P versus NP problem 1

P versus NP problemList of unsolved problems in computer science

If the solution to a problem can be quickly verified by a computer, can the computer also solve that problem quickly?

Millennium PrizeProblems

•• P versus NP problem•• Hodge conjecture• Poincaré conjecture (solved)•• Riemann hypothesis• Yang–Mills existence and mass gap• Navier–Stokes existence and smoothness•• Birch and Swinnerton-Dyer conjecture

•• v•• t• e [1]

Diagram of complexity classes provided that P ≠ NP. The existenceof problems within NP but outside both P and NP-complete, under

that assumption, was established by Ladner's theorem.[2]

The P versus NP problem is a major unsolvedproblem in computer science. Informally, it askswhether every problem whose solution can be quicklyverified by a computer can also be quickly solved by acomputer. It was introduced in 1971 by Stephen Cookin his seminal paper "The complexity of theoremproving procedures" and is considered by many to bethe most important open problem in the field.[3] It isone of the seven Millennium Prize Problems selectedby the Clay Mathematics Institute to carry aUS$1,000,000 prize for the first correct solution.

The informal term quickly used above means theexistence of an algorithm for the task that runs inpolynomial time. The general class of questions for which some algorithm can provide an answer in polynomial timeis called "class P" or just "P". For some questions, there is no known way to find an answer quickly, but if one isprovided with information showing what the answer is, it may be possible to verify the answer quickly. The class ofquestions for which an answer can be verified in polynomial time is called NP.

Consider the subset sum problem, an example of a problem that is easy to verify, but whose answer may be difficultto compute. Given a set of integers, does some nonempty subset of them sum to 0? For instance, does a subset of theset {−2, −3, 15, 14, 7, −10} add up to 0? The answer "yes, because {−2, −3, −10, 15} adds up to zero" can bequickly verified with three additions. However, there is no known algorithm to find such a subset in polynomial time(there is one, however, in exponential time, which consists of 2n-1 tries), and indeed such an algorithm can only existif P = NP; hence this problem is in NP (quickly checkable) but not necessarily in P (quickly solvable).An answer to the P = NP question would determine whether problems that can be verified in polynomial time, likethe subset-sum problem, can also be solved in polynomial time. If it turned out that P ≠ NP, it would mean that thereare problems in NP (such as NP-complete problems) that are harder to compute than to verify: they could not besolved in polynomial time, but the answer could be verified in polynomial time.

P versus NP problem 2

Aside from being an important problem in computational theory, a proof either way would have profoundimplications for mathematics, cryptography, algorithm research, artificial intelligence, game theory, multimediaprocessing, philosophy, economics and many other fields.

ContextThe relation between the complexity classes P and NP is studied in computational complexity theory, the part of thetheory of computation dealing with the resources required during computation to solve a given problem. The mostcommon resources are time (how many steps it takes to solve a problem) and space (how much memory it takes tosolve a problem).In such analysis, a model of the computer for which time must be analyzed is required. Typically such modelsassume that the computer is deterministic (given the computer's present state and any inputs, there is only onepossible action that the computer might take) and sequential (it performs actions one after the other).In this theory, the class P consists of all those decision problems (defined below) that can be solved on adeterministic sequential machine in an amount of time that is polynomial in the size of the input; the class NPconsists of all those decision problems whose positive solutions can be verified in polynomial time given the rightinformation, or equivalently, whose solution can be found in polynomial time on a non-deterministic machine.[4]

Clearly, P ⊆ NP. Arguably the biggest open question in theoretical computer science concerns the relationshipbetween those two classes:

Is P equal to NP?In a 2002 poll of 100 researchers, 61 believed the answer to be no, 9 believed the answer is yes, and 22 were unsure;8 believed the question may be independent of the currently accepted axioms and therefore is impossible to prove ordisprove.

NP-complete

Euler diagram for P, NP, NP-complete, and NP-hard set of problems

To attack the P = NP question the conceptof NP-completeness is very useful.NP-complete problems are a set of problemsto each of which any other NP-problem canbe reduced in polynomial time, and whosesolution may still be verified in polynomialtime. That is, any NP problem can betransformed into any of the NP-completeproblems. Informally, an NP-completeproblem is an NP problem that is at least as"tough" as any other problem in NP.

NP-hard problems are those at least as hardas NP problems, i.e., all NP problems canbe reduced (in polynomial time) to them. NP-hard problems need not be in NP, i.e., they need not have solutionsverifiable in polynomial time.

For instance, the boolean satisfiability problem is NP-complete by the Cook–Levin theorem, so any instance of anyproblem in NP can be transformed mechanically into an instance of the boolean satisfiability problem in polynomialtime. The boolean satisfiability problem is one of many such NP-complete problems. If any NP-complete problem isin P, then it would follow that P = NP. Unfortunately, many important problems have been shown to beNP-complete, and not a single fast algorithm for any of them is known.

P versus NP problem 3

Based on the definition alone it is not obvious that NP-complete problems exist. A trivial and contrivedNP-complete problem can be formulated as: given a description of a Turing machine M guaranteed to halt inpolynomial time, does there exist a polynomial-size input that M will accept? It is in NP because (given an input) itis simple to check whether M accepts the input by simulating M; it is NP-complete because the verifier for anyparticular instance of a problem in NP can be encoded as a polynomial-time machine M that takes the solution to beverified as input. Then the question of whether the instance is a yes or no instance is determined by whether a validinput exists.The first natural problem proven to be NP-complete was the boolean satisfiability problem. As noted above, this isthe Cook–Levin theorem; its proof that satisfiability is NP-complete contains technical details about Turingmachines as they relate to the definition of NP. However, after this problem was proved to be NP-complete, proof byreduction provided a simpler way to show that many other problems are also NP-complete, including the subset-sumproblem discussed earlier. Thus, a vast class of seemingly unrelated problems are all reducible to one another, andare in a sense "the same problem".

Harder problemsAlthough it is unknown whether P = NP, problems outside of P are known. A number of succinct problems(problems that operate not on normal input, but on a computational description of the input) are known to beEXPTIME-complete. Because it can be shown that P ⊊ EXPTIME, these problems are outside P, and so requiremore than polynomial time. In fact, by the time hierarchy theorem, they cannot be solved in significantly less thanexponential time. Examples include finding a perfect strategy for chess (on an N × N board) and some other boardgames.The problem of deciding the truth of a statement in Presburger arithmetic requires even more time. Fischer andRabin proved in 1974 that every algorithm that decides the truth of Presburger statements has a runtime of at least

for some constant c. Here, n is the length of the Presburger statement. Hence, the problem is known to needmore than exponential run time. Even more difficult are the undecidable problems, such as the halting problem. Theycannot be completely solved by any algorithm, in the sense that for any particular algorithm there is at least one inputfor which that algorithm will not produce the right answer; it will either produce the wrong answer, finish withoutgiving a conclusive answer, or otherwise run forever without producing any answer at all.

Problems in NP not known to be in P or NP-completeIt was shown by Ladner that if P ≠ NP then there exist problems in NP that are neither in P nor NP-complete. Suchproblems are called NP-intermediate problems. The graph isomorphism problem, the discrete logarithm problem andthe integer factorization problem are examples of problems believed to be NP-intermediate. They are some of thevery few NP problems not known to be in P or to be NP-complete.The graph isomorphism problem is the computational problem of determining whether two finite graphs areisomorphic. An important unsolved problem in complexity theory is whether the graph isomorphism problem is in P,NP-complete, or NP-intermediate. The answer is not known, but it is believed that the problem is at least notNP-complete. If graph isomorphism is NP-complete, the polynomial time hierarchy collapses to its second level.[5]

Since it is widely believed that the polynomial hierarchy does not collapse to any finite level, it is believed that graphisomorphism is not NP-complete. The best algorithm for this problem, due to Laszlo Babai and Eugene Luks has runtime 2O(√nlog(n)) for graphs with n vertices.The integer factorization problem is the computational problem of determining the prime factorization of a given integer. Phrased as a decision problem, it is the problem of deciding whether the input has a factor less than k. No efficient integer factorization algorithm is known, and this fact forms the basis of several modern cryptographic systems, such as the RSA algorithm. The integer factorization problem is in NP and in co-NP (and even in UP and

P versus NP problem 4

co-UP[6]). If the problem is NP-complete, the polynomial time hierarchy will collapse to its first level (i.e., NP =co-NP). The best known algorithm for integer factorization is the general number field sieve, which takes expectedtime

to factor an n-bit integer. However, the best known quantum algorithm for this problem, Shor's algorithm, does runin polynomial time. Unfortunately, this fact doesn't say much about where the problem lies with respect tonon-quantum complexity classes.

Does P mean "easy"?

The graph shows time (average of 100 instances in ms using a 933 MHz Pentium III)vs.problem size for knapsack problems for a state-of-the-art specialized algorithm.

Quadratic fit suggests that empirical algorithmic complexity for instances with50–10,000 variables is O((log(n))2).[7]

All of the above discussion has assumedthat P means "easy" and "not in P" means"hard", an assumption known asCobham's thesis. It is a common andreasonably accurate assumption incomplexity theory; however, it has somecaveats.

First, it is not always true in practice. Atheoretical polynomial algorithm mayhave extremely large constant factors orexponents thus rendering it impractical.On the other hand, even if a problem isshown to be NP-complete, and even if P≠ NP, there may still be effectiveapproaches to tackling the problem inpractice. There are algorithms for manyNP-complete problems, such as the knapsack problem, the traveling salesman problem and the boolean satisfiabilityproblem, that can solve to optimality many real-world instances in reasonable time. The empirical average-casecomplexity (time vs. problem size) of such algorithms can be surprisingly low. A famous example is the simplexalgorithm in linear programming, which works surprisingly well in practice; despite having exponential worst-casetime complexity it runs on par with the best known polynomial-time algorithms.

Second, there are types of computations which do not conform to the Turing machine model on which P and NP aredefined, such as quantum computation and randomized algorithms.

Reasons to believe P ≠ NPAccording to polls, many computer scientists believe that P ≠ NP. A key reason for this belief is that after decades ofstudying these problems no one has been able to find a polynomial-time algorithm for any of more than 3000important known NP-complete problems (see List of NP-complete problems). These algorithms were sought longbefore the concept of NP-completeness was even defined (Karp's 21 NP-complete problems, among the first found,were all well-known existing problems at the time they were shown to be NP-complete). Furthermore, the result P =NP would imply many other startling results that are currently believed to be false, such as NP = co-NP and P = PH.It is also intuitively argued that the existence of problems that are hard to solve but for which the solutions are easyto verify matches real-world experience.[8]

If P = NP, then the world would be a profoundly different place than we usually assume it to be. There would be no special value in "creative leaps," no fundamental gap between solving a problem and recognizing the

P versus NP problem 5

solution once it's found.— Scott Aaronson, MIT

On the other hand, some researchers believe that there is overconfidence in believing P ≠ NP and that researchersshould explore proofs of P = NP as well. For example, in 2002 these statements were made:

The main argument in favor of P ≠ NP is the total lack of fundamental progress in the area of exhaustivesearch. This is, in my opinion, a very weak argument. The space of algorithms is very large and we are only atthe beginning of its exploration. [...] The resolution of Fermat's Last Theorem also shows that very simplequestions may be settled only by very deep theories.—Moshe Y. Vardi, Rice UniversityBeing attached to a speculation is not a good guide to research planning. One should always try both directionsof every problem. Prejudice has caused famous mathematicians to fail to solve famous problems whosesolution was opposite to their expectations, even though they had developed all the methods required.—Anil Nerode, Cornell University

Consequences of the resolution of the problemOne of the reasons the problem attracts so much attention is the consequences of the answer. Either direction ofresolution would advance theory enormously, and perhaps have huge practical consequences as well.

P = NPA proof that P = NP could have stunning practical consequences, if the proof leads to efficient methods for solvingsome of the important problems in NP. It is also possible that a proof would not lead directly to efficient methods,perhaps if the proof is non-constructive, or the size of the bounding polynomial is too big to be efficient in practice.The consequences, both positive and negative, arise since various NP-complete problems are fundamental in manyfields.Cryptography, for example, relies on certain problems being difficult. A constructive and efficient solution[9] to anNP-complete problem such as 3-SAT would break most existing cryptosystems including:• public-key cryptography,[10] a foundation for many modern security applications such as secure economic

transactions over the Internet; and• symmetric ciphers such as AES or 3DES,[11] used for the encryption of communications data.These would need to be modified or replaced by information-theoretically secure solutions.On the other hand, there are enormous positive consequences that would follow from rendering tractable manycurrently mathematically intractable problems. For instance, many problems in operations research are NP-complete,such as some types of integer programming, and the travelling salesman problem, to name two of the most famousexamples. Efficient solutions to these problems would have enormous implications for logistics. Many otherimportant problems, such as some problems in protein structure prediction, are also NP-complete; if these problemswere efficiently solvable it could spur considerable advances in biology.But such changes may pale in significance compared to the revolution an efficient method for solving NP-completeproblems would cause in mathematics itself. Godel, in his early thoughts on computational complexity, noted that amechanical method that could solve any problem would revolutionize mathematics:[12][13]

If there really were a machine with φ(n) ∼ k ⋅ n (or even ∼ k ⋅ n2), this would have consequences of the greatest importance. Namely, it would obviously mean that in spite of the undecidability of the Entscheidungsproblem, the mental work of a mathematician concerning Yes-or-No questions could be completely replaced by a machine. After all, one would simply have to choose the natural number n so large that when the machine does not deliver a result, it makes no sense to think more about the

P versus NP problem 6

problem.Similarly, Stephen Cook says

...it would transform mathematics by allowing a computer to find a formal proof of any theorem whichhas a proof of a reasonable length, since formal proofs can easily be recognized in polynomial time.Example problems may well include all of the CMI prize problems.

Research mathematicians spend their careers trying to prove theorems, and some proofs have taken decades or evencenturies to find after problems have been stated—for instance, Fermat's Last Theorem took over three centuries toprove. A method that is guaranteed to find proofs to theorems, should one exist of a "reasonable" size, wouldessentially end this struggle.

P ≠ NPA proof that showed that P ≠ NP would lack the practical computational benefits of a proof that P = NP, but wouldnevertheless represent a very significant advance in computational complexity theory and provide guidance forfuture research. It would allow one to show in a formal way that many common problems cannot be solvedefficiently, so that the attention of researchers can be focused on partial solutions or solutions to other problems. Dueto widespread belief in P ≠ NP, much of this focusing of research has already taken place.Also P ≠ NP still leaves open the average-case complexity of hard problems in NP. For example, it is possible thatSAT requires exponential time in the worst case, but that almost all randomly selected instances of it are efficientlysolvable. Russell Impagliazzo has described five hypothetical "worlds" that could result from different possibleresolutions to the average-case complexity question.[14] These range from "Algorithmica", where P = NP andproblems like SAT can be solved efficiently in all instances, to "Cryptomania", where P ≠ NP and generating hardinstances of problems outside P is easy, with three intermediate possibilities reflecting different possibledistributions of difficulty over instances of NP-hard problems. The "world" where P ≠ NP but all problems in NPare tractable in the average case is called "Heuristica" in the paper. A Princeton University workshop in 2009 studiedthe status of the five worlds.[15]

Results about difficulty of proofAlthough the P = NP? problem itself remains open despite a million-dollar prize and a huge amount of dedicatedresearch, efforts to solve the problem have led to several new techniques. In particular, some of the most fruitfulresearch related to the P = NP problem has been in showing that existing proof techniques are not powerful enoughto answer the question, thus suggesting that novel technical approaches are required.As additional evidence for the difficulty of the problem, essentially all known proof techniques in computationalcomplexity theory fall into one of the following classifications, each of which is known to be insufficient to provethat P ≠ NP:

P versus NP problem 7

Classification Definition

Relativizingproofs

Imagine a world where every algorithm is allowed to make queries to some fixed subroutine called an oracle, and the running timeof the oracle is not counted against the running time of the algorithm. Most proofs (especially classical ones) apply uniformly in aworld with oracles regardless of what the oracle does. These proofs are called relativizing. In 1975, Baker, Gill, and Solovayshowed that P = NP with respect to some oracles, while P ≠ NP for other oracles.[16] Since relativizing proofs can only provestatements that are uniformly true with respect to all possible oracles, this showed that relativizing techniques cannot resolve P =NP.

Natural proofs In 1993, Alexander Razborov and Steven Rudich defined a general class of proof techniques for circuit complexity lower bounds,called natural proofs. At the time all previously known circuit lower bounds were natural, and circuit complexity was considered avery promising approach for resolving P = NP. However, Razborov and Rudich showed that, if one-way functions exist, then nonatural proof method can distinguish between P and NP. Although one-way functions have never been formally proven to exist,most mathematicians believe that they do, and a proof or disproof of their existence would be a much stronger statement than thequantification of P relative to NP. Thus it is unlikely that natural proofs alone can resolve P = NP.

Algebrizingproofs

After the Baker-Gill-Solovay result, new non-relativizing proof techniques were successfully used to prove that IP = PSPACE.However, in 2008, Scott Aaronson and Avi Wigderson showed that the main technical tool used in the IP = PSPACE proof,known as arithmetization, was also insufficient to resolve P = NP.

These barriers are another reason why NP-complete problems are useful: if a polynomial-time algorithm can bedemonstrated for an NP-complete problem, this would solve the P = NP problem in a way not excluded by the aboveresults.These barriers have also led some computer scientists to suggest that the P versus NP problem may be independentof standard axiom systems like ZFC (cannot be proved or disproved within them). The interpretation of anindependence result could be that either no polynomial-time algorithm exists for any NP-complete problem, andsuch a proof cannot be constructed in (e.g.) ZFC, or that polynomial-time algorithms for NP-complete problems mayexist, but it's impossible to prove in ZFC that such algorithms are correct. However, if it can be shown, usingtechniques of the sort that are currently known to be applicable, that the problem cannot be decided even with muchweaker assumptions extending the Peano axioms (PA) for integer arithmetic, then there would necessarily existnearly-polynomial-time algorithms for every problem in NP. Therefore, if one believes (as most complexity theoristsdo) that not all problems in NP have efficient algorithms, it would follow that proofs of independence using thosetechniques cannot be possible. Additionally, this result implies that proving independence from PA or ZFC usingcurrently known techniques is no easier than proving the existence of efficient algorithms for all problems in NP.

Claimed solutionsWhile the P versus NP problem is generally considered unsolved, many amateur and some professional researchershave claimed solutions. Woeginger (2010) has a comprehensive list. An August 2010 claim of proof that P ≠ NP, byVinay Deolalikar, researcher at HP Labs, Palo Alto, received heavy Internet and press attention after being initiallydescribed as "seem[ing] to be a relatively serious attempt" by two leading specialists. The proof has been reviewedpublicly by academics,[17] and Neil Immerman, an expert in the field, had pointed out two possibly fatal errors in theproof. As of 15 September 2010, Deolalikar was reported to be working on a detailed expansion of his attemptedproof. However, opinions expressed by several notable theoretical computer scientists indicate that the attemptedproof is neither correct nor a significant advancement in the understanding of the problem.[18]

P versus NP problem 8

Logical characterizationsThe P = NP problem can be restated in terms of expressible certain classes of logical statements, as a result of workin descriptive complexity.Consider all language of finite structures with a fixed signature including a linear order relation. Then, all suchlanguages in P can be expressed in first-order logic with the addition of a suitable least fixed-point combinator.Effectively, this, in combination with the order, allows the definition of recursive functions. As long as the signaturecontains at least one predicate or function in addition to the distinguished order relation, so that the amount of spacetaken to store such finite structures is actually polynomial in the number of elements in the structure, this preciselycharacterizes P.Similarly, NP is the set of languages expressible in existential second-order logic—that is, second-order logicrestricted to exclude universal quantification over relations, functions, and subsets. The languages in the polynomialhierarchy, PH, correspond to all of second-order logic. Thus, the question "is P a proper subset of NP" can bereformulated as "is existential second-order logic able to describe languages (of finite linearly ordered structures withnontrivial signature) that first-order logic with least fixed point cannot?".[19] The word "existential" can even bedropped from the previous characterization, since P = NP if and only if P = PH (as the former would establish thatNP = co-NP, which in turn implies that NP = PH).

Polynomial-time algorithmsNo algorithm for any NP-complete problem is known to run in polynomial time. However, there are algorithms forNP-complete problems with the property that if P = NP, then the algorithm runs in polynomial time (although withenormous constants, making the algorithm impractical). The following algorithm, due to Levin (without anycitation), is such an example below. It correctly accepts the NP-complete language SUBSET-SUM. It runs inpolynomial time if and only if P = NP:

// Algorithm that accepts the NP-complete language SUBSET-SUM.

//

// this is a polynomial-time algorithm if and only if P = NP.

//

// "Polynomial-time" means it returns "yes" in polynomial time when

// the answer should be "yes", and runs forever when it is "no".

//

// Input: S = a finite set of integers

// Output: "yes" if any subset of S adds up to 0.

// Runs forever with no output otherwise.

// Note: "Program number P" is the program obtained by

// writing the integer P in binary, then

// considering that string of bits to be a

// program. Every possible program can be

// generated this way, though most do nothing

// because of syntax errors.

FOR N = 1...∞ FOR P = 1...N

Run program number P for N steps with input S

IF the program outputs a list of distinct integers

AND the integers are all in S

P versus NP problem 9

AND the integers sum to 0

THEN

OUTPUT "yes" and HALT

If, and only if, P = NP, then this is a polynomial-time algorithm accepting an NP-complete language. "Accepting"means it gives "yes" answers in polynomial time, but is allowed to run forever when the answer is "no" (also knownas a semi-algorithm).This algorithm is enormously impractical, even if P = NP. If the shortest program that can solve SUBSET-SUM inpolynomial time is b bits long, the above algorithm will try at least 2b−1 other programs first.

Formal definitions for P and NPConceptually a decision problem is a problem that takes as input some string w over an alphabet Σ, and outputs "yes"or "no". If there is an algorithm (say a Turing machine, or a computer program with unbounded memory) that canproduce the correct answer for any input string of length n in at most cnk steps, where k and c are constantsindependent of the input string, then we say that the problem can be solved in polynomial time and we place it in theclass P. Formally, P is defined as the set of all languages that can be decided by a deterministic polynomial-timeTuring machine. That is,

where

and a deterministic polynomial-time Turing machine is a deterministic Turing machine M that satisfies the followingtwo conditions:1. M halts on all input w and2. there exists such that , where O refers to the big O notation and

NP can be defined similarly using nondeterministic Turing machines (the traditional way). However, a modernapproach to define NP is to use the concept of certificate and verifier. Formally, NP is defined as the set oflanguages over a finite alphabet that have a verifier that runs in polynomial time, where the notion of "verifier" isdefined as follows.Let L be a language over a finite alphabet, Σ.L ∈ NP if, and only if, there exists a binary relation and a positive integer k such that the followingtwo conditions are satisfied:

1. For all , such that (x, y) ∈ R and ; and2. the language over is decidable by a Turing machine in polynomial

time.A Turing machine that decides LR is called a verifier for L and a y such that (x, y) ∈ R is called a certificate ofmembership of x in L.In general, a verifier does not have to be polynomial-time. However, for L to be in NP, there must be a verifier thatruns in polynomial time.

P versus NP problem 10

ExampleLet

Clearly, the question of whether a given x is a composite is equivalent to the question of whether x is a member ofCOMPOSITE. It can be shown that COMPOSITE ∈ NP by verifying that it satisfies the above definition (if weidentify natural numbers with their binary representations).COMPOSITE also happens to be in P.[20]

Formal definition for NP-completenessThere are many equivalent ways of describing NP-completeness.Let L be a language over a finite alphabet Σ.L is NP-complete if, and only if, the following two conditions are satisfied:1. L ∈ NP; and2. any L′ in NP is polynomial-time-reducible to L (written as ), where if, and only if, the

following two conditions are satisfied:1. There exists f : Σ* → Σ* such that for all w in Σ* we have: ; and2. there exists a polynomial-time Turing machine that halts with f(w) on its tape on any input w.

Popular cultureThe film Travelling Salesman, by director Timothy Lanzone, is the story of four mathematicians hired by the USgovernment to solve the P vs. NP problem.In the CBS series Numbers, it was said that Charlie Epps spent the last three months of his mother's life in thegarage, unable to face her, working on P vs NP, misguidedly thinking that he may be able to cure her sickness.In the CBS series Elementary, Season 2, Episode 2, entitled "Solve for X" and originally broadcast October 3, 2013,the search for a solution to the problem and its value to the computer world is used as a motive for a doublehomicide.

References[1] http:/ / en. wikipedia. org/ w/ index. php?title=Template:Millennium_Problems& action=edit[2] R. E. Ladner "On the structure of polynomial time reducibility," Journal of the ACM, 22, pp. 151–171, 1975. Corollary 1.1. ACM site (http:/ /

portal. acm. org/ citation. cfm?id=321877& dl=ACM& coll=& CFID=15151515& CFTOKEN=6184618).[3] Lance Fortnow, The status of the P versus NP problem (http:/ / www. cs. uchicago. edu/ ~fortnow/ papers/ pnp-cacm. pdf), Communications

of the ACM 52 (2009), no. 9, pp. 78–86.[4] Sipser, Michael: Introduction to the Theory of Computation, Second Edition, International Edition, page 270. Thomson Course Technology,

2006. Definition 7.19 and Theorem 7.20.[5] Uwe Schöning, "Graph isomorphism is in the low hierarchy", Proceedings of the 4th Annual Symposium on Theoretical Aspects of Computer

Science, 1987, 114–124; also: Journal of Computer and System Sciences, vol. 37 (1988), 312–323[6] Lance Fortnow. Computational Complexity Blog: Complexity Class of the Week: Factoring (http:/ / weblog. fortnow. com/ 2002/ 09/

complexity-class-of-week-factoring. html). 13 September 2002.[7][7] Pisinger, D. 2003. "Where are the hard knapsack problems?" Technical Report 2003/08, Department of Computer Science, University of

Copenhagen, Copenhagen, Denmark[8][8] , point 9.[9][9] Exactly how efficient a solution must be to pose a threat to cryptography depends on the details. A solution of

UNIQ-math-0-17389af98f4a80be-QINU or better and a reasonable constant term would be catastrophic. On the other hand, a solution that isUNIQ-math-1-17389af98f4a80be-QINU or worse in almost all cases would not pose an immediate practical danger.

P versus NP problem 11

[10][10] See for a reduction of factoring to SAT. A 512 bit factoring problem (8400 MIPS-years when factored) translates to a SAT problem of63,652 variables and 406,860 clauses.

[11][11] See, for example, in which an instance of DES is encoded as a SAT problem with 10336 variables and 61935 clauses. A 3DES probleminstance would be about 3 times this size.

[12][12] History of this letter and its translation from[13] From pages 359–376 of Optimization Stories, M. Grotschel (editor), a special issue of ¨ Documenta Mathematica, published in August 2012

and distributed to attendees at the 21st International Symposium on Mathematical Programming in Berlin.[14] R. Impagliazzo, "A personal view of average-case complexity," (http:/ / cseweb. ucsd. edu/ ~russell/ average. ps) sct, pp.134, 10th Annual

Structure in Complexity Theory Conference (SCT'95), 1995[15] http:/ / intractability. princeton. edu/ blog/ 2009/ 05/ program-for-workshop-on-impagliazzos-worlds/[16] T. P. Baker, J. Gill, R. Solovay. Relativizations of the P =? NP Question. SIAM Journal on Computing, 4(4): 431–442 (1975)[17] Science News, "Crowdsourcing peer review" (http:/ / www. sciencenews. org/ index/ generic/ activity/ view/ id/ 63252/ title/

Crowdsourcing_peer_review)[18] Gödel’s Lost Letter and P=NP, Update on Deolalikar’s Proof that P≠NP (http:/ / rjlipton. wordpress. com/ 2010/ 08/ 10/

update-on-deolalikars-proof-that-p�np/ #comment-4885)[19] Elvira Mayordomo. "P versus NP" (http:/ / www. unizar. es/ acz/ 05Publicaciones/ Monografias/ MonografiasPublicadas/ Monografia26/

057Mayordomo. pdf) Monografías de la Real Academia de Ciencias de Zaragoza 26: 57–68 (2004).[20][20] AKS primality test

Notes

Further reading• Fraenkel, A. S.; Lichtenstein, D. Computing a Perfect Strategy for n*n Chess Requires Time Exponential in N.

(http:/ / www. pubzone. org/ dblp/ conf/ icalp/ FraenkelL81). doi: 10.1007/3-540-10843-2+23 (http:/ / dx. doi.org/ 10. 1007/ 3-540-10843-2+ 23).

• Garey, Michael; Johnson, David (1979). Computers and Intractability: A Guide to the Theory ofNP-Completeness. San Francisco: W. H. Freeman and Company. ISBN 0-7167-1045-5.

• Goldreich, Oded (2010). P, Np, and Np-Completeness. Cambridge: Cambridge University Press.ISBN 978-0-521-12254-2. Online drafts (http:/ / www. wisdom. weizmann. ac. il/ ~oded/ bc-drafts. html)

• Immerman, N. (1987). "Languages which capture complexity classes" (http:/ / citeseerx. ist. psu. edu/ viewdoc/summary?doi=10. 1. 1. 50. 4936). SIAM Journal of Computing 16 (4): 760–778. doi: 10.1137/0216051 (http:/ /dx. doi. org/ 10. 1137/ 0216051).

• Cormen, Thomas (2001). Introduction to Algorithms. Cambridge: MIT Press. ISBN 0-262-03293-7.• Papadimitriou, Christos (1994). Computational Complexity. Boston: Addison-Wesley. ISBN 0-201-53082-1.• Fortnow, L. (2009). "The Status of the P versus NP problem" (http:/ / cacm. acm. org/ magazines/ 2009/ 9/

38904-the-status-of-the-p-versus-np-problem/ fulltext). Communications of the ACM 52 (9): 78. doi:10.1145/1562164.1562186 (http:/ / dx. doi. org/ 10. 1145/ 1562164. 1562186).

• Fortnow, L.; Gasarch, W. "Computational complexity" (http:/ / weblog. fortnow. com).• Fortnow, Lance. The Golden Ticket: P, NP, and the Search for the Impossible (http:/ / press. princeton. edu/ titles/

9937. html) ISBN 9780691156491. Princeton University Press. Princeton, NJ (2013)

P versus NP problem 12

External links• The Clay Mathematics Institute Millennium Prize Problems (http:/ / www. claymath. org/ millennium/ )• The Clay Math Institute Official Problem Description (http:/ / www. claymath. org/ millennium/ P_vs_NP/

Official_Problem_Description. pdf) PDF (118 KB)• Gerhard J. Woeginger. The P-versus-NP page (http:/ / www. win. tue. nl/ ~gwoegi/ P-versus-NP. htm). A list of

links to a number of purported solutions to the problem. Some of these links state that P equals NP, some of themstate the opposite. It is probable that all these alleged solutions are incorrect.

• Complexity Zoo: Class P (http:/ / complexity-zoo. net/ Complexity_Zoo:P#p), Complexity Zoo: Class NP (http:/ /complexity-zoo. net/ Complexity_Zoo:N#np)

• Scott Aaronson 's Shtetl Optimized blog: Reasons to believe (http:/ / scottaaronson. com/ blog/ ?p=122), a list ofjustifications for the belief that P ≠ NP

Article Sources and Contributors 13

Article Sources and ContributorsP versus NP problem  Source: http://en.wikipedia.org/w/index.php?oldid=599282735  Contributors: 128.138.87.xxx, 194.117.133.xxx, 4Jays1034, 62.202.117.xxx, A bit iffy, Action JacksonIV, Adam McMaster, Adityad, Ahmadiesa abu, Alexwatson, Algebraist, Algebran, Alksentrs, Altenmann, Andejons, Andreas Kaufmann, AndrewWTaylor, Andris, Anog, Anonrabbi, AnonymousDissident, Anonymousacademic, Anonymousone2, Archibald Fitzchesterfield, Arichnad, Arthur Frayn, Arthur Rubin, Artinmm, Arvindn, Aseld, Asmeurer, AstroNomer, AxelBoldt, Azimuth1,Banus, Bdesham, Beb0s, Bender235, Bender2k14, BethNaught, Bevo74, Bgwhite, Bigtimepeace, Bihco, Bkell, Blamelooseradiolabel, Blazotron, Blokhead, Blotwell, Bluerasberry, Bluewaves,Bluhd, Bofa, Booyabazooka, Brianjd, Brighterorange, Budalla, C S, CALR, CBKAtTopsails, CBM, CRGreathouse, Calculuslover, Calvin 1998, Canon, Cf. Hay, ChandraKarChandra, CharlesMatthews, Chenopodiaceous, Chinju, Chocolateboy, Chrisjwmartin, Civil Engineer III, Ckplato, CloudNine, Cngoulimis, Cointyro, Cole Kitchen, Conversion script, Creidieki, Cybercobra, Cyde,DGG, DMacks, Damian Yerrick, DanielMayer, Dantheox, Daonguyen95, Dark Charles, David Eppstein, David Gerard, David.Monniaux, Davidhorman, Dayewalker, Dcoetzee, Deadcode,Debresser, Dekart, Delmet, Denisarona, Dereckson, Deret1987, Dillon256, Discospinster, Dissident, Djhulme, Dlakavi, DocWatson42, Docu, Donarreiskoffer, Doradus, Doulos Christos,Download, Dragon guy, DragonLord, Dspart, Duncan, Duncancumming, ELLinng, ESkog, Ebehn, Ebyabe, Eescardo, Eggishorn, Elizabeyth, EmersonLowry, EmilJ, Emurphy42, Enyokoyama,Epbr123, Epicgenius, Eric119, Evanh2008, Ewa5050, Fappah, Farzaneh, Favonian, Fcady2007, Feinoha, Feynman81, Fiachra10003, Frazzydee, Fred2013, Fredrik, Frungi, Gakrivas, Gandalf61,Gary, Gavia immer, Gdr, Giftlite, Glaisher, Gonzolito, GraemeL, GraemeMcRae, Graham87, Grimguard, GromXXVII, Gtcostello, Guoguo12, Gutworth, Gyllstromk, Hairy Dude, Hcsradek,Headbomb, Hideyuki, Hobophobe, Hofingerandi, Hritcu, Humanengr, Husky, Iain.dalton, Iamfaster, Ianhowlett, Icairns, Ihope127, Illuminatedwax, Iman.saleh, Intgr, InverseHypercube,IvanAndreevich, Ixfd64, JRSpriggs, Jacob grace, Jammycakes, Jan Hidders, Jayanta Sen, Jaybuffington, Jayme, Jeff G., Jeff3000, Jeremy W Powell, Jiayq, Jim1138, Jitse Niesen, Jlind0, Jmah,Jman0726, John Vandenberg, Jon Awbrey, Jonnty, Joshua Issac, Jossi, Jtwdog, Julesd, Justin Stafford, Justin W Smith, Kayau, Kelapstick, Kirill Borisenko, Kmote, Kolbasz, Kotniski, Kralizec!,Kufat, Kulvko, Kvikram, Kzzl, L Kensington, LC, LJosil, LOL, Lambiam, Landroo, Laurusnobilis, LeoNomis, Levelmartinmusatov, Libertyrights, Linket, Liron00, Logan, Lotu, LouScheffer,Lowellian, Luckas Blade, Lukeh1, MATThematical, MER-C, MSGJ, Magioladitis, Marknau, Markvs88, MartinMusatov, Materialscientist, Mathiastck, MatthewH, Max Longint, McGeddon,Mcsee, MelbourneStar, Mellum, Mentifisto, Mgiganteus1, Mgraham831, Michael Hardy, Mikaey, Mike Schwartz, Mindmatrix, Minesweeper, Minime12358, Minna Sora no Shita, Miym,Mpeisenbr, Mrmagoo2006, Ms2ger, Mshebanow, Msikma, Nageh, Najoj, Narsil, Navigatr85, Nbhatla, Nealmcb, Neoglez, Neutrality, Nicolaennio, Night Gyr, Nikkimaria, Nith Sahor, Noamz,NormDor, NuclearWarfare, O.Koslowski, Obradovic Goran, Occamster, Oddity-, Ohconfucius, Olivier, Omnipaedista, Osias, Otisjimmy1, Paddu, ParotWise, Patelhiren.101, Pcap, Perey,Peterjoel, Petr Matas, Phil Boswell, Phil Sandifer, Pichpich, Pinethicket, Pmadrid, PoolGuy, Poor Yorick, Predawn, Prochron, Prodego, Prosfilaes, Pxtreme75,QuantitativeFinanceKinderChocolate, Qwertyus, R.e.b., R6144, RTC, Raiden09, Rajpaj, Raven4x4x, Rbarreira, Rdnk, Remy B, Revent, Rhinestone K, Rhythm, Rich Farmbrough, Rick Block,Rjlipton, Rjwilmsi, Robbar, Robert K S, Robert Merkel, Robert The Rebuilder, RobinK, Robinh, Rockiesfan19, RomeW, Rorro, RoySmith, Rs561, Rspeer, Rspence1234, Rtc, Rupert Clayton,Ruud Koot, Ryan Postlethwaite, Sabb0ur, Salgueiro, ScWizard, Schneelocke, Scholarlyworks, Shawn comes, Shreevatsa, Sirreallysam, Sith Lord 13, Slawekb, Smj2118, Solomon7968,Speculatrix, Srinivasasha, Staecker, Stan Marian C-tin, Stardust8212, Staszek Lem, Stephen B Streater, Stfg, Stifle, Stopflingingthebull, Stuart P. Bentley, Subtilior, Sundar, Suruena, Svnsvn,Tbhotch, Tcotco, Telekid, Template namespace initialisation script, Teorth, Tetraedycal, The Obfuscator, The Thing That Should Not Be, Theatre gurl, Thehotelambush, Thomas Hard, ThorbjørnRavn Andersen, Thore, Tide rolls, Timc, Timrem, Timwi, Tizio, Tobias Bergemann, Tommy2010, Tong chuang, Trovatore, Ttonyb1, Twin Bird, Twri, UkPaolo, Ultimus, Unflavoured, Ustadny,Vadim Makarov, VernoWhitney, Versus22, Vivekk, Waldir, Washington Irving Esquire, WavePart, Wavelength, Webclient101, Wikiklrsc, Wikinick, Wikipediatrist, Wmahan, WookieInHeat,Wtt, Wtuvell, X7q, XVirus, Xiaodai, Yellowdesk, Ylloh, ^demon, 896 ,ماني anonymous edits

Image Sources, Licenses and ContributorsFile:Complexity classes.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Complexity_classes.svg  License: Public Domain  Contributors: Booyabazooka, Fæ, Mdd, Mike1024,NeverDoING, 1 anonymous editsFile:P np np-complete np-hard.svg  Source: http://en.wikipedia.org/w/index.php?title=File:P_np_np-complete_np-hard.svg  License: Creative Commons Attribution-Sharealike 3.0,2.5,2.0,1.0 Contributors: Behnam EsfahbodFile:KnapsackEmpComplexity.GIF  Source: http://en.wikipedia.org/w/index.php?title=File:KnapsackEmpComplexity.GIF  License: Public Domain  Contributors: Cngoulimis

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