

A033992


Numbers that are divisible by exactly three different primes.


33



30, 42, 60, 66, 70, 78, 84, 90, 102, 105, 110, 114, 120, 126, 130, 132, 138, 140, 150, 154, 156, 165, 168, 170, 174, 180, 182, 186, 190, 195, 198, 204, 220, 222, 228, 230, 231, 234, 238, 240, 246, 252, 255, 258, 260, 264, 266, 270, 273, 276, 280, 282, 285
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OFFSET

1,1


COMMENTS

This sequence and A000977 are identical through their first 32 terms, but A000977(33) = 210. [Comment edited by Jon E. Schoenfield, Dec 30 2014]


LINKS

T. D. Noe, Table of n, a(n) for n = 1..1000


FORMULA

omega(a(n)) = A001221(a(n)) = 3.  Jonathan Vos Post, Sep 20 2005
a(n) ~ 2n log n / (log log n)^2.  Charles R Greathouse IV, Jul 28 2016


EXAMPLE

220=2*2*5*11 is here but 210 is not; compare A000977.


MAPLE

A033992 := proc(n)
if (nops(numtheory[factorset](n)) = 3) then
RETURN(n)
fi: end: seq(A033992(n), n=1..500); # Jani Melik, Feb 24 2011


MATHEMATICA

Select[Range[0, 6! ], Length[FactorInteger[ # ]]==3&] (* Vladimir Joseph Stephan Orlovsky, Apr 22 2010 *)
Select[Range[300], PrimeNu[#]==3&] (* Harvey P. Dale, May 01 2013 *)


PROG

(Haskell)
a033992 n = a033992_list !! (n1)
a033992_list = filter ((== 3) . a001221) [1..]
 Reinhard Zumkeller, May 03 2013
(PARI) is(n)=omega(n)==3 \\ Charles R Greathouse IV, Apr 28 2015
(PARI) list(lim)=my(v=List(), pq); forprime(p=2, lim\6, forprime(q=2, min(p2, lim\p\2), pq=p*q; forprime(r=2, min(q1, lim\r), listput(


CROSSREFS

Cf. A000977, A007774, A000961, A033993, A051270, A112802.
Cf. A225228 (subsequence).
Sequence in context: A114816 A299991 A000977 * A214195 A308127 A091454
Adjacent sequences: A033989 A033990 A033991 * A033993 A033994 A033995


KEYWORD

nonn


AUTHOR

Labos Elemer


STATUS

approved



