Restrictions of Minimum Spanner Problems

A t-spanner of a graph G is a spanning subgraph H such that the distance between any two vertices in H is at most t times their distance in G. Spanners arise in the context of approximating the original graph with a sparse subgraph (Peleg, D., and Scha ffer, A. A. (1989), J. Graph. Theory 13 (1), 99 116). The MINIMUM t-SPANNER problem seeks to find a t-spanner with the minimum number of edges for the given graph. In this paper, we completely settle the complexity status of this problem for various values of t, on chordal graphs, split graphs, bipartite graphs and convex bipartite graphs. Our results settle an open question raised by L. Cai (1994, Discrete Appl. Math. 48, 187 194) and also greatly simplify some of the proofs presented by Cai and by L. Cai and M. Keil (1994, Networks 24, 233 249). We also give a factor 2 approximation algorithm for the MINIMUM 2-SPANNER problem on interval graphs. Finally, we provide approximation algorithms for the bandwidth minimization problem on convex bipartite graphs and split graphs using the notion of tree spanners. ] 1997 Academic Press article no. IC972641