In a graph
G, an
efficient dominating set is a subset
D of vertices such that
D is an independent set and each vertex outside
D has exactly one neighbor in
D. The
Efficient Dominating Set (ED) problem asks for the existence of an efficient dominating set in a given graph. The ED problem is known to be solvable in polynomial time for
P5-free graphs but
NP-complete for
P7-free graphs whereas for
P6-free graphs, its complexity was an open problem. Recently, Lokshtanov et al. and independently, Mosca showed that ED is solvable in polynomial time for
P6-free graphs.
In this paper, we show that the ED problem can be solved efficiently for two subclasses of P6-free graphs, namely for (P6, bull)-free graphs, and for (P6,S1,1,3)-free graphs; the time bounds for the two subclasses are much better than in the general case of P6-free graphs.