A025678 -id:A025678 - cp's OEIS frontend

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

David W. Wilson

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

Subsequence of A003586.

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

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 *)

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

A025694 Index of 2^n within sequence of numbers of form 2^i * 9^j.

Original entry on oeis.org

1, 2, 3, 4, 6, 8, 10, 13, 16, 19, 23, 27, 31, 36, 41, 46, 52, 58, 64, 70, 77, 84, 91, 99, 107, 115, 124, 133, 142, 152, 162, 172, 183, 194, 205, 217, 229, 241, 253, 266, 279, 292, 306, 320, 334, 349, 364, 379, 395, 411, 427, 444, 461, 478, 496, 514, 532, 550, 569, 588, 607

Offset: 0

David W. Wilson

Positions of zeros in A025678. - R. J. Mathar, Jul 06 2025

Robert Israel, Table of n, a(n) for n = 0..10000

f:= proc(n) local j,s,t;
      t:= n;
      for j from 1 do
         s:= ilog2(9^j);
         if s >= n then return t fi;
         t:= t + n-1-s;
      od
end proc:
map(f, [$1..100]); # Robert Israel, Dec 09 2016

a(n)=my(N=1); n+1+sum(i=1, n, logint(N<<=1, 9)); \\ Charles R Greathouse IV, Jan 11 2018

first(n)=my(s, N=1/2); vector(n+1, i, s+=logint(N<<=1, 9)+1) \\ Charles R Greathouse IV, Jan 11 2018

Offset corrected by Charles R Greathouse IV, Jan 11 2018

cp's OEIS Frontend

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

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