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Abstract: Given a graph , a defensive alliance of is a set of vertices satisfying the condition that for each , at least half of the vertices in the closed neighborhood of are in . A defensive alliance is called global if every vertex in is adjacent to at least one member of the defensive alliance . The global defensive alliance number of , denoted , is the minimum size around all the global defensive alliances of . In this paper, we present an efficient algorithm to determine the global defensive alliance numbers of trees, and further give formulas to decide the global defensive alliance numbers of complete -ary trees for . We also establish upper bounds and lower bounds for and , and show that the bounds are sharp for certain . [Copyright &y& Elsevier]

Abstract: Given a graph and two positive integers with an -labeling of is a function from the vertex set to the set of all nonnegative integers such that if and if . A -labeling is an -labeling such that no label is greater than . The -labeling number of , denoted by is the smallest number such that has a -labeling. When considering the digraph , we use in place of . We study the -labeling number of a digraph in this paper. We find some relations between the -labeling number of a graph and an orientation of , and give some results for the -labeling numbers of -partite digraphs. We also study the -labeling numbers for those graphs for which the underlying graphs are paths, cycles or trees. [Copyright &y& Elsevier]

Abstract: All-to-all communication occurs in many important applications in parallel processing. In this paper, we study the all-to-all broadcast number (the shortest time needed to complete the all-to-all broadcast) of graphs under the assumption that: each vertex can use all of its links at the same time, and each communication link is half duplex and can carry only one message at a unit of time. We give upper and lower bounds for the all-to-all broadcast number of graphs and give formulas for the all-to-all broadcast number of trees, complete bipartite graphs and double loop networks under this model. [Copyright &y& Elsevier]