Faster Algorithms for k -Medians in Trees

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In the k-median problem we are given a connected graph with non-negative weights associated with the nodes and lengths associated with the edges. The task is to compute locations of k facilities in order to minimize the sum of the weighted distances between each node and its closest facility. In this paper we consider the case when the graph is a tree. We show that this problem can be solved in time \(O(n {\mbox{\rm polylog}} (n))\) for the following cases: (i) directed trees (and any fixed k), (ii) balanced undirected trees, and (iii) undirected trees with k=3.