Approximating k-median via pseudo-approximation

We present a novel approximation algorithm for k-median that achieves an approximation guarantee of 1 + √3 + ε, improving upon the decade-old ratio of 3+ε. Our approach is based on two components, each of which, we believe, is of independent interest. First, we show that in order to give an α-approximation algorithm for k-median, it is sufficient to give a pseudo- approximation algorithm that finds an α-approximate solu- tion by opening k+O(1) facilities. This is a rather surprising result as there exist instances for which opening k + 1 facil- ities may lead to a significant smaller cost than if only k facilities were opened. Second, we give such a pseudo-approximation algorithm with α = 1+ √3+ε. Prior to our work, it was not even known whether opening k + o(k) facilities would help improve the approximation ratio. Copyright 2013 ACM.