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

1,1

Links

Table of n, a(n) for n=1..55.

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 *)

Prog

(Magma) [ n: n in [1..223] | not IsPrime(6*n-5) and not IsPrime(6*n-1) ]; // Bruno Berselli, Nov 09 2011
(PARI) for(n=1, 1e3, if(!isprime(6*n-5)&&!isprime(6*n-1), print1(n", "))) \\ Charles R Greathouse IV, Nov 10 2011

Crossrefs

Cf. A186243.

Sequence in context: A335210 A061934 A195683 * A163901 A044860 A272533

Adjacent sequences: A199713 A199714 A199715 * A199717 A199718 A199719

Keyword

nonn,easy

Author

Eleonora Echeverri-Toro, Nov 09 2011

Status

approved