A025624 Numbers of form 5^i*9^j, with i, j >= 0.

1, 5, 9, 25, 45, 81, 125, 225, 405, 625, 729, 1125, 2025, 3125, 3645, 5625, 6561, 10125, 15625, 18225, 28125, 32805, 50625, 59049, 78125, 91125, 140625, 164025, 253125, 295245, 390625, 455625, 531441, 703125, 820125, 1265625, 1476225, 1953125

Offset: 1

Author

David W. Wilson

Keywords

• easy
• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Subsequence of A003593.

Programs

Mathematica

f[upto_]:=Module[{maxi=Ceiling[Log[5,upto]],maxj=Ceiling[Log[9,upto]],s}, s=Union[Flatten[Outer[Times,5^Range[0,maxi], 9^Range[0,maxj]]]]; Select[s,#<=upto&]]; f[2000000] (* Harvey P. Dale, Mar 27 2011 *)

Formula

Sum_{n>=1} 1/a(n) = (5*9)/((5-1)*(9-1)) = 45/32. - Amiram Eldar, Sep 24 2020

a(n) ~ exp(sqrt(2*log(5)*log(9)*n)) / sqrt(45). - Vaclav Kotesovec, Sep 24 2020

a(n) = 5^A025654(n) * 9^A025679(n). - R. J. Mathar, Jul 06 2025