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Spanning galaxies in digraphs. (English) Zbl 1273.05087
Nešetřil, Jaroslav (ed.) et al., Extended abstracts of the 5th European conference on combinatorics, graph theory and applications, EuroComb’09, Bordeaux, France, September 7–11, 2009. Amsterdam: Elsevier. Electronic Notes in Discrete Mathematics 34, 139-143 (2009).
Summary: A star is an arborescence in which the root dominates all the other vertices. A galaxy is a vertex-disjoint union of stars. The directed star arboricity of a digraph \(D\), denoted by \(\mathsf{dst}(D)\), is the minimum number of galaxies needed to cover \(A(D)\). In this paper, we show that \(\mathsf{dst}(D)\leq \Delta(D)+1\) and that if \(D\) is acyclic then \(\mathsf{dst}(D)\leq \Delta(D)\). These results are proved by considering the existence of spanning galaxies in digraphs. Thus, we study the problem of deciding whether a digraph \(D\) has a spanning galaxy or not. We show that it is NP-complete (even when restricted to acyclic digraphs) but that it becomes polynomial-time solvable when restricted to strongly connected digraphs.
For the entire collection see [Zbl 1239.05008].
MSC:
05C20 Directed graphs (digraphs), tournaments
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