Competition Numbers, Quasi-line Graphs, and Holes

Abstract

The competition graph of an acyclic directed graph $D$ is the undirected graph on the same vertex set as $D$ in which two distinct vertices are adjacent if they have a common out-neighbor in $D$. The competition number of an undirected graph $G$ is the least number of isolated vertices that have to be added to $G$ to make it the competition graph of an acyclic directed graph. We resolve two conjectures concerning competition graphs. First, we prove a conjecture of Opsut by showing that the competition number of every quasi-line graph is at most 2. Recall that a quasi-line graph, also called a locally co-bipartite graph, is a graph for which the neighborhood of every vertex can be partitioned into at most two cliques. To prove this conjecture we devise an alternative characterization of quasi-line graphs to the one by Chudnovsky and Seymour. Second, we prove a conjecture of Kim by showing that the competition number of any graph is at most one greater than the number of holes in the graph. Our methods also allow us to prove a strengthened form of this conjecture recently proposed by Kim et al., showing that the competition number of any graph is at most one greater than the dimension of the subspace of the cycle space spanned by the holes.