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Numbers k such that 4^k + 31 is prime.
2

%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