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%I #22 Mar 01 2023 17:31:52
%S 21,63,147,189,441,567,1029,1323,1701,3087,3969,5103,7203,9261,11907,
%T 15309,21609,27783,35721,45927,50421,64827,83349,107163,137781,151263,
%U 194481,250047,321489,352947,413343,453789,583443,750141,964467
%N Numbers whose prime factors are 3 and 7.
%C Numbers k such that phi(k)/k = 4/7, where phi is the Euler totient function A000010. - _Lekraj Beedassy_, Jul 18 2008
%C Subsequence of A143203. - _Reinhard Zumkeller_, Sep 13 2011
%D J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 189, p. 57, Ellipses, Paris 2008.
%H Reinhard Zumkeller, <a href="/A033850/b033850.txt">Table of n, a(n) for n = 1..10000</a>
%F A143201(a(n)) = 5. - _Reinhard Zumkeller_, Sep 13 2011
%F Sum_{n>=1} 1/a(n) = 1/12. - _Amiram Eldar_, Dec 22 2020
%t Select[Range[10^6],Union[FactorInteger[#][[;;,1]]]=={3,7}&] (* _Harvey P. Dale_, Mar 01 2023 *)
%o (Haskell)
%o import Data.Set (singleton, deleteFindMin, insert)
%o a033850 n = a033850_list !! (n-1)
%o a033850_list = f (singleton (3*7)) where
%o f s = m : f (insert (3*m) $ insert (7*m) s') where
%o (m,s') = deleteFindMin s
%o -- _Reinhard Zumkeller_, Sep 13 2011
%Y Cf. A000010, A033845, A033846, A033847, A033848, A033849, A033851, A143203.
%K nonn
%O 1,1
%A _Jeff Burch_
%E Offset fixed by _Reinhard Zumkeller_, Sep 13 2011
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