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 A143207 Numbers with distinct prime factors 2, 3, and 5. 24
 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 Numbers of the form 2^i * 3^j * 5^k with i, j, k > 0. - Reinhard Zumkeller, Sep 13 2011 Integers k such that phi(k)/k = 4/15. - Artur Jasinski, Nov 07 2008 LINKS Vaclav Kotesovec, Table of n, a(n) for n = 1..10000 (terms 1..1000 from T. D. Noe) FORMULA A001221(a(n)) = 3; A020639(a(n)) = 2; A006530(a(n)) = 5; A143201(a(n)) = 6. a(n) = 30*A051037(n); A007947(a(n)) = A010869(n). - Reinhard Zumkeller, Sep 13 2011 a(n) ~ sqrt(30) * exp((6*log(2)*log(3)*log(5)*n)^(1/3)). - Vaclav Kotesovec, Sep 22 2020 Sum_{n>=1} 1/a(n) = 1/8. - Amiram Eldar, Sep 24 2020 MATHEMATICA a = {}; Do[If[EulerPhi[x]/x == 4/15, AppendTo[a, x]], {x, 1, 11664}]; a (* Artur Jasinski, Nov 07 2008 *) n = 10^4; Table[2^i*3^j*5^k, {i, 1, Log[2, n]}, {j, 1, Log[3, n/2^i]}, {k, 1, Log[5, n/(2^i*3^j)]}] // Flatten // Sort (* Amiram Eldar, Sep 24 2020 *) 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) 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 (PARI) is(n) = if(n%30, return(0)); my(f=factor(n, 6)[, 1]); f[#f]<6 \\ David A. Corneth, Sep 22 2020 (MAGMA) [n: n in [1..5000] | PrimeDivisors(n) eq [2, 3, 5]]; // Bruno Berselli, Sep 14 2015 CROSSREFS Cf. A069819. Subsequence of A143204 and of A051037. Sequence in context: A249674 A050519 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 October 22 05:01 EDT 2020. Contains 337950 sequences. (Running on oeis4.)