Centre for Discrete and Applicable Mathematics
	CDAM Research Report, LSE-CDAM-2004-10 May 2004

Abstract

A hypergraph  H = (V,E)  is a subtree hypergraph if there is a tree T on V such that each hyperedge of E induces a subtree of T. To find a minimum size transversal for a subtree hypergraph is, in general, NP-hard. In this paper, we show that if it is possible to decide if a set of vertices W in V is a transversal in time S(n) (where  n = |V|), then it is possible to find a minimum size transversal in  O(n³S(n)).

This result provides a polynomial algorithm for the Source Location Problem: a set of (k,l)-sources for a digraph  D = (V,A)  is a subset K of V such that for any v in V\K there are k arc-disjoint paths that each join a vertex of K to v and l arc-disjoint paths that each join v to K. The Source Location Problem is to find a minimum size set of (k,l)-sources. We show that this is a case of finding a transversal of a subtree hypergraph, and that in this case S(n) is polynomial.

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(This paper supersedes the earlier paper A Polynomial Algorithm for the Source Location Problem in Digraphs, which is obsolete.)

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