A025631 Numbers of form 7^i*9^j, with i, j >= 0.

1, 7, 9, 49, 63, 81, 343, 441, 567, 729, 2401, 3087, 3969, 5103, 6561, 16807, 21609, 27783, 35721, 45927, 59049, 117649, 151263, 194481, 250047, 321489, 413343, 531441, 823543, 1058841, 1361367, 1750329, 2250423, 2893401, 3720087, 4782969

Offset: 1

Author

David W. Wilson

Keywords

• easy
• nonn

Links

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

Crossrefs

Subsequence of A003594.

Programs

Mathematica

With[{nn=40},Take[Union[7^First[#] 9^Last[#]&/@Tuples[Range[0,nn/4],2]],nn]] (* Harvey P. Dale, Mar 06 2014 *)

Formula

Sum_{n>=1} 1/a(n) = (7*9)/((7-1)*(9-1)) = 21/16. - Amiram Eldar, Sep 24 2020

a(n) ~ exp(sqrt(2*log(7)*log(9)*n)) / sqrt(63). - Vaclav Kotesovec, Sep 24 2020

a(n) = 7^A025670(n) * 9^A025681(n). - R. J. Mathar, Jul 06 2025