Technical Report CS0920

Title: One for the Price of Two: A Unified Approach for Approximating Covering Problems
Authors: Reuven Bar-Yehuda
Abstract: We present a simple and unified approach for developing and analyzing approximation algorithms for covering problems. We illustrate this on approximation algorithms for the following problems: Vertex Cover, Set Cover, Feedback Vertex Set, Generalized Steiner Tree and related problems. The main idea can be phrased as follows: iteratively, pay two dollars (at most) to reduce the total optimum by one dollar (at least), so the rate of payment is no more than twice the rate of the optimum reduction. This implies a total payment (i.e., approximation cost ) $\leq$ twice the optimum cost. Our main contribution is based on a formal definition for covering problems, which includes all the above fundamental problems and others. We further extend the Bafna, Berman and Fujito Local-Ratio theorem. This extension eventually yields a short generic $r$-approximation algorithm which can generate most known approximation algorithms for most covering problems. Using our technique, we present a linear time 2-approximation algorithm for the Min Clique-Complement problem. The previous best result was a 4-approximation algorithm due to Hochbaum. Another extension of the Local-Ratio theorem to randomized algorithms gives a simple proof of Pitt's randomized approximation for Vertex Cover. Using this approach, we develop a modified greedy algorithm, which for Vertex Cover, gives an expected performance ratio $ \leq 2$.
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