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A171271 Numbers n such that phi(n)=2*phi(n-1). 4
3, 5, 17, 155, 257, 287, 365, 805, 1067, 2147, 3383, 4551, 6107, 7701, 8177, 9269, 11285, 12557, 12971, 16403, 19229, 19277, 20273, 25133, 26405, 27347, 29155, 29575, 35645, 36419, 38369, 39647, 40495, 47215, 52235, 54653, 65537, 84863 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Theorem: A prime p is in the sequence iff p is a Fermat prime.

Proof: If p=2^2^n+1 is prime (Fermat prime) then phi(p)=2^2^n=2* phi(2^2^n)=2*phi(p-1), so p is in the sequence. Now if p is a prime term of the sequence then phi(p)=2*phi(p-1) so p-1=2*phi(p-1) and we deduce that p-1=2^m hence p is a Fermat prime.

LINKS

Hiroaki Yamanouchi, Table of n, a(n) for n = 1..5416

MATHEMATICA

Select[Range[85000], EulerPhi[ # ]==2EulerPhi[ #-1]&]

CROSSREFS

Cf. A019434, A171262.

Sequence in context: A084723 A107312 A083213 * A056826 A058910 A023394

Adjacent sequences:  A171268 A171269 A171270 * A171272 A171273 A171274

KEYWORD

easy,nonn

AUTHOR

Farideh Firoozbakht, Feb 23 2010

STATUS

approved

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Last modified December 18 07:25 EST 2014. Contains 252098 sequences.