A168638 Number of distinct prime divisors of n is 2 or 3.

6, 10, 12, 14, 15, 18, 20, 21, 22, 24, 26, 28, 30, 33, 34, 35, 36, 38, 39, 40, 42, 44, 45, 46, 48, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 80, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, 100, 102, 104, 105, 106

Offset: 1

Author

Juri-Stepan Gerasimov, Dec 01 2009

Keywords

• nonn

Examples

			2310=2*3*5*7*11 has 5 prime factors, hence it is not here, but it is part of A064040.
		

Links

• G. C. Greubel, Table of n, a(n) for n = 1..1000

Programs

Mathematica

Select[Range[106],MemberQ[{2,3},PrimeNu[#]]&] (* Jayanta Basu, Jun 04 2013 *)

PARI

is(n)=my(o=omega(n)); o==2 || o==3 \\ Charles R Greathouse IV, Jul 28 2016

Formula

a(n) ~ A033992(n) ~ 2n log n / (log log n)^2. - Charles R Greathouse IV, Jul 28 2016