A factor 2 approximation algorithm for the generalized Steiner network problem

We present a factor 2 approximation algorithm for finding a minimum-cost su bgraph having at least a specified number of edges in each cut. This class of problems includes, among others, the generalized Steiner network problem , which is also known as the survivable network design problem. Our algorit hm first solves the linear relaxation of this problem, and then iteratively rounds off the solution. The key idea in rounding off is that in a basic s olution of the LP relaxation, at least one edge gets included at least to t he extent of half. We include this edge into our integral solution and solv e the residual problem.