login
A104280
Numbers n such that 2n+1 is not prime and 2n-1 is prime.
2
4, 7, 10, 12, 16, 19, 22, 24, 27, 31, 34, 37, 40, 42, 45, 49, 52, 55, 57, 64, 66, 70, 76, 79, 82, 84, 87, 91, 97, 100, 106, 112, 115, 117, 121, 126, 129, 132, 136, 139, 142, 147, 154, 157, 159, 166, 169, 175, 177, 180, 184, 187, 190, 192, 195, 199, 201, 205, 211, 217
OFFSET
1,1
MATHEMATICA
Select[ Range[219], !PrimeQ[2# + 1] && PrimeQ[2# - 1] &] (* Robert G. Wilson v, Apr 18 2005 *)
PROG
(Magma) [n: n in [0..250]| 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.
Equals A136798/2.
Sequence in context: A080734 A310676 A320929 * A083032 A284933 A020965
KEYWORD
easy,nonn
AUTHOR
Alexandre Wajnberg, Apr 17 2005
EXTENSIONS
More terms from Robert G. Wilson v, Apr 18 2005
STATUS
approved