Using $p$-row graphs to study $p$-competition graphs

Abstract

For a positive integer $p$, the $p$-competition graph of a digraph $D$ is a graph which has the same vertex set as $D$ and an edge between distinct vertices $x$ and $y$ if and only if $x$ and $y$ have at least $p$ common out-neighbors in $D$. A graph is said to be a $p$-competition graph if it is the $p$-competition graph of a digraph. Given a graph $G$, we call the set of positive integers $p$ such that $G$ is a $p$-competition the competition-realizer of a graph $G$. In this paper, we introduce the notion of $p$-row graph of a matrix which generalizes the existing notion of row graph. We call the graph obtained from a graph $G$ by identifying each pair of adjacent vertices which share the same closed neighborhood the condensation of $G$. Using the notions of $p$-row graph and condensation of a graph, we study competition-realizers for various graphs to extend results given by Kim {\it et al.}~[$p$-competition graphs, {\it Linear Algebra Appl.} {\bf 217} (1995) 167--178]. Especially, we find all the elements in the competition-realizer for each caterpillar.