A199717 Numbers k such that 6*k-1 is composite, but 6*k-5 is prime.

6, 11, 13, 24, 26, 27, 31, 34, 36, 41, 46, 48, 56, 57, 62, 63, 69, 71, 73, 88, 91, 92, 96, 97, 101, 102, 104, 106, 111, 116, 119, 122, 123, 126, 132, 136, 139, 154, 166, 167, 171, 173, 174, 176, 178, 179, 187, 188, 189, 193, 196, 201, 206, 207, 209, 216, 221

Offset: 1

Author

Eleonora Echeverri-Toro, Nov 09 2011

Keywords

• nonn
• easy

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A186243.

Programs

Magma

[ p div 6 +1: p in PrimesUpTo(1326) | not IsPrime(p+4) and p mod 6 eq 1 ]; // Bruno Berselli, Nov 09 2011

Mathematica

Select[Range[221], PrimeQ[6 # - 5] && ! PrimeQ[6 # - 1] &] (* T. D. Noe, Nov 09 2011 *)

PARI

is(n)=isprime(6*n-5) && !isprime(6*n-1) \\ Charles R Greathouse IV, Jun 13 2017