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A reduction is the transformation of one problem into another problem. A example of using a reduction would be to be to show if a problem P is undecidable. This would be achieved by transforming or performing a reduction of a decision problem $P$ into an undecidable problem. If this can be achieved then we have shown that this problem P is undecidable.

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Nontrivial membership in NP

Integer Programming. Showing that if there is an integer solution then there is a polynomial size integer solution is quite involved. See Christos Papadimitriou, "On the Complexity of Integer Prog …
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15 votes
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Does NP-completeness/hardness have to be constructive?

Yes, there are such sets, take any $\mathsf{NP}$-intermediate set (any set that is provably $\mathsf{NP}$-intermediate assuming $\mathsf{P}\neq\mathsf{NP}$), e.g. construct one from SAT using Ladner's …
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10 votes

Advanced techniques for determining complexity lower bounds

The techniques depend on the model and the type of resource we want to get a lower bound on. Note that to prove a lower bound on the complexity of a problem we have to first fix a mathematical model o …
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9 votes

Should reductions make us more or less optimistic for the tractability of a problem?

However, although they are technically reductions we often don't refer to these as such. What we mean by a reduction is usually a reduction to some (NP-)hard problem. … If you accept these conjectures then it is completely reasonable to look at reductions proving universality of a problem for NP as the problem being hard. …
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