A122488 Numbers k such that 1 + 2k + 3k^2 is semiprime.

1, 3, 4, 5, 6, 8, 9, 10, 11, 15, 20, 21, 22, 29, 30, 33, 35, 36, 40, 42, 45, 47, 48, 50, 54, 57, 59, 60, 64, 66, 69, 72, 75, 76, 77, 78, 80, 81, 82, 84, 86, 89, 95, 96, 100, 102, 108, 110, 111, 112, 113, 116, 123, 125, 130, 131, 132, 134, 136, 143, 146, 147, 148, 152, 153

Offset: 1

Author

Zak Seidov, Oct 20 2006

Keywords

• nonn

Examples

			k=3 is a term because 1 + 2k + 3k^2 = 34 = 2*17 (semiprime);
k=4 is a term because 1 + 2k + 3k^2 = 57 = 3*19 (semiprime); etc.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A086285 (numbers k such that 1 + 2k + 3k^2 is prime).

Programs

Maple

filter:= n -> numtheory:-bigomega(1+2*n+3*n^2)=2:
select(filter, [$1..200]); # Robert Israel, Nov 03 2022

Mathematica

Select[Range[160],PrimeOmega[1+2#+3#^2]==2&] (* Harvey P. Dale, Aug 22 2011 *)