A112776 Numbers k such that 6k+5 is semiprime.

5, 10, 12, 15, 19, 23, 25, 26, 30, 33, 34, 35, 36, 47, 49, 50, 53, 54, 55, 56, 60, 61, 62, 65, 67, 68, 72, 78, 80, 82, 85, 87, 88, 90, 91, 96, 101, 103, 104, 105, 111, 114, 115, 117, 118, 121, 122, 124, 125, 127, 129, 130, 131, 133, 135, 141, 144, 148, 149, 150

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. A108172 (resulting semiprimes).

Programs

Magma

IsSemiprime:=func; [n: n in [2..150] | IsSemiprime(6*n+5)]; // Vincenzo Librandi, Sep 22 2012

Mathematica

Select[Range[3000], Plus@@Last/@FactorInteger[6# + 5] == 2&] (* Vincenzo Librandi, Sep 22 2012 *)

PARI

is(n)=bigomega(6*n+5)==2 \\ Charles R Greathouse IV, Feb 08 2017

Formula

a(n) = (A108172(n) - 5)/6.