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
KEYWORD
nonn
AUTHOR
EXTENSIONS
Offset fixed by Reinhard Zumkeller, Sep 13 2011
STATUS
approved