Tree 3-spanners of diameter at most 5
  Tree spanners approximate distances within graphs; a subtree of a graph is a tree $t$-spanner of the graph if and only if for every pair of vertices their distance in the subtree is at most $t$ times their distance in the graph. When a graph contains a subtree of diameter at most $t$, then trivially admits a tree $t$-spanner. Now, determining whether a graph admits a tree $t$-spanner of diameter at most $t+1$ is an NP complete problem, when $t\geq 4$, and it is tractable, when $t\leq 3$. Although it is not known whether it is tractable to decide graphs that admit a tree 3-spanner of any diameter, an efficient algorithm to determine graphs that admit a tree 3-spanner of diameter at most 5 is presented. Moreover, it is proved that if a graph of diameter at most 3 admits a tee 3-spanner, then it admits a tree 3-spanner of diameter at most 5. Hence, this algorithm decides tree 3-spanner admissibility of diameter at most 3 graphs. 
Ioannis Papoutsakis
cs.DM cs.DS