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A039772 phi(a(n)) and (a(n)-1) have a common factor, are distinct and a(n) is even. 4
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Also a(n) 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

LINKS

Table of n, a(n) for n=1..52.

Romeo Meštrović, Generalizations of Carmichael numbers I, arXiv:1305.1867v1 [math.NT], May 4, 2013.

Eric Weisstein, Link to a section of The World of Mathematics. Fermat Pseudoprime.

EXAMPLE

phi(28)=12, gcd(12,27)=3.

CROSSREFS

Cf. A000010.

Sequence in context: A046419 A063770 A161923 * A181792 A181793 A224545

Adjacent sequences:  A039769 A039770 A039771 * A039773 A039774 A039775

KEYWORD

nonn,easy

AUTHOR

Olivier Gérard

STATUS

approved

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Last modified December 3 01:12 EST 2016. Contains 278694 sequences.