Given a graph G=(V,E) and a positive integer D , we consider the problem of finding a minimum number of new edges E' such that the augmented graph G'=(V,E\cup E') is biconnected and has diameter no greater than D. In this note we show that this problem is NP‐hard for all fixed D , by employing a reduction from the DOMINATING SET problem. We prove that the problem remains NP‐hard even for forests and trees, but in this case we present approximation algorithms with worst‐case bounds 3 (for even D ) and 6 (for odd D ). A closely related problem of finding a minimum number of edges such that the augmented graph has diameter no greater than D has been shown to be NP‐hard by Schoone et al. [21] when D=3 , and by Li et al. [17] when D=2.
