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In [[arithmetic]], an [[Odd number|odd]] [[composite number|composite]] [[integer]] ''n'' is called an '''Euler pseudoprime''' to base ''a'', if ''a'' and ''n'' are [[coprime]], and | |||
: <math>a^{(n-1)/2} \equiv \pm 1\pmod{n}</math> | |||
(where ''mod'' refers to the [[modular arithmetic|modulo]] operation). | |||
The motivation for this definition is the fact that all [[prime number]]s ''p'' satisfy the above equation which can be deduced from [[Fermat's little theorem]]. Fermat's theorem asserts that if ''p'' is prime, and coprime to ''a'', then ''a''<sup>''p''−1</sup> = 1 (mod ''p''). Suppose that ''p''>2 is prime, then ''p'' can be expressed as 2''q'' + 1 where ''q'' is an integer. Thus; ''a''<sup>(2''q''+1) − 1</sup> = 1 (mod ''p'') which means that ''a''<sup>2''q''</sup> − 1 = 0 (mod ''p''). This can be factored as (''a''<sup>''q''</sup> − 1)(''a''<sup>''q''</sup> + 1) = 0 (mod ''p'') which is equivalent to ''a''<sup>(''p''−1)/2</sup> = ±1 (mod ''p''). | |||
The equation can be tested rather quickly, which can be used for probabilistic [[prime testing|primality testing]]. These tests are twice as strong as tests based on Fermat's little theorem. | |||
Every Euler pseudoprime is also a Fermat [[pseudoprime]]. It is not possible to produce a definite test of primality based on whether a [[number]] is an Euler pseudoprime because there exist ''absolute Euler pseudoprimes'', numbers which are Euler pseudoprimes to every base relatively prime to themselves. The absolute Euler pseudoprimes are a [[subset]] of the absolute Fermat pseudoprimes, or [[Carmichael number]]s, and the smallest absolute Euler pseudoprime is [[1729 (number)|1729]] = 7×13×19. | |||
The slightly stronger condition that | |||
: <math> a^{(n-1)/2} \equiv \left(\frac{a}{n}\right) \pmod n</math> | |||
where ''n'' is an odd composite, the [[greatest common divisor]] of ''a'' and ''n'' equals 1, and (''a''/''n'') is the [[Jacobi symbol]], is the more common definition of an Euler pseudoprime. | |||
See, for example, page 115 of the book by Koblitz listed below, page 90 of the book by Riesel, or page 1003 of.<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> | |||
A discussion of numbers of this form can be found at [[Euler–Jacobi pseudoprime]]. | |||
==See also== | |||
* [[Probable prime]] | |||
==References== | |||
{{reflist}} | |||
*M. Koblitz, "A Course in Number Theory and Cryptography", Springer-Verlag, 1987. | |||
*H. Riesel, "Prime numbers and computer methods of factorisation", Birkhäuser, Boston, Mass., 1985. | |||
{{Classes of natural numbers}} | |||
{{DEFAULTSORT:Euler Pseudoprime}} | |||
[[Category:Pseudoprimes]] | |||
Revision as of 21:12, 29 November 2013
In arithmetic, an odd composite integer n is called an Euler pseudoprime to base a, if a and n are coprime, and
(where mod refers to the modulo operation).
The motivation for this definition is the fact that all prime numbers p satisfy the above equation which can be deduced from Fermat's little theorem. Fermat's theorem asserts that if p is prime, and coprime to a, then ap−1 = 1 (mod p). Suppose that p>2 is prime, then p can be expressed as 2q + 1 where q is an integer. Thus; a(2q+1) − 1 = 1 (mod p) which means that a2q − 1 = 0 (mod p). This can be factored as (aq − 1)(aq + 1) = 0 (mod p) which is equivalent to a(p−1)/2 = ±1 (mod p).
The equation can be tested rather quickly, which can be used for probabilistic primality testing. These tests are twice as strong as tests based on Fermat's little theorem.
Every Euler pseudoprime is also a Fermat pseudoprime. It is not possible to produce a definite test of primality based on whether a number is an Euler pseudoprime because there exist absolute Euler pseudoprimes, numbers which are Euler pseudoprimes to every base relatively prime to themselves. The absolute Euler pseudoprimes are a subset of the absolute Fermat pseudoprimes, or Carmichael numbers, and the smallest absolute Euler pseudoprime is 1729 = 7×13×19.
The slightly stronger condition that
where n is an odd composite, the greatest common divisor of a and n equals 1, and (a/n) is the Jacobi symbol, is the more common definition of an Euler pseudoprime. See, for example, page 115 of the book by Koblitz listed below, page 90 of the book by Riesel, or page 1003 of.[1] A discussion of numbers of this form can be found at Euler–Jacobi pseudoprime.
See also
References
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- M. Koblitz, "A Course in Number Theory and Cryptography", Springer-Verlag, 1987.
- H. Riesel, "Prime numbers and computer methods of factorisation", Birkhäuser, Boston, Mass., 1985.
Template:Classes of natural numbers
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