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| A '''primality test''' is an [[algorithm]] for determining whether an input number is [[prime number|prime]]. Amongst other fields of [[mathematics]], it is used for [[cryptography]]. Unlike [[integer factorization]], primality tests do not generally give [[prime factor]]s, only stating whether the input number is prime or not. Factorization is thought to be a computationally difficult problem, whereas primality testing is comparatively easy (its [[Run-time complexity|running time]] is [[Polynomial time|polynomial]] in the size of the input). Some primality tests ''prove'' that a number is prime, while others like [[Miller–Rabin primality test|Miller–Rabin]] prove that a number is [[composite number|composite]]. Therefore the latter might be called ''compositeness tests'' instead of primality tests.
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| ==Naive methods==
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| {{unreferenced section|date=August 2013}}
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| The simplest primality test is ''[[trial division]]'': Given an input number ''n'',
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| check whether any integer ''m'' from 2 to ''n'' − 1 evenly [[divisibility|divides]] ''n'' (the division leaves no [[remainder]]). If ''n'' is divisible by any ''m'' then ''n'' is [[Composite_number|composite]], otherwise it is [[Prime_number|prime]].
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| For example, to test whether 17 is prime, test whether 17 is divisible by 2, or 3, or 4, 5, 6, ..., 16. Since a prime is only divisible by 1 and itself, if we reach 16 without finding a [[divisor]], then we have proven that 17 is prime. However, we don't actually have to check all numbers up to ''n''. Let's look at another example: all the divisors of 100: | |
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| :2, 4, 5, 10, 20, 25, 50
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| here we see that the largest factor is 100/2 = 50. This is true for all ''n'': all divisors are less than or equal to ''n''/2. We can do better though. If we take a closer look at the divisors, we will see that some of them are redundant. If we write the list differently:
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| :100 = 2 × 50 = 4 × 25 = 5 × 20 = 10 × 10 = 20 × 5 = 25 × 4 = 50 × 2
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| it becomes obvious. Once we reach 10, which is <math>\scriptstyle\sqrt {100}</math>, the divisors just flip around and repeat. Therefore we can further eliminate testing divisors greater than <math>\scriptstyle\sqrt n</math>. We can also eliminate all the even numbers greater than 2, since if an even number can divide ''n'', so can 2.
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| The algorithm can be improved further by observing that all primes are of the form 6''k'' ± 1, with the exception of 2 and 3. This is because all integers can be expressed as (6''k'' + ''i'') for some integer ''k'' and for ''i'' = −1, 0, 1, 2, 3, or 4; 2 divides (6''k'' + 0), (6''k'' + 2), (6''k'' + 4); and 3 divides (6''k'' + 3). So a more efficient method is to test if ''n'' is divisible by 2 or 3, then to check through all the numbers of form 6''k'' ± 1 <math>\scriptstyle{}\leq\sqrt n</math>. This is 3 times as fast as testing all ''m''.
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| Generalising further, it can be seen that all primes are of the form ''c''#''k'' + ''i'' for ''i'' < ''c''# where ''i'' represents the numbers that are [[coprime]] to [[primorial|c#]] and where ''c'' and ''k'' are integers. For example, let ''c'' = 6. Then ''c''# = 2 <math>\cdot</math> 3 <math>\cdot</math> 5 = 30. All integers are of the form 30''k'' + ''i'' for ''i'' = 0, 1, 2,...,29 and ''k'' an integer. However, 2 divides 0, 2, 4,...,28 and 3 divides 0, 3, 6,...,27 and 5 divides 0, 5, 10,...,25. So all prime numbers are of the form 30''k'' + ''i'' for ''i'' = 1, 7, 11, 13, 17, 19, 23, 29 (i.e. for ''i'' < 30 such that [[greatest common divisor|gcd]](''i'',30) = 1). Note that if ''i'' and 30 are not coprime, then 30''k'' + ''i'' is divisible by a prime divisor of 30, namely 2, 3 or 5, and is therefore not prime.
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| As ''c'' → ∞, the number of values that ''c''#''k'' + ''i'' can take over a certain range decreases, and so the time to test ''n'' decreases. For this method, it is also necessary to check for divisibility by all primes that are less than ''c''. Observations analogous to the preceding can be applied [[recursion|recursively]], giving the [[Sieve of Eratosthenes]].
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| A good way to speed up these methods (and all the others mentioned below) is to pre-compute and store a list of all primes up to a certain bound, say all primes up to 200. (Such a list can be computed with the [[Sieve of Eratosthenes]] or by an algorithm that tests each incremental ''m'' against all known primes < <math>\scriptstyle\sqrt{m}</math>). Then, before testing ''n'' for primality with a serious method, ''n'' can first be checked for divisibility by any prime from the list. If it is divisible by any of those numbers then it is composite, and any further tests can be skipped.
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| A simple, but very inefficient primality test uses [[Wilson's theorem]], which states that ''p'' is prime if and only if:
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| :<math>(p-1)! \equiv -1\pmod p \,</math>
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| Although this method requires about ''p'' modular multiplications, rendering it impractical, theorems about primes and modular residues form the basis of many more practical methods.
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| === Python implementation ===
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| <syntaxhighlight lang="python">
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| def is_prime(n):
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| if n < 2:
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| return False
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| if n in (2, 3):
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| return True
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| if n % 2 == 0 or n % 3 == 0:
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| return False
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| max_divisor = int(n ** 0.5) # square root of n
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| divisor = 5
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| while divisor <= max_divisor:
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| if n % divisor == 0 or n % (divisor + 2) == 0:
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| return False
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| divisor += 6
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| return True
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| </syntaxhighlight>
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| ==Probabilistic tests==
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| Most popular primality tests are [[randomized algorithm|probabilistic tests]]. These tests use, apart from the tested number ''n'', some other numbers ''a'' which are chosen at random from some [[sample space]]; the usual randomized primality tests never report a prime number as composite, but it is possible for a composite number to be reported as prime. The probability of error can be reduced by repeating the test with several independently chosen values of ''a''; for two commonly used tests, for ''any'' composite ''n'' at least half the ''a''{{'}}s detect ''n''{{'}}s compositeness, so ''k'' repetitions reduce the error probability to at most 2<sup>−''k''</sup>, which can be made arbitrarily small by increasing ''k''.
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| The basic structure of randomized primality tests is as follows:
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| #Randomly pick a number ''a''.
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| #Check some equality (corresponding to the chosen test) involving ''a'' and the given number ''n''. If the equality fails to hold true, then ''n'' is a composite number, ''a'' is known as a ''witness'' for the compositeness, and the test stops.
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| #Repeat from step 1 until the required accuracy is achieved.
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| After one or more iterations, if ''n'' is not found to be a composite number, then it can be declared [[Probable prime|probably prime]].
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| The simplest probabilistic primality test is the [[Fermat primality test]] (actually a compositeness test). It works as follows:
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| :Given an integer ''n'', choose some integer ''a'' coprime to ''n'' and calculate ''a<sup>n</sup>''<sup> − 1</sup> [[modular arithmetic|modulo]] ''n''. If the result is different from 1, then ''n'' is composite. If it is 1, then ''n'' may or may not be prime.
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| If ''a<sup>n</sup>''<sup> - 1</sup> (modulo ''n'') is 1 but ''n'' is not prime, then ''n'' is called a
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| [[pseudoprime]] to base ''a''. In practice, we observe that, if
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| ''a<sup>n</sup>''<sup> - 1</sup> (modulo ''n'')
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| is 1, then ''n'' is usually prime. But here is a counterexample:
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| if ''n'' = 341 and ''a'' = 2, then
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| : <math>2^{340} \equiv 1\pmod{341}</math>
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| even though 341 = 11·31 is composite. In fact, 341 is the smallest pseudoprime base 2 (see Figure 1 of
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| <ref name="PSW">{{cite journal|coauthors=[[John L. Selfridge]], [[Samuel S. Wagstaff, Jr.]]|title=The pseudoprimes to 25·10<sup>9</sup>|journal=Mathematics of Computation|date=July 1980|volume=35|issue=151|pages=1003–1026|url=http://www.math.dartmouth.edu/~carlp/PDF/paper25.pdf|author = [[Carl Pomerance]]| doi=10.1090/S0025-5718-1980-0572872-7 }}</ref>). | |
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| There are only 21853 pseudoprimes base 2 that are less than 25·10<sup>9</sup> (see page 1005 of <ref name="PSW"/>). This means that, for ''n'' up to 25·10<sup>9</sup>, if ''2<sup>n</sup>''<sup> - 1</sup> (modulo ''n'') equals 1, then ''n'' is prime, unless ''n'' is one of these 21853 pseudoprimes.
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| The Fermat primality test is only a heuristic test; some composite numbers ([[Carmichael number]]s) will be declared "probably prime" no matter what witness is chosen. Nevertheless, it is often used if a rapid screening of numbers is needed, for instance in the key generation phase of the [[RSA (algorithm)|RSA public key cryptographic algorithm]]. | |
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| The [[Miller–Rabin primality test]] and [[Solovay–Strassen primality test]] are more sophisticated variants which detect all composites (once again, this means: for ''every'' composite number ''n'', at least 3/4 (Miller–Rabin) or 1/2 (Solovay–Strassen) of numbers ''a'' are witnesses of compositeness of ''n''). These are also compositeness tests.
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| The Miller–Rabin primality test works as follows:
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| Given an integer ''n'', choose some integer ''a'' < ''n''. Let 2<sup>''s''</sup>''d'' = ''n'' − 1 where ''d'' is odd. If
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| :<math>
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| a^{d} \not\equiv 1\pmod{n}
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| </math>
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| and
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| :<math>
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| a^{2^rd} \not\equiv -1\pmod{n}</math> for all <math>0 \le r \le s-1
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| </math>
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| then ''n'' is composite and ''a'' is a witness for the compositeness. Otherwise, ''n'' may or may not be prime.
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| The Miller-Rabin test is a [[strong pseudoprime]] test (see,<ref name="PSW"/> page 1004).
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| The Solovay–Strassen primality test uses another equality: Given an odd number ''n'', choose some integer ''a'' < ''n'', if
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| :<math> a^{(n-1)/2} \not\equiv \left(\frac{a}{n}\right) \pmod n</math>, where <math>\left(\frac{a}{n}\right)</math> is the [[Jacobi symbol]],
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| then ''n'' is composite and ''a'' is a witness for the compositeness. Otherwise, ''n'' may or may not be prime.
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| The Solovay-Strassen test is an [[Euler pseudoprime]] test (see,<ref name="PSW"/> page 1003).
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| For each individual value of ''a'', the Solovay-Strassen test is weaker than the Miller-Rabin test. For example, if ''n'' = 1905 and ''a'' = 2, then the Miller-Rabin test
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| shows that ''n'' is composite, but the Solovay-Strassen test does not. This is because 1905 is an Euler
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| pseudoprime base 2 but not a strong pseudoprime base 2; this is illustrated in Figure 1 of.<ref name="PSW"/>
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| These two primality tests are simple and are much faster than other general primality tests. One method of improving efficiency further in some cases is the [[Frobenius pseudoprime|Frobenius pseudoprimality test]]; a round of this test takes about three times as long as a round of Miller–Rabin, but achieves a probability bound comparable to seven rounds of Miller–Rabin.
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| The Frobenius test is a generalization of the [[Lucas pseudoprime]] test. One can also combine a Miller-Rabin type test with a Lucas pseudoprime test to get a primality test that has no known counterexamples. That is, this combined test has no known composite ''n'' for which the test reports that ''n'' is probably prime. One such test is the [[Baillie-PSW primality test]], several variations of which are described in.<ref name="lpsp">{{cite journal|coauthors=[[Samuel S. Wagstaff, Jr.]]|title=Lucas Pseudoprimes|journal=Mathematics of Computation|date=October 1980|volume=35|issue=152|pages=1391–1417|url=http://mpqs.free.fr/LucasPseudoprimes.pdf|author=Robert Baillie| mr=583518| doi=10.1090/S0025-5718-1980-0583518-6 }}</ref>
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| [[Leonard Adleman]] and Ming-Deh Huang presented an errorless (but expected polynomial-time) variant of the [[Elliptic curve primality proving|elliptic curve primality test]]. Unlike the other probabilistic tests, this algorithm produces a [[primality certificate]], and thus can be used to prove that a number is prime.<ref name=AH92>{{cite book | first1=Leonard M. | last1=Adleman | author1-link=Leonard Adleman | first2=Ming-Deh | last2=Huang | title=Primality testing and Abelian varieties over finite field | series=Lecture notes in mathematics | volume=1512 | year=1992 | isbn=3-540-55308-8 | publisher=[[Springer-Verlag]] }}</ref> The algorithm is prohibitively slow in practice.
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| If [[quantum computer]]s were available, primality could be tested [[Big O notation|asymptotically faster]] than by using classical computers. A combination of [[Shor's algorithm]], an integer factorization method, with the [[Pocklington primality test]] could solve the problem in <math>O(\log^3 n\log\log n \log\log\log n)</math>.<ref>{{cite arXiv |eprint=quant-ph/9508005 |last1=Chau |first1=H. F. |last2=Lo |first2=H. -K. |title=Primality Test Via Quantum Factorization |class=quant-ph |year=1995 }}</ref> | |
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| ==Fast deterministic tests==
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| Near the beginning of the 20th century, it was shown that a result of [[Fermat's little theorem]] could be used to test for primality. This resulted in the [[Pocklington primality test]].<ref>{{MathWorld |urlname=PocklingtonsTheorem |title=Pocklington's Theorem}}</ref> However, as this test requires a partial factorization of ''n'' − 1 the running time was still quite slow in the worst case. The first [[deterministic algorithm|deterministic]] primality test significantly faster than the naive methods was the [[Adleman–Pomerance–Rumely primality test|cyclotomy test]]; its runtime can be proven to be [[Big O notation|O]]((log ''n'')<sup>''c'' log log log ''n''</sup>), where ''n'' is the number to test for primality and ''c'' is a constant independent of ''n''. Many further improvements were made, but none could be proven to have polynomial running time. (Note that running time is measured in terms of the size of the input, which in this case is ~ log ''n'', that being the number of bits needed to represent the number ''n''.) The [[Elliptic curve primality proving|elliptic curve primality test]] can be proven to run in O((log ''n'')<sup>6</sup>), but only if some still unproven (but widely assumed to be true) statements of [[analytic number theory]] are used{{Which|date=April 2010}}. Similarly, under the [[generalized Riemann hypothesis]], the Miller–Rabin test can be turned into a deterministic version (called [[Miller–Rabin primality test#Deterministic variants of the test|Miller's test]]) with runtime [[big O notation#Extensions to the Bachmann–Landau notations|Õ]]((log ''n'')<sup>4</sup>).<ref>{{cite journal |doi=10.1016/S0022-0000(76)80043-8 |author=[[Gary L. Miller (mathematician)|Gary L. Miller]] |title=Riemann's Hypothesis and Tests for Primality |journal=[[Journal of Computer and System Sciences]] |volume=13 |issue=3 |pages=300–317 |year=1976}}</ref> In practice, this algorithm is slower than the other two for sizes of numbers that can be dealt with at all. Because the implementation of these methods is rather difficult and creates a risk of programming errors, the slower but simpler tests are often preferred.
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| In 2002 the first provably polynomial time test for primality was invented by [[Manindra Agrawal]], [[Neeraj Kayal]] and [[Nitin Saxena]]. The [[AKS primality test]], runs in Õ((log ''n'')<sup>12</sup>) (improved to Õ((log ''n'')<sup>7.5</sup>) in the published revision of their paper), which can be further reduced to Õ((log ''n'')<sup>6</sup>) if the [[Sophie Germain prime|Sophie Germain conjecture]] is true.<ref name="AKS">Manindra Agrawal, Neeraj Kayal, Nitin Saxena, "[http://www.cse.iitk.ac.in/users/manindra/algebra/primality_v6.pdf PRIMES is in P]", ''Annals of Mathematics'' 160 (2004), no. 2, pp. 781–793.</ref> Subsequently, Lenstra and Pomerance presented a version of the test which runs in time Õ((log ''n'')<sup>6</sup>) unconditionally.<ref>Carl Pomerance and Hendrik W. Lenstra. [http://www.math.dartmouth.edu/~carlp/PDF/complexity12.pdf Primality testing with Gaussian periods]</ref>
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| ==Complexity==
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| In [[computational complexity theory]], the formal language corresponding to the prime numbers is denoted as PRIMES. It is easy to show that PRIMES is in '''[[Co-NP]]''': its complement COMPOSITES is in '''NP''' because one can decide compositeness by nondeterministically guessing a factor.
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| In 1975, [[Vaughan Pratt]] showed that there existed a certificate for primality that was checkable in polynomial time, and thus that PRIMES was in [[NP (complexity)|NP]], and therefore in '''NP ∩ coNP'''. See [[primality certificate]] for details.
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| The subsequent discovery of the Solovay–Strassen and Miller–Rabin algorithms put PRIMES in [[RP (complexity)|coRP]]. In 1992, the Adleman–Huang algorithm<ref name=AH92/> reduced the complexity to [[ZPP (complexity)|ZPP]] = '''RP''' ∩ '''coRP''', which superseded Pratt's result.
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| The cyclotomy test of Adleman, [[Carl Pomerance|Pomerance]], and Rumely from 1983 put PRIMES in '''QP''' ([[quasi-polynomial time]]), which is not known to be comparable with the classes mentioned above. | |
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| Because of its tractability in practice, polynomial-time algorithms assuming the Riemann hypothesis, and other similar evidence, it was long suspected but not proven that primality could be solved in polynomial time. The existence of the [[AKS primality test]] finally settled this long-standing question and placed PRIMES in '''[[P (complexity)|P]]'''. However, PRIMES is not known to be [[P-complete]], and it is not known whether it lies in classes lying inside '''P''' such as [[NC (complexity)|NC]] or [[L (complexity)|L]]. It is known that PRIMES is not in [[AC0|AC<sup>0</sup>]].<Ref>E. Allender, M. Saks, and I.E. Shparlinski, A lower bound for primality, ''J. Comp. Syst. Sci.'' '''62''' (2001), pp. 356–366.</ref>
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| ==Number-theoretic methods==
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| Certain number-theoretic methods exist for testing whether a number is prime, such as the [[Lucas primality test|Lucas test]] and [[Proth's theorem|Proth's test]]. These tests typically require factorization of ''n'' + 1, ''n'' − 1, or a similar quantity, which means that they are not useful for general-purpose primality testing, but they are often quite powerful when the tested number ''n'' is known to have a special form.
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| The Lucas test relies on the fact that the [[multiplicative order]] of a number ''a'' modulo ''n'' is ''n'' − 1 for a prime ''n'' when ''a'' is a [[primitive root modulo n]]. If we can show ''a'' is primitive for ''n'', we can show ''n'' is prime.
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| ==References==
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| <references/>
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| {{Refbegin}}
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| * {{cite book|author = [[Richard Crandall]] and [[Carl Pomerance]] | year = 2005 | title = Prime Numbers: A Computational Perspective | publisher = Springer | edition = 2nd | isbn = 0-387-25282-7}} Chapter 3: Recognizing Primes and Composites, pp. 109–158. Chapter 4: Primality Proving, pp. 159–190. Section 7.6: Elliptic curve primality proving (ECPP), pp. 334–340.
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| * {{cite book|first=Donald|last=Knuth|authorlink=Donald Knuth|title=The Art of Computer Programming|volume=Volume 2: ''Seminumerical Algorithms''|edition=Third|publisher=Addison–Wesley|year=1997|isbn=0-201-89684-2|pages=391–396|chapter=section 4.5.4}}
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| * {{cite book|author=[[Thomas H. Cormen]], [[Charles E. Leiserson]], [[Ronald L. Rivest]], and [[Clifford Stein]]|title=[[Introduction to Algorithms]]|edition=Second|publisher=MIT Press and McGraw–Hill|year=2001|isbn=0-262-03293-7|chapter=Section 31.8: Primality testing|pages=887–896}}
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| * {{cite book|author = [[Christos Papadimitriou]] | year = 1993 | title = Computational Complexity | publisher = Addison Wesley | edition = 1st | isbn = 0-201-53082-1| chapter= Section 10.2: Primality|pages=222–227}}
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| {{Refend}}
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| ==External links==
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| *[http://www.excelexchange.com/prime_number_test.html Excel Formula] ExcelExchange
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| *[http://computacion.cs.cinvestav.mx/~mruiz/cursos/maestria/csac.html Solovay-Strassen] Implementation of the Solovay-Strassen primality test in Maple
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| *[http://cr.yp.to/primetests.html Distinguishing prime numbers from composite numbers], D.J. Bernstein
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| *[http://primes.utm.edu/ The Prime Pages]
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| *[http://www.mathpages.com/home/kmath473.htm Lucas Primality Test with Factored ''N'' − 1] at MathPages
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| *[http://www.primaboinca.com/ PRIMABOINCA] is a research project that uses Internet-connected computers to search for a [[counterexample]] to some conjectures. The first conjecture ([[Agrawal's conjecture]]) was the basis for the formulation of the first deterministic prime test algorithm in polynomial time ([[AKS algorithm]]).
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| {{Number theoretic algorithms}}
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| {{DEFAULTSORT:Primality Test}}
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| [[Category:Primality tests|*]]
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| [[Category:Asymmetric-key algorithms]]
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| {{Link FA|es}}
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