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NumberTheory`PrimeQ`

This package implements primality proving. If ProvablePrimeQ[n] returns True, then the number can be mathematically proven to be prime. In addition, PrimeQCertificate[n] prints a certificate that can be used to verify that is prime or composite. Note that the built-in primality testing function PrimeQ does not actually give a proof that a number is prime. However, as of this writing, there are no known examples where PrimeQ fails.

Proving primality or compositeness.

The functions provided in this package not only prove primality, but they also generate a certificate of primality. A certificate of primality is a relatively short set of data that can be easily used to prove primality. The word easily means that using the data to prove primality is much easier and faster than generating the data in the first place. As a simple example of a certificate, the factors of a composite number provide a certificate of compositeness. Multiplying the numbers together to show that they are the factors is much easier than finding the factors. The advantage of providing certificates is that the user does not have to trust the internal mechanism of the algorithm that generated the certificate. It is fairly easy to write a program (in any system) that checks that the certificate provides a proof of primality.

This loads the package.

In[1]:= <<NumberTheory`PrimeQ`

PrimeQ indicates that the number 1093 is prime.

In[2]:= PrimeQ[1093]

Out[2]=

ProvablePrimeQ gives the same result, but it has generated a certificate.

In[3]:= ProvablePrimeQ[1093]

Out[3]=

This prints the certificate.

In[4]:= PrimeQCertificate[1093]

Out[4]=

This prints the certificate directly.

In[5]:= ProvablePrimeQ[1093, Certificate->True]

Out[5]=

The certificate of primality used in this package for large n is based on the theory of elliptic curves. The basic idea was discovered by S. Goldwasser and J. Kilian, "Almost All Primes Can Be Quickly Certified," in Proc. 18th STOC, 1986, pp. 316-329. Their algorithm was only of theoretical significance, however. A. O. L. Atkin found a way to make it practical by using ideas from complex multiplication, which is a branch of number theory that combines the fields of complex analysis, Galois theory and modular forms. This method was implemented very successfully by F. Morain in "Implementation of the Atkin-Goldwasser-Kilian Primality Testing Algorithm," INRIA Research Report, # 911, October 1988. In November 1989, Morain was able to give a primality proof for a 1065-digit number, the first titanic prime ( digits) to be tested without special purpose algorithms. Since that time Atkin and Morain have written an extensive treatise on the algorithm: A. O. L. Atkin and F. Morain, "Elliptic Curves and Primality Proving," Mathematics of Computation, 1993, 29-68.

For an introduction to primality testing and elliptic curves, see D. Bressoud, Factorization and Primality Testing, Springer-Verlag, 1989.

This package can also be used to generate certificates of compositeness for composite numbers. These certificates are based on showing that simple properties that are true of prime numbers are not true for the given number. For example, PrimeQCertificate[3837523] returns the certificate {2, 3837522, 3837523}, which is intended to show that is not equal to 1.

ProvablePrimeQ[n] returns True or False depending on whether n is prime or not. The certificate for primality or compositeness is generated by PrimeQCertificate[n]. ProvablePrimeQ calls PrimeQCertificate and stores the result, so it does not take any extra time to create a certificate once ProvablePrimeQ has returned an answer. The certificate generated by PrimeQCertificate can be checked by PrimeQCertificateCheck. This function recognizes whether the certificate asserts primality or compositeness and then uses the certificate to verify the assertion.

This prints the certificate of the prime number.

In[6]:= ProvablePrimeQ[1093,
Certificate -> True]

Out[6]=

This verifies primality using the certificate.

In[7]:= PrimeQCertificateCheck[Last[%], 1093]

Out[7]=

Here is the certificate for a composite number. You can recognize a certificate of compositeness because it will always be a list of three integers.

In[8]:= PrimeQCertificate[1093 * 3511]

Out[8]=

This verifies the compositeness.

In[9]:= PrimeQCertificateCheck[%, 3837523]

Out[9]=

The built-in function PrimeQ first tests for divisibility using small primes, then uses the Miller-Rabin strong pseudoprime test base 2 and base 3, and then uses the Lucas test. As of 1998, this procedure is known to be correct only for , and it is conceivable that for larger it could claim a composite number to be prime. However, it is a mathematical theorem that when PrimeQ[n] returns False, the number is genuinely composite. Thus PrimeQ[n] can only fail if is composite but PrimeQ declares it to be prime. It is important to note that PrimeQ is deterministic; no computations based on random numbers are involved.

This package is not meant to replace the built-in primality tester PrimeQ but rather to allow one to be completely secure that a number is truly prime. The package should be used only to certify results after all the number theoretical work has been done. For example, it would be a mistake to use ProvablePrimeQ as a primality test for an integer factoring algorithm. Rather, only when the complete factorization has been achieved (using PrimeQ for primality testing) would one use ProvablePrimeQ to certify the primality of the prime factors given by the algorithm. The reason for this is that PrimeQ will be, in general, several orders of magnitude faster than ProvablePrimeQ.

As noted above, there is a possibility that PrimeQ is incorrect, that is, it asserts that a number is prime when it is really composite. It is unclear whether ProvablePrimeQ always detects this; but if it does, a warning message is generated indicating a counterexample to PrimeQ, and False is returned. This behavior can be simulated by artificially setting PrimeQ[n] to True for a known composite .

Here PrimeQ is artificially set to give an incorrect answer. ProvablePrimeQ detects that PrimeQ is incorrect, prints a warning message, and returns the correct answer.

In[10]:= (Unprotect[PrimeQ];
PrimeQ[38200901201] = True;
ProvablePrimeQ[38200901201])

SqrtMod::fail: Warning: $IterationLimit exceeded; PrimeQ[38200901201] may be incorrect.



PrimeQCertificate::qrsqrtmod:

Failure of elliptic curves certificate for p = 38200901201. Unable to find quadratic representation 4*p = u^2 + 7*v^2 due to failure of SqrtMod[-7 , p].



PrimeQCertificate::false:

Warning: PrimeQCertificate has detected a counterexample to PrimeQ:

PowerMod[3, 38200901200, 38200901201] != 1.

Out[10]=False

Options for ProvablePrimeQ and PrimeQCertificate.

When is larger than the value of the option SmallPrime, ProvablePrimeQ uses the Atkin-Morain test as described above. This primality proving algorithm is suboptimal if the number is within the range of efficient factoring algorithms. When this is the case, a method of V. Pratt is preferable: V. Pratt, "Every Prime Has a Succinct Certificate," SIAM Journal of Computation 4 (1975), pages 214-220. An implementation of this algorithm in Mathematica is described in S. Wagon, Mathematica in Action, W. H. Freeman, 1991, Section 8.7.

Note that you must use the same value of SmallPrime when you check a certificate using PrimeQCertificate that you used when you generated it using ProvablePrimeQ or PrimeQCertificate.

Since the default value of SmallPrime is larger than the given number, Pratt's certificate of primality is returned.

In[11]:= PrimeQCertificate[3511]

Out[11]=

For large numbers, the certificate can be quite long and involved.

In[12]:= PrimeQCertificate[10^20 + 39]

Out[12]=