Let γ(G) denote the domination number of a simple graph G. Then Vizing conjectured that γ(G) γ(H)<=γ(G×H), where G×H is the graph product. While the full conjecture remains open, Clark and Suen have proved the looser result γ(G) γ(H)<=2γ(G×H).

domination number | Vizing's theorem