On feedback vertex set in reducible flow hypergraphs

Abstract

A directed hypergraph H = (V, A) is a finite set of vertices V and a set of hyper-arcs A, where each hyper-arc is an ordered pair of nonempty subsets of vertices. A flow hypergraph H = (V, A, s) is a triple, such that (V, A) is a directed hypergraph, s e V is a distinguished vertex such that s reaches every vertex of V. Reducible flow hypergraphs are a generalization of Hecht and Ullman’s reducible flowgraphs. The feedback vertex set (fvs) decision problem has a directed hypergraph H and an integer k ≥ 0 as input and the question is whether there is V'⊆V, |V' |≤k such that H\\V' is an acyclic directed hypergraph. It is known that fvs is polynomial time solvable for reducible flowgraphs. In this article we prove that fvs is NP-complete for reducible flow hypergraphs showing a reduction from 3-satisfiability problem with at most 3 occurrences per variable (3sat3-). We exhibit a polynomial-time ∆-approximation for fvs in reducible flow hypergraphs, where ∆ is the maximum number of hyper-arcs adjacent to a vertex of H.