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A168638 Number of distinct prime divisors of n is 2 or 3. 2
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

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

FORMULA

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

EXAMPLE

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

MATHEMATICA

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

PROG

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

CROSSREFS

Sequence in context: A325265 A105642 A064040 * A024619 A335080 A323304

Adjacent sequences:  A168635 A168636 A168637 * A168639 A168640 A168641

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Dec 01 2009

STATUS

approved

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Last modified July 4 18:38 EDT 2020. Contains 335448 sequences. (Running on oeis4.)