Abstract-We consider the single-source (or single-sink) buyat-bulk problem with an unknown concave cost function. We want to route a set of demands along a graph to or from a designated root node, and the cost of routing x units of flow along an edge is proportional to some concave, non-decreasing function f such that f (0) = 0. We present a polynomial time algorithm that finds a distribution over trees such that the expected cost of a tree for any f is within an O(1)-factor of the optimum cost for that f . The previous best simultaneous approximation for this problem, even ignoring computation time, was O(log |D|), where D is the multi-set of demand nodes. We design a simple algorithmic framework using the ellipsoid method that finds an O(1)-approximation if one exists, and then construct a separation oracle using a novel adaptation of the Guha, Meyerson, and Munagala [10] algorithm for the single-sink buy-atbulk problem that proves an O(1) approximation is possible for all f . The number of trees in the support of the distribution constructed by our algorithm is at most 1 + log |D|.
In a related paper by REF , the authors construct (in polynomial time) a set of overlay trees from a given general graph such that the expected cost of a tree for any f is within an O(1)-factor of the optimum cost for that f .
An Oblivious O(1)-Approximation for Single Source Buy-at-Bulk
2009 50th Annual IEEE Symposium on Foundations of Computer Science
2009 50th Annual IEEE Symposium on Foundations of Computer Science
Mathematics Computer Science