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 A019481 #17 Sep 08 2022 08:44:44
%S A019481 4,12,37,115,358,1115,3473,10818,33697,104963,326950,1018419,3172281,
%T A019481 9881362,30779529,95875387,298642966,930245227,2897627873,9025842914,
%U A019481 28114666161,87574585651,272786737286,849705465187,2646753961545,8244393875250
%N A019481 a(n) = 3*a(n-1) + a(n-2) - 2*a(n-3) (agrees with A019480 for n <= 19 only).
%D A019481 R. K. Guy, personal communication.
%H A019481 Vincenzo Librandi, <a href="/A019481/b019481.txt">Table of n, a(n) for n = 0..1000</a>
%H A019481 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,1,-2)
%F A019481 From _R. J. Mathar_, Aug 22 2008: (Start)
%F A019481 O.g.f.: (4 - 3*x^2)/(1 - 3*x - x^2 + 2*x^3).
%F A019481 a(n) = 4*A100058(n) - 3*A100058(n-2). (End)
%t A019481 LinearRecurrence[{3,1, -2}, {4, 12, 37}, 30] (* _Harvey P. Dale_, Jan 17 2017 *)
%o A019481 (Magma) I:=[4, 12, 37]; [n le 3 select I[n] else 3*Self(n-1)+Self(n-2)-2*Self(n-3): n in [1..30]]; // _Vincenzo Librandi_, Jan 18 2017
%K A019481 nonn,easy
%O A019481 0,1
%A A019481 _N. J. A. Sloane_