A199716 Numbers k such that 6k-5 and 6k-1 are both composite.

16, 20, 21, 35, 37, 50, 51, 54, 55, 61, 66, 68, 76, 79, 81, 83, 86, 89, 90, 105, 112, 115, 118, 121, 125, 128, 130, 131, 134, 141, 142, 145, 146, 149, 150, 151, 153, 156, 160, 161, 165, 168, 180, 181, 186, 190, 191, 195, 200, 202, 208, 211, 212, 219, 223

Offset: 1

Author

Eleonora Echeverri-Toro, Nov 09 2011

Keywords

• nonn
• easy

Crossrefs

Cf. A186243.

Programs

Magma

[ n: n in [1..223] | not IsPrime(6*n-5) and not IsPrime(6*n-1) ]; // Bruno Berselli, Nov 09 2011

Mathematica

Select[Range[223], ! PrimeQ[6#-5] && ! PrimeQ[6#-1] &] (* T. D. Noe, Nov 09 2011 *)
Select[Range[250],AllTrue[6#+{-5,-1},CompositeQ]&] (* Requires Mathematica version 10 or later *)  (* Harvey P. Dale, Oct 11 2018 *)

PARI

for(n=1,1e3,if(!isprime(6*n-5)&&!isprime(6*n-1),print1(n", "))) \\ Charles R Greathouse IV, Nov 10 2011