# Difference between revisions of "Pseudo-prime"

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− | + | Traditionally, a composite natural number $n$ is called a pseudo-prime if $2^{n-1} \equiv 1$ modulo $n$, for it has long been known that primes have this property. (The term is apparently due to D.H. Lehmer.) There are infinitely many such $n$, the first five being | |

+ | $$ | ||

+ | 341,\,561,\,645,\,1105,\,1387\ . | ||

+ | $$ | ||

More recently, the concept has been extended to include any composite number that acts like a prime in some realization of a [[Probabilistic primality test|probabilistic primality test]]. That is, it satisfies some easily computable necessary, but not sufficient, condition for primality. Pseudo-primes in this larger sense include: | More recently, the concept has been extended to include any composite number that acts like a prime in some realization of a [[Probabilistic primality test|probabilistic primality test]]. That is, it satisfies some easily computable necessary, but not sufficient, condition for primality. Pseudo-primes in this larger sense include: | ||

− | 1) ordinary base- | + | 1) ordinary base-$b$ pseudo-primes, satisfying $b^{n-1}\equiv1$ modulo $n$; |

− | 2) Euler base- | + | 2) Euler base-$b$ pseudo-primes, whose [[Jacobi symbol|Jacobi symbol]] with $b$ satisfies |

+ | $$ | ||

+ | b^{(n-1)/2} \equiv\left({\frac{b}{n}}\right) = \pm 1 | ||

+ | $$ | ||

− | + | 3) strong base-$b$ pseudo-primes, for which the sequence $b^{s.2^i}$ modulo $n$ for $i=0,\ldots, r$ is either always $1$, or contains $-1$. (Here $n-1 = 2^r.s$ with $s$ odd.) | |

− | |||

− | 3) strong base- | ||

For each <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027023.png" />, the implications 3)<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027024.png" />2)<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027025.png" />1) hold. A number <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027026.png" /> that is an ordinary base-<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027027.png" /> pseudo-prime for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027028.png" /> prime to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027029.png" /> is called a [[Carmichael number|Carmichael number]]. Analogous numbers for the other two categories do not exist. | For each <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027023.png" />, the implications 3)<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027024.png" />2)<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027025.png" />1) hold. A number <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027026.png" /> that is an ordinary base-<img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027027.png" /> pseudo-prime for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027028.png" /> prime to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/p/p110/p110270/p11027029.png" /> is called a [[Carmichael number|Carmichael number]]. Analogous numbers for the other two categories do not exist. |

## Revision as of 17:10, 25 August 2013

Traditionally, a composite natural number $n$ is called a pseudo-prime if $2^{n-1} \equiv 1$ modulo $n$, for it has long been known that primes have this property. (The term is apparently due to D.H. Lehmer.) There are infinitely many such $n$, the first five being $$ 341,\,561,\,645,\,1105,\,1387\ . $$

More recently, the concept has been extended to include any composite number that acts like a prime in some realization of a probabilistic primality test. That is, it satisfies some easily computable necessary, but not sufficient, condition for primality. Pseudo-primes in this larger sense include:

1) ordinary base-$b$ pseudo-primes, satisfying $b^{n-1}\equiv1$ modulo $n$;

2) Euler base-$b$ pseudo-primes, whose Jacobi symbol with $b$ satisfies $$ b^{(n-1)/2} \equiv\left({\frac{b}{n}}\right) = \pm 1 $$

3) strong base-$b$ pseudo-primes, for which the sequence $b^{s.2^i}$ modulo $n$ for $i=0,\ldots, r$ is either always $1$, or contains $-1$. (Here $n-1 = 2^r.s$ with $s$ odd.)

For each , the implications 3)2)1) hold. A number that is an ordinary base- pseudo-prime for all prime to is called a Carmichael number. Analogous numbers for the other two categories do not exist.

For a thorough empirical study of pseudo-primes, see [a4]. Lists of pseudo-primes to various small bases can be found in [a6].

The concept of a pseudo-prime has been generalized to include primality tests based on finite fields and elliptic curves (cf. also Finite field; Elliptic curve). For reviews of this work, see [a3], [a5].

The complementary concept is also of interest. The base is called a (Fermat) witness for if is composite but not a base- pseudo-prime. Euler and strong witnesses are similarly defined. If , the smallest strong witness for , grows sufficiently slowly, there is a polynomial-time algorithm for primality. It is known that is not bounded [a2], but if an extended version of the Riemann hypothesis (cf. Riemann hypotheses) holds, then [a1].

#### References

[a1] | E. Bach, "Analytic methods in the analysis and design of number-theoretic algorithms" , MIT (1985) |

[a2] | W.R. Alford, A. Granville, C. Pomerance, "On the difficulty of finding reliable witnesses" , Algorithmic Number Theory, First Internat. Symp., ANTS-I , Lecture Notes in Computer Science , 877 , Springer (1994) pp. 1–16 |

[a3] | F. Morain, "Pseudoprimes: a survey of recent results" , Proc. Eurocode '92 , Springer (1993) pp. 207–215 |

[a4] | C. Pomerance, J.L. Selfridge, S.S. Wagstaff, Jr., "The pseudoprimes to " Math. Comp. , 35 (1980) pp. 1003–1026 |

[a5] | P. Ribenboim, "The book of prime number records" , Springer (1989) (Edition: Second) |

[a6] | N.J.A. Sloane, S. Plouffe, "The encyclopedia of integer sequences" , Acad. Press (1995) |

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Pseudo-prime.

*Encyclopedia of Mathematics.*URL: http://encyclopediaofmath.org/index.php?title=Pseudo-prime&oldid=16100