A set D of vertices in a graph G is said to be a dominating set if every
vertex in V −D is adjacent to some vertex in D. We call D a total dominating
set for G if every vertex in V is adjacent to some vertex in D.(i.e.,N(D) = V ).
The minimum cardinality of a dominating set (a total dominating set) of G
is denoted by
(G), (
t(G)) and is called the domination number (the total
domination number)of G. It is clear that
(G) ≤
t(G) ≤ 2
(G) for any graph
G without isolated vertices.