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On Strong Domination in Graphs

Johannes H. Hattingh 1, Michael A. Henning2
1Department of Mathematics Rand Afrikaans University P.O. Box 524 Auckland Park 2006 South Africa
2 Department of Mathematics University of Natal Private Bag X01 Pietermaritzburg 3209 South Africa

Abstract

Let G=(V,E) be a graph. A vertex u strongly dominates a vertex v if uvE and deg(u)>deg(v). A set SV is a strong dominating set of G if every vertex in VS is strongly dominated by at least one vertex of S.

The minimum cardinality among all strong dominating sets of G is called the strong domination number of G and is denoted by γst(G). This parameter was introduced by Sampathkumar and Pushpa Latha in [4].

In this paper, we investigate sharp upper bounds on the strong domination number for a tree and a connected graph. We show that for any tree T of order p>2 that is different from the tree obtained from a star K1,3 by subdividing each edge once,
γst(T)4p17
and this bound is sharp.

For any connected graph G of order p3, it is shown that γst(G)2(p1)3 and this bound is sharp. We show that the decision problem corresponding to the computation of γst is NP-complete, even for bipartite or chordal graphs.