## [Computational Complexity] Favorite Theorems: NP-Completeness

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• March Edition This month we honor the papers that gave us the first NP-complete problems and marked the official beginning of the P versus NP question. The P
Message 1 of 1 , Apr 23, 2005
March Edition

This month we honor the papers that gave us the first NP-complete problems and marked the official beginning of the P versus NP question. The P and NP notation did not originate with these papers but I will use them anyway for clarity.

Steve Cook, The Complexity of Theorem-Proving Procedures, STOC 1971.

Suppose a nondeterministic Turing machine M accepts a set S of strings within time Q(n), where Q(n) is a polynomial. Given an input w for M, we will construct a propositional formula A(w) in conjunctive normal form such that A(w) is satisfiable iff M accepts.
In this paper Cook gives the first formal treatment of the P versus NP problem. He gives several examples of NP problems and notes his notion of NP is equivalent to a class of extended positive rudimentary relations due to James Bennett. He introduces P-reducibility (now often called Cook reductions) where one reduces a problem A to a problem B by solving A using a polynomial-time machine with access to an oracle for B. His main theorems show that Tautology and Subgraph Isomorphism are hard for NP under P-reducibility. The quote above came from the beginning of the proof for Tautology.
The theorems suggest that it is fruitless to search for a polynomial decision procedure for the subgraph problem, since success would bring polynomial decision procedures to many other apparently intractable problems…The theorems suggest that Tautology is a good candidate for a set not in P, and I feel it is worth spending considerable effort trying to prove this conjecture. Such a proof would be a major breakthrough in complexity theory.
Indeed.

Leonid Levin, Universal'nyie Perebornyie Zadachi (Universal Search Problems), Problemy Peredachi Informatsii 1973. Translation in appendix of the Trakhtenbrot survey.

If we assume that there exists some (even if artificially formulated) problem of the search type that is unsolvable by simple (in terms of volume of computation) algorithms, then it can be shown that many "classical" search problems have the same property.
Levin claims that any NP problem reduces to any of a list of problems including tautology, subgraph isomorphism, tiling, set cover and a few others. As was the Russian tradition of the times, Levin's paper does not have fully formal definitions or any proofs. Still he has similar results and the same insight as Cook that one can reduce any NP problem to specific natural problems.
All of these problems are solved by trivial algorithms entailing the sequential scanning of all possibilities. The operating time of the algorithms, however, is exponential, and mathematicians nurture the conviction that it is impossible to find simpler algorithms…but not one has yet succeeded in proving it.
In today's world results proven today get transmitted around the world in minutes. But given the technological and even more important the political situation of the 70's, Cook and Levin did not learn of each other's work until years later. Today we give them both credit for taking the P versus NP problem out of the abstract and connecting it to solving natural problems. Now only three decades later the P versus NP problem has become one of the great open questions in all of mathematics.

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Posted by Lance to Computational Complexity at 4/23/2005 12:09:00 PM

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