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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 (list; graph; refs; listen; history; text; internal format)
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 !! (n-1)

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(p-2, lim\p\2), pq=p*q; forprime(r=2, min(q-1, 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

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Last modified December 8 18:37 EST 2019. Contains 329865 sequences. (Running on oeis4.)