Optimal Distributed Covering Algorithms
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We present a time-optimal deterministic distributed algorithm for approximating a minimum weight vertex cover in hypergraphs of rank f. This problem is equivalent to the Minimum Weight Set Cover problem in which the frequency of every element is bounded by f. The approximation factor of our algorithm is (f+ϵ). Our algorithm runs in the CONGEST model and requires O(Δ/ Δ) rounds, for constants ϵ∈(0,1] and f∈ N^+. This is the first distributed algorithm for this problem whose running time does not depend on the vertex weights nor the number of vertices. For constant values of f and ϵ, our algorithm improves over the (f+ϵ)-approximation algorithm of KMW06 whose running time is O(Δ + W), where W is the ratio between the largest and smallest vertex weights in the graph. Our algorithm also achieves an f-approximation for the problem in O(f n) rounds, improving over the classical result of KVY94 that achieves a running time of O(f^2 n). Finally, for weighted vertex cover (f=2) our algorithm achieves a deterministic running time of O( n), matching the randomized previously best result of KY11. We also show that integer covering-programs can be reduced to the Minimum Weight Set Cover problem in the distributed setting. This allows us to achieve an (f+ϵ)-approximate integral solution in O(Δ/Δ+(f· M)^1.01ϵ^-1(Δ)^0.01) rounds, where f bounds the number of variables in a constraint, Δ bounds the number of constraints a variable appears in, and M={1, 1/a_}, where a_min is the smallest normalized constraint coefficient. This improves over the results of KMW06 for the integral case, which runs in O(ϵ^-4· f^4· f·(M·Δ)) rounds.
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