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Local ratio: A unified framework for approximation algorithms. In Memoriam: Shimon Even 1935-2004

Published: 01 December 2004 Publication History

Abstract

The local ratio technique is a methodology for the design and analysis of algorithms for a broad range of optimization problems. The technique is remarkably simple and elegant, and yet can be applied to several classical and fundamental problems (including covering problems, packing problems, and scheduling problems). The local ratio technique uses elementary math and requires combinatorial insight into the structure and properties of the problem at hand. Typically, when using the technique, one has to invent a weight function for a problem instance under which every "reasonable" solution is "good." The local ratio technique is closely related to the primal-dual schema, though it is not based on weak LP duality (which is the basis of the primal-dual approach) since it is not based on linear programming.In this survey we, introduce the local ratio technique and demonstrate its use in the design and analysis of algorithms for various problems. We trace the evolution path of the technique since its inception in the 1980's, culminating with the most recent development, namely, fractional local ratio, which can be viewed as a new LP rounding technique.

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Ranan B. Banerji

The authors describe this survey as a small textbook, and I agree wholeheartedly. But, it is a textbook summarizing about 80 related papers in 41 pages, so the interested reader is required to read it slowly, and with a lot of thought. Nevertheless, the guidance provided is excellent. The paper primarily considers graph-theoretic and set-theoretic problems. Most of the problems are nondeterministic polynomial time (NP) hard, although pedagogy may have demanded that a few other problems be addressed by the very generalized methods the authors use, to tie a wide range of optimization problems together under one model. An r -approximate algorithm for the solution of an optimization problem is defined as one that yields a solution whose value is within a factor r of the optimal cost ( r will be greater than 1 for minimization problems and less than 1 for maximization problems). Local ratio algorithms apply in the case where a solution can be represented by a vector, and the solution's cost can be represented by the scalar product of the solution vector and a "weight vector" gleaned from the problem. The representation of a problem in this form is where the ingenuity of the designer is required. Many graph problems are classified into the general class of "graph editing" problems. The authors address solutions of partial problems whose weight and solution vectors can be seen as the sum of two partial weight and solution vectors. The local ratio theorem shows that, if the partial solutions are r -approximate, then so is their sum. The authors present a generic r -approximate solution as consisting of the recursion of a three-way "if-then" algorithm, with the three "then" parts of the algorithm called weight decomposition, problem reduction, and optimum solution. The above concepts and their applications are illustrated using many examples. The examples are used to introduce the concepts, and also to illustrate their application in a step-by-step manner. The authors discuss the challenges associated with the application of the model as they arise, and illustrate the way the challenges are met. In my experience, most approximate algorithms are described in the literature as being developed on an ad hoc basis for a specific problem. This survey is a laudable effort toward tying them into a uniform approach, and will be appreciated by disciplined and dedicated readers. Online Computing Reviews Service

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cover image ACM Computing Surveys
ACM Computing Surveys  Volume 36, Issue 4
December 2004
129 pages
ISSN:0360-0300
EISSN:1557-7341
DOI:10.1145/1041680
Issue’s Table of Contents
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Published: 01 December 2004
Published in CSUR Volume 36, Issue 4

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  1. Approximation algorithms
  2. fractional local ratio
  3. local ratio technique

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