%I #19 Feb 23 2024 13:42:06
%S 2,4,8,22,24,26,98,228,246,808,1556,1792,1978,3424,69738,108376,169584
%N Numbers k such that 4^k + 21 is prime.
%C Half of even terms of A057201. - _Elmo R. Oliveira_, Nov 29 2023
%e For k = 26, 4^26 + 21 = 4503599627370517 is prime.
%t Select[Range[3000], PrimeQ[4^# + 21] &]
%o (Magma) [n: n in [0..700] | IsPrime(4^n+21)];
%o (PARI) for(n=1, 1e3, if(isprime(4^n+21), print1(n", "))) \\ _Altug Alkan_, Sep 18 2015
%Y Cf. A057201, A156983, A253772.
%K nonn,more
%O 1,1
%A _Robert Price_, Sep 18 2015