- INSTANCE: Directed Graph , edge costs , root , set of terminals of size .
- SOLUTION: A directed tree in rooted at such that and contains at most branching nodes
- COST FUNCTION:
- OBJECTIVE:
Minimize.
*Hardness:*NP-hard to approximate within for every when is not fixed, even for planar graphs with unit edge costs [106]*Comment:*When both and are fixed, deciding existence of a feasible solution is in [106].

2015-04-27 Revision: 288 PDF version