Let G = (V, E) be a graph and S = {s{sub}1, s{sub}2,..., s{sub}k} be a subset of V. An attack on S is any k mutually disjoint sets A= {A{sub}1, A{sub}2,..., A{sub}k} such that A{sub}i{is contained in}Ns{sub}i = S for 1 ≤ i ≤ k, and a defense of S is any k mutually disjoint sets D= {D{sub}1, D{sub}2, ..., D{sub}k} such that D{sub}i{is contained in}Ns{sub}i ∩ S for 1 ≤ i ≤ k, where Nv denotes the closed neighborhood of v. Then attack A is said to be defendable if there exists a defense D such that Di{sub} ≥ A{sub}i for 1≤i ≤ k, and S is secure if every attack on S is defendable. The security number of G is the cardinality of a smallest secure set of G. In this paper, we show that any outerplanar graph has security number at most 3.
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