A033850 Numbers whose prime factors are 3 and 7.

21, 63, 147, 189, 441, 567, 1029, 1323, 1701, 3087, 3969, 5103, 7203, 9261, 11907, 15309, 21609, 27783, 35721, 45927, 50421, 64827, 83349, 107163, 137781, 151263, 194481, 250047, 321489, 352947, 413343, 453789, 583443, 750141, 964467

Offset

1,1

Comments

Numbers k such that phi(k)/k = 4/7, where phi is the Euler totient function A000010. - Lekraj Beedassy, Jul 18 2008

Subsequence of A143203. - Reinhard Zumkeller, Sep 13 2011

References

J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 189, p. 57, Ellipses, Paris 2008.

Links

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

Formula

A143201(a(n)) = 5. - Reinhard Zumkeller, Sep 13 2011

Sum_{n>=1} 1/a(n) = 1/12. - Amiram Eldar, Dec 22 2020

Mathematica

Select[Range[10^6], Union[FactorInteger[#][[;; , 1]]]=={3, 7}&] (* Harvey P. Dale, Mar 01 2023 *)

Prog

(Haskell)
import Data.Set (singleton, deleteFindMin, insert)
a033850 n = a033850_list !! (n-1)
a033850_list = f (singleton (3*7)) where
   f s = m : f (insert (3*m) $ insert (7*m) s') where
     (m, s') = deleteFindMin s
-- Reinhard Zumkeller, Sep 13 2011

Crossrefs

Cf. A000010, A033845, A033846, A033847, A033848, A033849, A033851, A143203.

Sequence in context: A143203 A082060 A025525 * A251213 A170930 A113622

Adjacent sequences: A033847 A033848 A033849 * A033851 A033852 A033853

Keyword

nonn

Author

Jeff Burch

Extensions

Offset fixed by Reinhard Zumkeller, Sep 13 2011

Status

approved