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Numbers k such that 4^k + 19 is prime.
2

%I #19 Nov 28 2023 16:01:47

%S 1,3,15,81,327,357,685,831,861,1405,38571,78127,216487,546121,622615

%N Numbers k such that 4^k + 19 is prime.

%F a(n) = A057221(n)/2. - _Joerg Arndt_, Aug 28 2015

%e 15 is a term because 4^15 + 19 = 1073741843 is prime.

%t Select[Range[3000], PrimeQ[4^# + 19] &]

%o (Magma) [n: n in [0..700] | IsPrime(4^n+19)];

%o (PARI) is(n)=ispseudoprime(4^n+19) \\ _Charles R Greathouse IV_, Jun 06 2017

%Y Cf. A057221, A104068, A253772 (similar sequence).

%K nonn,more

%O 1,2

%A _Vincenzo Librandi_, Jan 16 2015

%E Three more term (using A057221) from _Joerg Arndt_, Aug 28 2015

%E a(14)-a(15) derived from A057221 by _Elmo R. Oliveira_, Nov 28 2023