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Unformatted text preview: CMSC 451: Reductions & NPcompleteness Slides By: Carl Kingsford Department of Computer Science University of Maryland, College Park Based on Section 8.1 of Algorithm Design by Kleinberg & Tardos. Reductions as tool for hardness We want prove some problems are computationally difficult. As a first step, we settle for relative judgements: Problem X is at least as hard as problem Y To prove such a statement, we reduce problem Y to problem X : If you had a black box that can solve instances of problem X, how can you solve any instance of Y using polynomial number of steps, plus a polynomial number of calls to the black box that solves X? Polynomial Reductions • If problem Y can be reduced to problem X , we denote this by Y ≤ P X . • This means “ Y is polynomaltime reducible to X .” • It also means that X is at least as hard as Y because if you can solve X , you can solve Y . • Note: We reduce to the problem we want to show is the harder problem. Polynomial Problems Suppose: • Y ≤ P X , and • there is an polynomial time algorithm for X . Then, there is a polynomial time algorithm for Y . Why? Polynomial Problems Suppose: • Y ≤ P X , and • there is an polynomial time algorithm for X . Then, there is a polynomial time algorithm for Y . Why? Because polynomials compose. Call X Call X We’ve Seen Reductions Before Examples of Reductions: • Max Bipartite Matching ≤ P Max Network Flow . • Image Segmentation ≤ P MinCut ....
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This note was uploaded on 01/13/2012 for the course CMSC 423 taught by Professor Staff during the Fall '07 term at Maryland.
 Fall '07
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