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 A160383 #19 Feb 06 2021 17:02:10
%S A160383 0,0,0,1,0,0,0,1,0,0,0,1,1,1,1,2,0,0,0,1,0,0,0,1,0,0,0,1,1,1,1,2,0,0,
%T A160383 0,1,0,0,0,1,0,0,0,1,1,1,1,2,1,1,1,2,1,1,1,2,1,1,1,2,2,2,2,3,0,0,0,1,
%U A160383 0,0,0,1,0,0,0,1,1,1,1,2,0,0,0,1,0,0,0,1,0,0,0,1,1,1,1,2,0,0,0,1,0,0,0,1,0
%N A160383 Number of 3's in base-4 representation of n.
%H A160383 F. T. Adams-Watters and 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 A160383 Recurrence relation: a(0) = 0, a(4m+3) = 1+a(m), a(4m) = a(4m+1) = a(4m+2) = a(m).
%F A160383 G.f.: (1/(1-z))*Sum_{m>=0} (z^(3*4^m)*(1 - z^(4^m))/(1 - z^(4^(m+1)))).
%F A160383 Morphism: 0, j -> j,j,j,j+1; e.g., 0 -> 0001 -> 0001000100011112 -> ...
%o A160383 (PARI) a(n) = #select(x->(x==3), digits(n, 4)); \\ _Michel Marcus_, Mar 24 2020
%Y A160383 Cf. A007090 (base 4), A160380 (0's), A160381 (1's), A160382 (2's).
%Y A160383 Cf. A283316 (mod 2).
%K A160383 nonn,base,easy
%O A160383 0,16
%A A160383 _Frank Ruskey_, Jun 05 2009