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 A102865 #22 Feb 02 2021 22:42:43
%S A102865 1,3,9,35,137,547,2185,8739,34953,139811,559241,2236963,8947849,
%T A102865 35791395,143165577,572662307,2290649225,9162596899,36650387593,
%U A102865 146601550371,586406201481,2345624805923,9382499223689,37529996894755,150119987579017,600479950316067
%N A102865 Base-4 digits are, in order, the first n terms of the sequence (1, 3, 21, 203, 2021, 20203, 202021, 2020203, 20202021, 202020203, ... ).
%H A102865 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (4,1,-4).
%F A102865 4^n = a(n) + A037576(n) for n >= 1.
%F A102865 a(n) + a(n+1) = A039301(n+2).
%F A102865 a(n) = 4*a(n-1) + a(n-2) - 4*a(n-3). - _Harvey P. Dale_, Mar 23 2012
%F A102865 G.f.: 1 + x*(3-3*x-4*x^2)/((1-x)*(1+x)*(1-4*x)). - _Colin Barker_, Aug 28 2012
%t A102865 FromDigits[IntegerDigits[#],4]&/@(NestList[FromDigits[Flatten[ IntegerDigits[#]/.{3->{2,1},1->{0,3}}]]&,1,30]) (* or *) LinearRecurrence[{4,1,-4},{1,3,9},31](* _Harvey P. Dale_, Mar 23 2012 *)
%Y A102865 Cf. A037576.
%K A102865 base,easy,nonn
%O A102865 0,2
%A A102865 _Creighton Dement_, Mar 01 2005
%E A102865 More terms from _Harvey P. Dale_, Mar 23 2012