A033847 Numbers whose prime factors are 2 and 7.

14, 28, 56, 98, 112, 196, 224, 392, 448, 686, 784, 896, 1372, 1568, 1792, 2744, 3136, 3584, 4802, 5488, 6272, 7168, 9604, 10976, 12544, 14336, 19208, 21952, 25088, 28672, 33614, 38416, 43904, 50176, 57344, 67228, 76832, 87808, 100352, 114688

Offset

1,1

Comments

Numbers k such that phi(k) = (3/7)*k - Benoit Cloitre, Apr 19 2002

Subsequence of A143204. - Reinhard Zumkeller, Sep 13 2011

Links

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

Formula

A143201(a(n)) = 6. - Reinhard Zumkeller, Sep 13 2011

Sum_{n>=1} 1/a(n) = 1/6. - Amiram Eldar, Dec 22 2020

Mathematica

With[{nn=20}, Select[Union[Flatten[Table[2^n 7^k, {n, nn}, {k, nn}]]], #<=2^nn 7&]] (* Harvey P. Dale, Nov 25 2020 *)

Prog

(Haskell)
import Data.Set (singleton, deleteFindMin, insert)
a033847 n = a033847_list !! (n-1)
a033847_list = f (singleton (2*7)) where
   f s = m : f (insert (2*m) $ insert (7*m) s') where
     (m, s') = deleteFindMin s
-- Reinhard Zumkeller, Sep 13 2011

Crossrefs

Cf. A033845, A033846, A033848, A033849, A033850, A033851, A143204.

Sequence in context: A033011 A321371 A077265 * A067295 A212890 A019552

Adjacent sequences: A033844 A033845 A033846 * A033848 A033849 A033850

Keyword

nonn

Author

Jeff Burch

Extensions

Offset fixed by Reinhard Zumkeller, Sep 13 2011

Status

approved