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 A033140 #23 Sep 08 2022 08:44:51
%S A033140 1,4,16,65,260,1040,4161,16644,66576,266305,1065220,4260880,17043521,
%T A033140 68174084,272696336,1090785345,4363141380,17452565520,69810262081,
%U A033140 279241048324,1116964193296,4467856773185,17871427092740,71485708370960
%N A033140 Base-4 digits are, in order, the first n terms of the periodic sequence with initial period 1,0,0.
%H A033140 Vincenzo Librandi, <a href="/A033140/b033140.txt">Table of n, a(n) for n = 1..1000</a>
%H A033140 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (4,0,1,-4).
%F A033140 a(n) = +4*a(n-1) +a(n-3) -4*a(n-4). G.f.: x / ( (x-1)*(4*x-1)*(1+x+x^2) ). - R. J. Mathar, Jan 08 2011
%t A033140 LinearRecurrence[{4,0,1,-4},{1,4,16,65},30] (* _Vincenzo Librandi_, Jun 21 2012 *)
%t A033140 Table[FromDigits[PadRight[{},n,{1,0,0}],4],{n,30}] (* _Harvey P. Dale_, Apr 15 2018 *)
%o A033140 (Magma) I:=[1, 4, 16, 65]; [n le 4 select I[n] else 4*Self(n-1)+Self(n-3)-4*Self(n-4): n in [1..30]]; // _Vincenzo Librandi_, Jun 21 2012
%K A033140 nonn,base,easy
%O A033140 1,2
%A A033140 _Clark Kimberling_