exactly $\lambda$ common out-neighbors. In 1975, Plesn\'{i}k [Graphs with a homogeneity, 1975. {\it Glasnik Mathematicki} 10:9-23] proved that any $(t,1)$-liking digraph is the complete digraph on $t+1$ vertices for each $t\geq 3$. Choi {\it et al}. [A digraph version of the Friendship Theorem, 2025. {\it Discrete mathematics}, 348(1), 114238] showed that a $(2,1)$-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 $(t,\lambda)$-liking digraphs with $t \geq \lambda+2$ and giving some equivalent conditions for a $(t,\lambda)$-liking digraph being a complete digraph on $t+\lambda$ vertices.