A175235 Prime p such that 2^p + 21 is also prime.

3, 5, 7, 11, 19, 61, 163, 911, 997, 16657, 36551

Offset: 1

Author

Vincenzo Librandi, Mar 09 2010

Keywords

• nonn
• more

Comments

a(12) > 100000. - Dana Jacobsen, Oct 03 2015

a(12) > 1173095 using A057201. - Michael S. Branicky, Sep 07 2024

Examples

			For p=3, 2^3+21=29; p=5, 2^5+21=53; p=7, 2^7+21=149.
		

Programs

Magma

[p: p in PrimesUpTo(4000) | IsPrime(2^p+21) ];

Mathematica

Select[Prime[Range[10000]], PrimeQ[(2^# + 21)] &] (* Vincenzo Librandi, Oct 04 2015 *)

PARI

is(n)=prime(n)&&prime(2^n+21) \\ Anders Hellström, Oct 03 2015

Perl

use ntheory ":all"; use Math::GMP qw/:constant/; forprimes { my $n = 2**$+21; say if is_prime($n) } 20000; # _Dana Jacobsen, Oct 03 2015

Formula

A000040 INTERSECT A057201. - R. J. Mathar, May 02 2010

Extensions

a(10)-a(11) from Dana Jacobsen, Oct 03 2015