%I #27 Feb 02 2024 00:03:53
%S 3,5,17,257,65537,83623937,4294967297,6992962672132097
%N Numbers such that 2*phi(n-2) = n-1.
%C Numbers such that A074057(n) = 0.
%C Note that the first 5 terms are Fermat primes (A019434), while a(6) is prime but is not a Fermat number, and a(7) is (F5 from A000215) but is not prime. Note also that F6, F7, F8 and F9 do not belong to this sequence.
%F a(n) = A050474(n) + 2. - _Jaroslav Krizek_, Feb 25 2015
%o (PARI) is_A232720(n) = 2*eulerphi(n-2) == n-1
%Y Cf. A000010, A074057.
%K nonn,more
%O 1,1
%A _Michel Marcus_, Nov 28 2013
%E a(8) from _Jaroslav Krizek_, Feb 25 2015