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A271659 Primes p such that phi(phi(p-1)+1) = phi(phi(p-2)+1). 5
3, 5, 17, 53, 83, 257, 701, 977, 1877, 2027, 2837, 3257, 5189, 6521, 9257, 11717, 13367, 13457, 18947, 22937, 27611, 56813, 65537, 81509, 137363, 157037, 277643, 307397, 491537, 589409, 728417, 928787, 983777, 1143953, 1206581, 1397933, 1659587, 2822717 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The first 5 known Fermat primes from A019434 are in the sequence.

LINKS

Table of n, a(n) for n=1..38.

EXAMPLE

53 is term because  phi(phi(53-1)+1) = phi(phi(52)+1) = phi(24+1) = phi(25) = 20 = phi(phi(53-2)+1) = phi(phi(51)+1) = phi(32+1) = phi(33).

MATHEMATICA

Select[Prime@ Range[10^5], EulerPhi[EulerPhi[# - 1] + 1] == EulerPhi[EulerPhi[# - 2] + 1] &] (* Michael De Vlieger, Apr 12 2016 *)

PROG

(MAGMA) [n: n in [3..10^7] | IsPrime(n) and EulerPhi(EulerPhi(n-2)+1) eq EulerPhi(EulerPhi(n-1) +1)]

(PARI) lista(nn) = forprime(p=3, nn, if(eulerphi(eulerphi(p-1)+1) == eulerphi(eulerphi(p-2)+1), print1(p, ", "))); \\ Altug Alkan, Apr 12 2016

CROSSREFS

Cf. A019434, A271655, A271656, A271657, A271658, A271660.

Sequence in context: A215106 A006483 A177960 * A049540 A097144 A219108

Adjacent sequences:  A271656 A271657 A271658 * A271660 A271661 A271662

KEYWORD

nonn

AUTHOR

Jaroslav Krizek, Apr 12 2016

STATUS

approved

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Last modified October 19 20:05 EDT 2018. Contains 316378 sequences. (Running on oeis4.)