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 A037520 #15 Jul 21 2021 13:17:43
%S A037520 2,7,21,65,196,588,1766,5299,15897,47693,143080,429240,1287722,
%T A037520 3863167,11589501,34768505,104305516,312916548,938749646,2816248939,
%U A037520 8448746817,25346240453,76038721360,228116164080,684348492242
%N A037520 Base 3 digits are, in order, the first n terms of the periodic sequence with initial period 2,1,0.
%H A037520 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,0,1,-3)
%F A037520 G.f.: x*(2+x) / ( (x-1)*(3*x-1)*(1+x+x^2) ). - _R. J. Mathar_, Nov 21 2011
%t A037520 With[{c=PadRight[{},27,{2,1,0}]},Table[FromDigits[Take[c,n],3], {n,25}]] (* _Harvey P. Dale_, Nov 08 2011 *)
%K A037520 nonn,base,easy
%O A037520 1,1
%A A037520 _Clark Kimberling_