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A039772
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Even numbers k such that phi(k) and k-1 are distinct and have a common factor > 1.
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8
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28, 52, 66, 70, 76, 112, 124, 130, 148, 154, 172, 176, 186, 190, 196, 208, 232, 238, 244, 246, 268, 276, 280, 286, 292, 304, 310, 316, 322, 344, 364, 366, 370, 388, 396, 406, 412, 418, 426, 430, 436, 442, 448, 490, 496, 506, 508, 520, 532, 556, 568, 574
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OFFSET
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1,1
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COMMENTS
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Also this sequence is the union of all possible even Fermat pseudoprimes q to some prime base p>q such that q does not divide p-1. Note that all even nonprime divisors of p-1 are the Fermat pseudoprimes to prime base p. E.g. q = {4,6,12,18,28,36} is a set of even Fermat pseudoprimes to prime base p = 37, but only number q = 28 from this set does not divide p-1 = 36. - Alexander Adamchuk, Jun 16 2007
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LINKS
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EXAMPLE
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phi(28)=12, gcd(12,27)=3.
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MAPLE
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select(t -> igcd(numtheory:-phi(t), t-1)>1, [seq(n, n=2..1000, 2)]); # Robert Israel, May 15 2017
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MATHEMATICA
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PROG
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(PARI) isok(n) = !(n%2) && (gcd(eulerphi(n), n-1) != 1); \\ Michel Marcus, Mar 15 2019
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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