The Steiner k-cut problem

We consider the Steiner k-cut problem which generalizes both the k-cut problem and the multiway cut problem. The Steiner k-cut problem is defined as follows. Given an edge-weighted undirected graph G = (V, E), a subset of vertices X subset of V called terminals, and an integer k = textbackslash{}Xtextbackslash{}, the objective is to find a minimum weight set of edges whose removal results in k disconnected components, each of which contains at least one terminal. We give two approximation algorithms for the problem: a greedy (2 - 2/k) approximation based on Gomory-Hu trees, and a (2 - 2/textbackslash{}Xtextbackslash{})-approximation based on rounding a linear program. We use the insight from the rounding to develop an exact bidirected formulation for the global minimum cut problem (the k-cut problem with k = 2).