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A152154
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Positive residues of Pepin's Test for Fermat numbers using either 5 or 10 for the base.
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4
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2, 0, 16, 256, 65536, 3484838166, 17225898269543404863, 6964187975677595099156927503004398881, 14553806122642016769237504145596730952769427034161327480375008633175279343120
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OFFSET
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0,1
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COMMENTS
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For n=0 or n>=2 the Fermat Number F(n) is prime if and only if 5^((F(n) - 1)/2) is congruent to -1 (mod F(n)).
5 was the base originally used by Pepin. The base 10 gives the same results.
Any positive integer k for which the Jacobi symbol (k|F(n)) is -1 can be used as the base instead.
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REFERENCES
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M. Krizek, F. Luca & L. Somer, 17 Lectures on Fermat Numbers, Springer-Verlag NY 2001, pp. 42-43.
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LINKS
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FORMULA
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a(n) = 5^((F(n) - 1)/2) (mod F(n)), where F(n) is the n-th Fermat Number
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EXAMPLE
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a(4) = 5^(32768) (mod 65537) = 65536 = -1 (mod F(4)), therefore F(4) is prime.
a(5) = 5^(2147483648) (mod 4294967297) = 3484838166 (mod F(5)), therefore F(5) is composite.
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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Dennis Martin (dennis.martin(AT)dptechnology.com), Nov 27 2008
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STATUS
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approved
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