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A143207 Numbers with distinct prime factors 2, 3, and 5. 18
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; text; internal format)
OFFSET

1,1

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. - Artur Jasinski, 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). - Reinhard Zumkeller, Sep 13 2011

LINKS

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

MATHEMATICA

a = {}; Do[If[EulerPhi[x]/x == 4/15, AppendTo[a, x]], {x, 1, 11664}]; a (* Artur Jasinski, Nov 07 2008 *)

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

(PARI) is(n)=if(n%30, return(0)); my(g=30); while(g>1, n/=30; g=gcd(n, 30)); n==1 \\ Charles R Greathouse IV, Sep 14 2015

(PARI) list(lim)=my(v=List(), s, t); for(i=1, logint(lim\6, 5), t=5^i; for(j=1, logint(lim\t\2, 3), s=t*3^j; while((s<<=1)<=lim, listput(v, s)))); Set(v) \\ Charles R Greathouse IV, Sep 14 2015

(MAGMA) [n: n in [1..5000] | PrimeDivisors(n) eq [2, 3, 5]]; // Bruno Berselli, Sep 14 2015

CROSSREFS

Cf. A069819.

Sequence in context: A050519 A249674 A069819 * A108454 A235483 A064783

Adjacent sequences:  A143204 A143205 A143206 * A143208 A143209 A143210

KEYWORD

nonn,easy

AUTHOR

Reinhard Zumkeller, Aug 12 2008

EXTENSIONS

New name from Charles R Greathouse IV, Sep 14 2015

STATUS

approved

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Last modified April 28 18:26 EDT 2017. Contains 285579 sequences.