A271658 - OEIS

%I #11 Sep 08 2022 08:46:16

%S 5,7,11,17,19,59,127,227,257,647,971,3259,3929,4721,5531,6869,11719,

%T 18097,22511,25847,40037,53987,65027,65537,65539,65699,76667,80279,

%U 195659,307399,368609,491539,1349251,1973627,2259197,2702317,2822719,3218417,3502007

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

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

%e 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).

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

%o (Magma) [n: n in [4..5*10^7] | IsPrime(n) and EulerPhi(n-3) eq EulerPhi(EulerPhi(n-2)-1)]

%o (PARI) lista(nn) = forprime(p=5, nn, if(eulerphi(p-3) == eulerphi(eulerphi(p-2)-1), print1(p, ", "))); \\ _Altug Alkan_, Apr 12 2016

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

%K nonn

%O 1,1

%A _Jaroslav Krizek_, Apr 12 2016