On the hardness of approximating spanners

A k-spanner of a connected graph G = (V, E) is a subgraph Gʹ consisting of all the vertices of V and a subset of the edges, with the additional property that the distance between any two vertices in Gʹ is larger than that distance in G by no more than a factor of k. This paper concerns the hardness of finding spanners with the number of edges close to the optimum. It is proved that for every fixed k approximating the spanner problem is at least as hard as approximating the set cover problem We also consider a weighted version of the spanner problem. We prove that in the case k = 2 the problem admits an O(log n)-ratio approximation, and in the case k ≥ 5, there is no 2^{log1-ϵ n}-ratio approximation, for any ϵ > 0, unless NP ⊆ DTIME(n^{polylog n}).