Graphs having many holes but with small competition numbers

Abstract

The competition number k ( G ) of a graph G is the smallest number k such that G together with k isolated vertices added is the competition graph of an acyclic digraph. A chordless cycle of length at least 4 of a graph is called a hole of the graph. The number of holes of a graph is closely related to its competition number as the competition number of a graph which does not contain a hole is at most one and the competition number of a complete bipartite graph K ⌊ n 2 ⌋ , ⌈ n 2 ⌉ which has so many holes that no more holes can be added is the largest among those of graphs with n vertices. In this paper, we show that even if a connected graph G has many holes, the competition number of G can be as small as 2 under some assumption. In addition, we show that, for a connected graph G with exactly h holes and at most one non-edge maximal clique, if all the holes of G are pairwise edge-disjoint and the clique number ω = ω ( G ) of G satisfies 2 ≤ ω ≤ h + 1 , then the competition number of G is at most h − ω + 3 .