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A258454 Numbers n such that phi(n) = 2*phi(n-2). 0
3, 4, 5, 8, 17, 32, 257, 512, 527, 992, 1952, 2522, 5252, 6512, 7412, 10376, 23432, 23717, 26732, 27302, 35114, 36632, 37442, 45872, 47027, 49022, 51092, 65537, 78899, 84242, 92432, 98432, 98672, 114767, 115292, 131072, 227222, 231167, 240977, 328352, 369272 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: a prime p is in the sequence iff p is a Fermat prime (A019434).

This is not correct: the first non-Fermat prime term is 83623937 = 2^18*11*29 + 1. - Joerg Arndt, Oct 11 2015

LINKS

Table of n, a(n) for n=1..41.

EXAMPLE

phi(32) = 16 = 2*phi(30) = 2*8, so 32 is in the sequence.

MATHEMATICA

Select[Range@ 400000, EulerPhi@ # == 2 EulerPhi[# - 2] &] (* Michael De Vlieger, Sep 25 2015 *)

PROG

(MAGMA) [n: n in [3..10000000] | EulerPhi(n) eq 2*EulerPhi(n-2)]

(PARI) for(n=1, 1e6, if(eulerphi(n) == 2*eulerphi(n-2), print1(n", "))); \\ Altug Alkan, Sep 26 2015

CROSSREFS

Cf. A171271, A171262.

Sequence in context: A216888 A106048 A219038 * A176776 A049931 A058983

Adjacent sequences:  A258451 A258452 A258453 * A258455 A258456 A258457

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, Sep 24 2015

STATUS

approved

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Last modified August 24 23:01 EDT 2019. Contains 326314 sequences. (Running on oeis4.)