Stability, domination and irredundance in a graph

In a graph G , a set X is called a stable set if any two vertices of X are nonadjacent. A set X is called a dominating set if every vertex of V – X is joined to at least one vertex of X. A set X is called an irredundant set if every vertex of X , not isolated in X , has at least one proper neighbor, that is a vertex of V – X joined to it but to no other vertex of X. Let α′ and α, γ, and Γ, ir and IR , denote respectively the minimum and maximum cardinalities of a maximal stable set, a minimal dominating set, and a maximal irredundant set. It is known that ir ⩽ γ ⩽ α′ ⩽ α ⩽ Γ ⩽ IR and that if G does not contain any induced subgraph isomorphic to K 1,3 , then γ = α′. Here we prove that if G contains no induced subgraph isomorphic to K 1,3 or to the graph H of figure 1, then ir = γ = α′. We prove also that if G contains no induced subgraph isomorphic to K 1,3 , to H , or to the graph h of figure 3, then Γ = IR. Finally, we improve a result of Bollobas and Cockayne about sufficient conditions for γ = ir in terms of forbidden subgraphs.