A242332 Numbers k such that k^2 + 4 is a semiprime.

0, 9, 19, 21, 23, 25, 31, 41, 43, 51, 53, 55, 63, 69, 71, 75, 77, 79, 83, 91, 93, 105, 107, 109, 113, 119, 123, 129, 131, 133, 143, 145, 149, 151, 153, 157, 165, 171, 173, 175, 181, 185, 187, 191, 195, 197, 201, 209, 221, 223, 225, 227, 241, 249, 251, 257, 259

Offset: 1

Author

Vincenzo Librandi, May 14 2014

Keywords

• nonn

Comments

The semiprimes of this form are: 4, 85, 365, 445, 533, 629, 965, 1685, 1853, 2605, 2813, 3029, 3973, 4765, 5045, 5629, 5933, 6245, ...

Links

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

Crossrefs

Cf. A005473, A007591, A085722, A242330, A242331, A242333.

Programs

Magma

IsSemiprime:=func; [n: n in [0..300] | IsSemiprime(s) where s is n^2+4];

Mathematica

Select[Range[0, 300], PrimeOmega[#^2 + 4] == 2 &]