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A143207
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Numbers with exactly 3 distinct prime factors not greater than 5.
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18
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30, 60, 90, 120, 150, 180, 240, 270, 300, 360, 450, 480, 540, 600, 720, 750, 810, 900, 960, 1080, 1200, 1350, 1440, 1500, 1620, 1800, 1920, 2160, 2250, 2400, 2430, 2700, 2880, 3000, 3240, 3600, 3750, 3840, 4050, 4320, 4500, 4800, 4860
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| A001221(a(n))=3; A020639(a(n))=2; A006530(a(n))=5;
A143201(a(n)) = 6;
subsequence of A143204 and of A051037.
Successive k such that EulerPhi[x]/x=4/15. [From Artur Jasinski (grafix(AT)csl.pl), Nov 07 2008]
Numbers of the form 2^i * 3^j * 5^k with i, j, k > 0; a(n)=30*A051037(n); A007947(a(n))=A010869(n); subsequence of A143204. [Reinhard Zumkeller, Sep 13 2011]
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LINKS
| T. D. Noe, Table of n, a(n) for n=1..1000
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MATHEMATICA
| a = {}; Do[If[EulerPhi[x]/x == 4/15, AppendTo[a, x]], {x, 1, 11664}]; a [From Artur Jasinski (grafix(AT)csl.pl), Nov 07 2008]
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PROG
| (Haskell)
import Data.Set (singleton, deleteFindMin, insert)
a143207 n = a143207_list !! (n-1)
a143207_list = f (singleton (2*3*5)) where
f s = m : f (insert (2*m) $ insert (3*m) $ insert (5*m) s') where
(m, s') = deleteFindMin s
-- Reinhard Zumkeller, Sep 13 2011
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CROSSREFS
| Cf. A069819.
Sequence in context: A056954 A050519 A069819 * A108454 A064783 A053014
Adjacent sequences: A143204 A143205 A143206 * A143208 A143209 A143210
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KEYWORD
| nonn
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AUTHOR
| Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Aug 12 2008
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