A112775 Numbers k such that 6k+1 is semiprime.

4, 8, 9, 14, 15, 19, 20, 22, 24, 28, 31, 34, 36, 39, 41, 42, 43, 44, 48, 49, 50, 53, 59, 60, 65, 67, 69, 71, 74, 75, 78, 80, 82, 84, 85, 86, 88, 89, 92, 93, 94, 97, 98, 108, 109, 111, 113, 114, 116, 117, 120, 124, 127, 130, 132, 133, 136, 139, 140, 144, 145, 148, 149

Offset: 1

Author

Jonathan Vos Post and Ray Chandler, Oct 15 2005

Keywords

• easy
• nonn

Links

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

Crossrefs

Cf. A112771 (resulting semiprimes).

Programs

Magma

IsSemiprime:=func; [n: n in [2..150] | IsSemiprime(6*n+1)]; // Vincenzo Librandi, Sep 22 2012

Mathematica

Select[Range[200],Plus@@Last/@FactorInteger[6#+1]==2&] (* Vincenzo Librandi, Sep 22 2012 *)

Formula

a(n) = (A112771(n) - 1)/6.