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A033847 Numbers whose prime factors are 2 and 7.

%I A033847 #25 Dec 22 2020 02:15:45
%S A033847 14,28,56,98,112,196,224,392,448,686,784,896,1372,1568,1792,2744,3136,
%T A033847 3584,4802,5488,6272,7168,9604,10976,12544,14336,19208,21952,25088,
%U A033847 28672,33614,38416,43904,50176,57344,67228,76832,87808,100352,114688
%N A033847 Numbers whose prime factors are 2 and 7.
%C A033847 Numbers k such that phi(k) = (3/7)*k - _Benoit Cloitre_, Apr 19 2002
%C A033847 Subsequence of A143204. - _Reinhard Zumkeller_, Sep 13 2011
%H A033847 Reinhard Zumkeller, <a href="/A033847/b033847.txt">Table of n, a(n) for n = 1..10000</a>
%F A033847 A143201(a(n)) = 6. - _Reinhard Zumkeller_, Sep 13 2011
%F A033847 Sum_{n>=1} 1/a(n) = 1/6. - _Amiram Eldar_, Dec 22 2020
%t A033847 With[{nn=20},Select[Union[Flatten[Table[2^n 7^k,{n,nn},{k,nn}]]],#<=2^nn 7&]] (* _Harvey P. Dale_, Nov 25 2020 *)
%o A033847 (Haskell)
%o A033847 import Data.Set (singleton, deleteFindMin, insert)
%o A033847 a033847 n = a033847_list !! (n-1)
%o A033847 a033847_list = f (singleton (2*7)) where
%o A033847    f s = m : f (insert (2*m) $ insert (7*m) s') where
%o A033847      (m,s') = deleteFindMin s
%o A033847 -- _Reinhard Zumkeller_, Sep 13 2011
%Y A033847 Cf. A033845, A033846, A033848, A033849, A033850, A033851, A143204.
%K A033847 nonn
%O A033847 1,1
%A A033847 _Jeff Burch_
%E A033847 Offset fixed by _Reinhard Zumkeller_, Sep 13 2011