%I #16 Nov 28 2023 18:35:03
%S 2,6,18,270,422,596,6068,42140,64178,158732,1509278
%N Numbers k such that 4^k + 31 is prime.
%C The next terms are > 1.5*10^6.
%C Contains exactly the halved even terms of A247952.
%F a(n) = A247952(n)/2. - _Elmo R. Oliveira_, Nov 28 2023
%e For k = 18, 4^18 + 31 = 68719476767 is prime.
%t Select[Range[0, 250000], PrimeQ[4^# + 31] &]
%o (Magma) [n: n in [0..700] | IsPrime(4^n+31)]; // _Vincenzo Librandi_, Oct 06 2015
%o (PARI) for(n=1, 1e3, if(isprime(4^n+31), print1(n", "))) \\ _Altug Alkan_, Oct 06 2015
%Y Cf. A247952, A253772 (similar sequence).
%K nonn,more
%O 1,1
%A _Robert Price_, Oct 05 2015
%E a(11) derived from A247952 by _Elmo R. Oliveira_, Nov 28 2023