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Abstract
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In a graph G, a vertex dominates itself and its neighbors. A set S of
vertices in a graph G is a double dominating set if S dominates every vertex of G at
least twice. The double domination number 'YX2(G) is the minimum cardinality of a
double dominating set in G. The annihilation number a(G) is the largest integer k
such that the sum of the first k terms of the non-decreasing degree sequence of G
is at most the number of edges in G. In this paper, we show that for any tree T of
order n 2: 2, different fl'Om P4, 'Yx2(T) ~ 3a(~)+1 •
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