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

1,2

Links

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

Example

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

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 *)

Prog

(Magma) [n: n in [1..220]| not IsPrime(2*n-1) and IsPrime(2*n+1)] // Vincenzo Librandi, Jan 28 2011
(PARI) is(n)=isprime(2*n+1) && !isprime(2*n-1) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

Cf. A040040 and others.

Sequence in context: A314388 A022798 A022808 * A184919 A063247 A184928

Adjacent sequences: A104276 A104277 A104278 * A104280 A104281 A104282

Keyword

easy,nonn

Author

Alexandre Wajnberg, Apr 17 2005

Extensions

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

Status

approved