A253774 Numbers k such that 4^k + 19 is prime.

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

Offset: 1

Author

Vincenzo Librandi, Jan 16 2015

Keywords

• nonn
• more

Examples

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

Crossrefs

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

Programs

Magma

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

Mathematica

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

PARI

is(n)=ispseudoprime(4^n+19) \\ Charles R Greathouse IV, Jun 06 2017

Formula

a(n) = A057221(n)/2. - Joerg Arndt, Aug 28 2015

Extensions

Three more term (using A057221) from Joerg Arndt, Aug 28 2015

a(14)-a(15) derived from A057221 by Elmo R. Oliveira, Nov 28 2023