A033849 Numbers whose prime factors are 3 and 5.

15, 45, 75, 135, 225, 375, 405, 675, 1125, 1215, 1875, 2025, 3375, 3645, 5625, 6075, 9375, 10125, 10935, 16875, 18225, 28125, 30375, 32805, 46875, 50625, 54675, 84375, 91125, 98415, 140625, 151875, 164025, 234375, 253125, 273375, 295245

Offset

1,1

Comments

Numbers k such that phi(k) = (8/15)*k. - Benoit Cloitre, Apr 19 2002

Subsequence of A143202. - Reinhard Zumkeller, Sep 13 2011

Links

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

Formula

From Reinhard Zumkeller, Sep 13 2011: (Start)

A143201(a(n)) = 3.

a(n) = 15*A003593(n). (End)

Sum_{n>=1} 1/a(n) = 1/8. - Amiram Eldar, Dec 22 2020

Mathematica

Sort[Flatten[Table[Table[3^j*5^k, {j, 1, 10}], {k, 1, 10}]]] (* Geoffrey Critzer, Dec 07 2014 *)
Select[Range[300000], FactorInteger[#][[All, 1]]=={3, 5}&] (* Harvey P. Dale, Oct 19 2022 *)

Prog

(Haskell)
import Data.Set (singleton, deleteFindMin, insert)
a033849 n = a033849_list !! (n-1)
a033849_list = f (singleton (3*5)) where
   f s = m : f (insert (3*m) $ insert (5*m) s') where
     (m, s') = deleteFindMin s
-- Reinhard Zumkeller, Sep 13 2011

Crossrefs

Cf. A033845, A033846, A033847, A033848, A033850, A033851, A143201, A143202.

Subsequence of A256617.

Sequence in context: A298462 A066763 A164788 * A194715 A060536 A014634

Adjacent sequences: A033846 A033847 A033848 * A033850 A033851 A033852

Keyword

nonn

Author

Jeff Burch

Extensions

Offset and typo in data fixed by Reinhard Zumkeller, Sep 13 2011

Status

approved