Inapproximability of H-Transversal/Packing

Given an undirected graph G = (VG, EG) and a fixed pattern graph H = (VH , EH) with k vertices, we consider the H-Transversal and H-Packing problems. The former asks to find the smallest S ⊆ VG such that the subgraph induced by VG \ S does not have H as a subgraph, and the latter asks to find the maximum number of pairwise disjoint k-subsets S1, ..., Sm ⊆ VG such that the subgraph induced by each Si has H as a subgraph. We prove that if H is 2-connected, H-Transversal and H-Packing are almost as hard to approximate as general k-Hypergraph Vertex Cover and k-Set Packing, so it is NP-hard to approximate them within a factor of Ω(k) and Ω̃(k) respectively. We also show that there is a 1-connected H where H-Transversal admits an O(log k)-approximation algorithm, so that the connectivity requirement cannot be relaxed from 2 to 1. For a special case of H-Transversal where H is a (family of) cycles, we mention the implication of our result to the related Feedback Vertex Set problem, and give a different hardness proof for directed graphs. 1998 ACM Subject Classification F.2.2 Nonnumerical Algorithms and Problems