A025611 Numbers of form 2^i*9^j, with i, j >= 0.

1, 2, 4, 8, 9, 16, 18, 32, 36, 64, 72, 81, 128, 144, 162, 256, 288, 324, 512, 576, 648, 729, 1024, 1152, 1296, 1458, 2048, 2304, 2592, 2916, 4096, 4608, 5184, 5832, 6561, 8192, 9216, 10368, 11664, 13122, 16384, 18432, 20736, 23328, 26244, 32768, 36864, 41472

Offset: 1

Author

David W. Wilson

Keywords

• easy
• nonn

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Subsequence of A003586.

Programs

Maple

N:= 10^5: # for terms <= N
sort([seq(seq(2^i*9^j,i=0..ilog2(N/9^j)),j=0..floor(log[9](N)))]); # Robert Israel, Mar 23 2020

Mathematica

Take[Union[2^First[#] 9^Last[#]&/@Tuples[Range[0,20],{2}]],50] (* Harvey P. Dale, May 26 2011 *)
n = 10^5; Flatten[Table[2^i*9^j, {i, 0, Log[2, n]}, {j, 0, Log[9, n/2^i]}]] // Sort (* Amiram Eldar, Sep 24 2020 *)

Formula

Sum_{n>=1} 1/a(n) = (2*9)/((2-1)*(9-1)) = 9/4. - Amiram Eldar, Sep 24 2020

a(n) ~ exp(sqrt(2*log(2)*log(9)*n)) / sqrt(18). - Vaclav Kotesovec, Sep 24 2020

a(n) = 2^A025638(n) * 9^A025678(n). - R. J. Mathar, Jul 06 2025