A156983 Primes of the form 2^k + 21.

23, 29, 37, 53, 149, 277, 2069, 32789, 65557, 524309, 17592186044437, 281474976710677, 2251799813685269, 4503599627370517, 2305843009213693973, 11692013098647223345629478661730264157247460343829

Offset: 1

Author

Edwin Dyke (ed.dyke(AT)btinternet.com), Feb 20 2009

Keywords

• nonn

Examples

			23 = 2^1 + 21 is in the sequence; 29 = 2^3 + 21 is in the sequence.
		

Crossrefs

Cf. A000040, A019434 (2^(2^k) + 1), A057201, A057733 (2^k + 3), A123250 (2^k + 5).

Programs

Magma

[ a: n in [1..200] | IsPrime(a) where a is 2^n+21 ]; // Vincenzo Librandi, Nov 27 2010

Mathematica

Delete[Union[Table[If[PrimeQ[2^n + 21], 2^n + 21, 0], {n, 1, 500}]],1]

Formula

a(n) = 2^A057201(n) + 21. - Elmo R. Oliveira, Nov 08 2023

Extensions

More terms from Vincenzo Librandi, Apr 29 2010