A104279 Numbers n such that 2n+1 is prime and 2n-1 is not prime.

1, 5, 8, 11, 14, 18, 20, 23, 26, 29, 33, 35, 39, 41, 44, 48, 50, 53, 56, 63, 65, 68, 74, 78, 81, 83, 86, 89, 95, 98, 105, 111, 113, 116, 119, 125, 128, 131, 134, 138, 140, 146, 153, 155, 158, 165, 168, 173, 176, 179, 183, 186, 189, 191, 194, 198, 200, 204, 209, 215

Offset: 1

Author

Alexandre Wajnberg, Apr 17 2005

Keywords

• easy
• nonn

Examples

			a(1)=1 because 3 is prime and 1 is not prime.
a(2)=5 because 11 is prime and 9 is not prime.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A040040 and others.

Programs

Magma

[n: n in [1..220]| not IsPrime(2*n-1) and IsPrime(2*n+1)] // Vincenzo Librandi, Jan 28 2011

Maple

filter:= n -> isprime(2*n+1) and not isprime(2*n-1):
select(filter, [$1..300]); # Robert Israel, Apr 27 2020

Mathematica

Select[ Range[218], PrimeQ[2# + 1] && !PrimeQ[2# - 1] &] (* Robert G. Wilson v, Apr 18 2005 *)

PARI

is(n)=isprime(2*n+1) && !isprime(2*n-1) \\ Charles R Greathouse IV, Jun 13 2017

Extensions

More terms from Robert G. Wilson v, Apr 18 2005