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%I #11 Sep 08 2022 08:46:16
%S 5,17,257,317,977,2237,3257,5189,8537,9533,11717,13367,22937,37847,
%T 65537,76871,307397,428957,491537,589409,610913,983777,1055063,
%U 1207097,1340789,1659587,2822717,3137357,3264887,3488333,4994117,5577827,6475457,7378373,7479089
%N Primes p such that phi(phi(p-2)-1) = phi(phi(p-1)-1).
%C The first 4 known Fermat primes > 3 from A019434 are in the sequence.
%e 317 is a term because phi(phi(317-2)-1) = phi(phi(315)-1) = phi(144-1) = phi(143) = 120 = phi(phi(317-1)-1) = phi(phi(316)-1) = phi(156-1) = phi(155).
%t Select[Prime@ Range[3, 10^6], EulerPhi[EulerPhi[# - 2] - 1] == EulerPhi[EulerPhi[# - 1] - 1] &] (* _Michael De Vlieger_, Apr 12 2016 *)
%o (Magma) [n: n in [4..5*10^7] | IsPrime(n) and EulerPhi(EulerPhi(n-2)-1) eq EulerPhi(EulerPhi(n-1)-1)]
%o (PARI) lista(nn) = forprime(p=5, nn, if(eulerphi(eulerphi(p-2)-1) == eulerphi(eulerphi(p-1)-1), print1(p, ", "))); \\ _Altug Alkan_, Apr 12 2016
%Y Cf. A019434, A271655, A271656, A271657, A271658, A271659.
%K nonn
%O 1,1
%A _Jaroslav Krizek_, Apr 12 2016
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