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Unformatted text preview: CSC 30155 Polynomial Time Reductions Dr. Ka Lok Man On Behalf of Dr. Daniel Hughes daniel.hughes@xjtlu.edu.cn Polynomialtime Reducibility l Another important idea in the theory of NPcompleteness is the idea of transforming one problem instance into another problem instance. We formalize this idea below. l We say that a problem L , defining some decision problem, is polynomialtime reducible to a problem M , if: there is a function f , computable in polynomial time, that takes an instance s of the problem L , and transforms it into an instance f(s) of the problem M such that s has answer yes if and only if f(s) has answer yes. l In other words, we can transform an input for one decision problem into an appropriate input for another decision problem . Furthermore, the first problem has a yes solution if and only if the second decision problem has a yes solution. l We use notation L poly M to signify that problem L is polynomial time reducible to problem M . NPhardness l We say that a problem M , defining some decision problem, is NPhard if every other problem L in NP is polynomialtime reducible to M . So this means that: l M is NPhard if for every L NP , L poly M . l If a language M is NPhard and it belongs to NP itself, then M is NPcomplete . l NPcomplete problems are some of the hardest problems in NP, as far as polynomial reducibility is concerned. Example: CookLevin Theorem l The circuit sat problem tests a given Boolean circuit for satisfiability, accepting it only if it is satisfiable. l The CookLevin Theorem states that CircuitSAT is NPcomplete. l Proof idea: The computation steps of any (reasonable) algorithm can be simulated by layers in appropriately constructed (polynomial time and size) Boolean Circuit. Other NPcomplete problems l We have just noted that there is at least one NP complete problem. l Using polynomial time reducibility we can show existence of other NPcomplete problems. l The following useful result also helps to prove NP completeness in many cases: Other NPcomplete problems l Suppose that we have a new problem X and we think that X is NPcomplete . How can we do this? l First, show that X NP , i.e. show that X has a polynomialtime nondeterministic algorithm, or equivalently, show that X has an efficient certifier. l Secondly, take a known NPcomplete problem Y , and demonstrate a polynomial time reduction from Y to X , i.e. show that Y poly X . Types of Reduction l Let M be an NPcomplete problem. There are three types of reduction: l Restriction: Noting that a known NPcomplete problem is a special case of our problem L. l Local replacement: Dividing instances of M and L into basic units, and then showing how each basic unit of M can be mapped to a basic unit of L....
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 Spring '11
 GaryLi
 Databases

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