cp's OEIS Frontend

A033851 Numbers whose prime factors are 5 and 7.

%I A033851 #31 Dec 22 2020 03:49:08
%S A033851 35,175,245,875,1225,1715,4375,6125,8575,12005,21875,30625,42875,
%T A033851 60025,84035,109375,153125,214375,300125,420175,546875,588245,765625,
%U A033851 1071875,1500625,2100875,2734375,2941225,3828125,4117715,5359375,7503125
%N A033851 Numbers whose prime factors are 5 and 7.
%C A033851 Numbers k such that phi(k)/k == 24/35. - _Artur Jasinski_, Nov 09 2008
%C A033851 Subsequence of A143202. - _Reinhard Zumkeller_, Sep 13 2011
%H A033851 Reinhard Zumkeller, <a href="/A033851/b033851.txt">Table of n, a(n) for n = 1..10000</a>
%F A033851 a(n) = 35*A003595(n). - _Artur Jasinski_, Nov 09 2008
%F A033851 A143201(a(n)) = 3. - _Reinhard Zumkeller_, Sep 13 2011
%F A033851 Sum_{n>=1} 1/a(n) = 1/24. - _Amiram Eldar_, Dec 22 2020
%t A033851 a = {}; Do[If[EulerPhi[x]/x == 24/35, AppendTo[a, x]], {x, 1, 10000}]; a (* _Artur Jasinski_, Nov 09 2008 *)
%t A033851 Take[With[{nn=10},Sort[Flatten[Table[5^i 7^j,{i,nn},{j,nn}]]]],40] (* _Harvey P. Dale_, Feb 09 2013 *)
%o A033851 (Haskell)
%o A033851 import Data.Set (singleton, deleteFindMin, insert)
%o A033851 a033851 n = a033851_list !! (n-1)
%o A033851 a033851_list = f (singleton (5*7)) where
%o A033851    f s = m : f (insert (5*m) $ insert (7*m) s') where
%o A033851      (m,s') = deleteFindMin s
%o A033851 -- _Reinhard Zumkeller_, Sep 13 2011
%Y A033851 Cf. A003595, A147571-A147575, A147576-A147580. [_Artur Jasinski_, Nov 09 2008]
%Y A033851 Cf. A033845, A033846, A033847, A033848, A033849, A033850, A143201, A143202.
%Y A033851 Subsequence of A256617.
%K A033851 nonn
%O A033851 1,1
%A A033851 _Jeff Burch_
%E A033851 Offset fixed by _Reinhard Zumkeller_, Sep 13 2011