PCP and Hardness of Approximation

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چکیده

A decision problem or a language L is one where all strings x are partitioned into those that are good inputs and the bad ones. In general, we are really interested in optimization problems, where for a problem A we consider all potential solutions w, and every input x determines a subset of all possible solutions that are good for x. Furthermore, every input x imposes a ranking among all good solutions according to the optimized parameter ρA(x,w); and our problem A calls for the best (either minimal or maximal) solution according to the optimized parameter ρA. An optimization algorithm MA returns a best solution. Denote by ρA(x) the optimal solution, namely the value ρA(x,w) for the best w. An approximation algorithm MA for a given optimization problem A comes up with a solution w that is not necessarily best for x, nevertheless, deviates from the optimal by a known approximation ratio c. Namely, say A is a maximization problem, then the outcome of MA is a good solution for x satisfying: ρA(x,MA(x)) ρA(x) > c

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تاریخ انتشار 2009