directed spanners via flow-based linear programs
We examine directed spanners through flow-based linear programming relaxations. We design an ~O(n 2/3 )-approximation algorithm for the directed k-spanner problem that works for all k ? 1, which is the first sublinear approximation for arbitrary edge-lengths. Even in the more restricted setting of unit edge-lengths, our algorithm improves over the previous ~O(n 1-1/k ) approximation [BGJRW09] when k ? 4. For the special case of k=3 we design a different algorithm achieving an ~O(?n)-approximation, improving the previous ~O(n 2/3 ) [EP05,BGJRW09] (independently of our work, an ~O(n 1-1/? k/2? ) was recently devised [BRR10]). Both of our algorithms easily extend to the fault-tolerant setting, which has recently attracted attention but not from an approximation viewpoint. We also prove a nearly matching integrality gap of ~?(n 1/3 - ? ) for every constant ? > 0. A virtue of all our algorithms is that they are relatively simple. Technically, we introduce a new yet natural flow-based relaxation, and show how to approximately solve it even when its size is not polynomial. The main challenge is to design a rounding scheme that "coordinates" the choices of flow-paths between the many demand pairs while using few edges overall. We achieve this, roughly speaking, by randomization at the level of vertices .
spanners linear programming approximation algorithms