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 A037521 #33 Jun 28 2023 22:11:33
%S A037521 2,9,36,146,585,2340,9362,37449,149796,599186,2396745,9586980,
%T A037521 38347922,153391689,613566756,2454267026,9817068105,39268272420,
%U A037521 157073089682,628292358729,2513169434916,10052677739666,40210710958665
%N A037521 Base-4 digits are, in order, the first n terms of the periodic sequence with initial period 2, 1, 0.
%H A037521 Vincenzo Librandi, <a href="/A037521/b037521.txt">Table of n, a(n) for n = 1..1000</a>
%H A037521 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (4,0,1,-4).
%F A037521 a(n) = floor(4^(n+1)/7). - _Mircea Merca_, Dec 26 2010
%F A037521 G.f.: x*(2+x) / ( (1-x)*(1-4*x)*(1+x+x^2) ).
%F A037521 a(n) = 4*a(n-1) + a(n-3) - 4*a(n-4).
%F A037521 a(n) = 4^(n+1)/7 - 1/3 - (-1)^(n mod 3)*A167380(4 + (n mod 3))/21 = 2*A033140(n) + A033140(n-1). - _R. J. Mathar_, Jan 08 2011
%p A037521 seq(floor(4^(n+1)/7),n=1..30); # _Mircea Merca_, Dec 26 2010
%t A037521 Table[FromDigits[PadRight[{}, n, {2, 1, 0}], 4], {n, 30}] (* _Harvey P. Dale_, Jul 03 2017 *)
%o A037521 (Magma) [Floor(4^(n+1)/7) : n in [1..30]]; // _Vincenzo Librandi_, Jun 25 2011
%K A037521 nonn,base,easy
%O A037521 1,1
%A A037521 _Clark Kimberling_