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 A036461 #15 Apr 04 2018 15:43:34
%S A036461 1,0,2,0,2,2,2,2,4,0,4,2,4,2,6,4,2,4,6,2,6,4,6,4,8,2,10,4,4,8,6,8,8,8,
%T A036461 8,6,10,8,10,10,6,6,12,8,10,14,8,10,10,12,16,8,12,18,10,10,14,10,14,
%U A036461 14,16,10,16,12,16,16,14,16,14,18,20,12,20,10,22,12,26,8,20,12,22,14,16
%N A036461 Number of 1 digits in base 3 representation of 2^n.
%C A036461 The number of 1's in the base 3 representation of any even(odd) number is even(odd).
%H A036461 Robert Israel, <a href="/A036461/b036461.txt">Table of n, a(n) for n = 0..10000</a>
%p A036461 seq(numboccur(1,convert(2^n,base,3)),n=0..100); # _Robert Israel_, Apr 04 2018
%t A036461 Table[DigitCount[2^n,3,1],{n,0,120}] (* _Harvey P. Dale_, Mar 14 2011 *)
%o A036461 (PARI) a(n) = #select(x->(x==1), digits(2^n, 3)); \\ _Michel Marcus_, Apr 04 2018
%Y A036461 Cf. A020915 (number of digits), A104320 (number of 0's), A260683 (number of 2's).
%K A036461 base,nonn
%O A036461 0,3
%A A036461 _David W. Wilson_