OFFSET
1,1
COMMENTS
Numbers k such that Sum_{d prime divisor of k} 1/d = 7/10. - Benoit Cloitre, Apr 13 2002
Numbers k such that phi(k) = (2/5)*k. - Benoit Cloitre, Apr 19 2002
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..10000
FORMULA
a(n) = 10*A003592(n).
A143201(a(n)) = 4. - Reinhard Zumkeller, Sep 13 2011
Sum_{n>=1} 1/a(n) = 1/4. - Amiram Eldar, Dec 22 2020
MAPLE
A033846 := proc(n)
if (numtheory[factorset](n) = {2, 5}) then
RETURN(n)
fi: end: seq(A033846(n), n=1..50000); # Jani Melik, Feb 24 2011
MATHEMATICA
Take[Union[Times@@@Select[Flatten[Table[Tuples[{2, 5}, n], {n, 2, 15}], 1], Length[Union[#]]>1&]], 45] (* Harvey P. Dale, Dec 15 2011 *)
PROG
(PARI) isA033846(n)=factor(n)[, 1]==[2, 5]~ \\ Charles R Greathouse IV, Feb 24 2011
(Haskell)
import Data.Set (singleton, deleteFindMin, insert)
a033846 n = a033846_list !! (n-1)
a033846_list = f (singleton (2*5)) where
f s = m : f (insert (2*m) $ insert (5*m) s') where
(m, s') = deleteFindMin s
-- Reinhard Zumkeller, Sep 13 2011
(Magma) [n:n in [1..100000]| Set(PrimeDivisors(n)) eq {2, 5}]; // Marius A. Burtea, May 10 2019
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
Offset fixed by Reinhard Zumkeller, Sep 13 2011
STATUS
approved