A104280 - OEIS

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A104280

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

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Table of n, a(n) for n=1..60.

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.