A k ‐spanner of a connected graph G=(V,E) is a subgraph G' 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 G' is larger than the distance in G by no more than a factor of k . This paper concerns the hardness of finding spanners with a 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, and prove an essential difference between the approximability of the case k=2 and the case k\geq 5 .
