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Numbers k such that 4^k + 21 is prime.
1

%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