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

Author

Jeff Burch

Keywords

• nonn

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

Crossrefs

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

Programs

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

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 *)

Formula

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

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

Extensions

Offset fixed by Reinhard Zumkeller, Sep 13 2011