This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A025631 #17 Jul 06 2025 10:19:43
%S A025631 1,7,9,49,63,81,343,441,567,729,2401,3087,3969,5103,6561,16807,21609,
%T A025631 27783,35721,45927,59049,117649,151263,194481,250047,321489,413343,
%U A025631 531441,823543,1058841,1361367,1750329,2250423,2893401,3720087,4782969
%N A025631 Numbers of form 7^i*9^j, with i, j >= 0.
%H A025631 Amiram Eldar, <a href="/A025631/b025631.txt">Table of n, a(n) for n = 1..10000</a>
%F A025631 Sum_{n>=1} 1/a(n) = (7*9)/((7-1)*(9-1)) = 21/16. - _Amiram Eldar_, Sep 24 2020
%F A025631 a(n) ~ exp(sqrt(2*log(7)*log(9)*n)) / sqrt(63). - _Vaclav Kotesovec_, Sep 24 2020
%F A025631 a(n) = 7^A025670(n) * 9^A025681(n). - _R. J. Mathar_, Jul 06 2025
%t A025631 With[{nn=40},Take[Union[7^First[#] 9^Last[#]&/@Tuples[Range[0,nn/4],2]],nn]] (* _Harvey P. Dale_, Mar 06 2014 *)
%Y A025631 Subsequence of A003594.
%K A025631 easy,nonn
%O A025631 1,2
%A A025631 _David W. Wilson_