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 A160381 #9 Jan 26 2020 16:37:16
%S A160381 0,1,0,0,1,2,1,1,0,1,0,0,0,1,0,0,1,2,1,1,2,3,2,2,1,2,1,1,1,2,1,1,0,1,
%T A160381 0,0,1,2,1,1,0,1,0,0,0,1,0,0,0,1,0,0,1,2,1,1,0,1,0,0,0,1,0,0,1,2,1,1,
%U A160381 2,3,2,2,1,2,1,1,1,2,1,1,2,3,2,2,3,4,3,3,2,3,2,2,2,3,2,2,1,2,1,1,2,3,2,2,1
%N A160381 Number of 1's in base-4 representation of n.
%H A160381 F. T. Adams-Watters, F. Ruskey, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL12/Ruskey2/ruskey14.html">Generating Functions for the Digital Sum and Other Digit Counting Sequences</a>, JIS 12 (2009) 09.5.6
%F A160381 Recurrence relation: a(0) = 0, a(4m+1) = 1+a(m), a(4m) = a(4m+2) = a(4m+3) = a(m).
%F A160381 Generating function: (1/(1-z))*Sum_{m>=0} (z^(4^m)*(1 - z^(4^m))/(1 - z^(4^(m+1)))).
%F A160381 Morphism: 0, j -> j,j+1,j,j; e.g., 0 -> 0100 -> 0100121101000100 -> ...
%t A160381 DigitCount[Range[0,120],4,1] (* _Harvey P. Dale_, Aug 28 2018 *)
%K A160381 nonn,base,easy
%O A160381 0,6
%A A160381 _Frank Ruskey_, Jun 05 2009