A271658 Primes p such that phi(p-3) = phi(phi(p-2)-1).

5, 7, 11, 17, 19, 59, 127, 227, 257, 647, 971, 3259, 3929, 4721, 5531, 6869, 11719, 18097, 22511, 25847, 40037, 53987, 65027, 65537, 65539, 65699, 76667, 80279, 195659, 307399, 368609, 491539, 1349251, 1973627, 2259197, 2702317, 2822719, 3218417, 3502007

Offset

1,1

Comments

The first 4 known Fermat primes > 3 from A019434 are in the sequence.

Links

Table of n, a(n) for n=1..39.

Example

257 is a term because phi(257-3) = phi(254) = 126 = phi(phi(257-2)-1) = phi(phi(255)-1) = phi(128-1) = phi(127).

Mathematica

Select[Prime@ Range[3, 10^6], EulerPhi[# - 3] == EulerPhi[EulerPhi[# - 2] - 1] &] (* Michael De Vlieger, Apr 12 2016 *)

Prog

(Magma) [n: n in [4..5*10^7] | IsPrime(n) and EulerPhi(n-3) eq EulerPhi(EulerPhi(n-2)-1)]
(PARI) lista(nn) = forprime(p=5, nn, if(eulerphi(p-3) == eulerphi(eulerphi(p-2)-1), print1(p, ", "))); \\ Altug Alkan, Apr 12 2016

Crossrefs

Cf. A019434, A271655, A271656, A271657, A271659, A271660.

Sequence in context: A110587 A192281 A191065 * A282739 A072249 A076665

Adjacent sequences: A271655 A271656 A271657 * A271659 A271660 A271661

Keyword

nonn

Author

Jaroslav Krizek, Apr 12 2016

Status

approved