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 A133665 #22 Jan 02 2024 08:55:28
%S A133665 1,3,-6,-33,21,318,129,-2733,-3894,20703,55749,-130578,-632319,542883,
%T A133665 6233754,1347807,-54755979,-66886242,425917569,1027893747,-2805364374,
%U A133665 -12056408097,13191871269,121699544142,2972702721,-1092323194557,-1119077519046,8711831231967
%N A133665 a(n) = a(n-1) - 9*a(n-2), a(0) = 1, a(1) = 3.
%H A133665 Harvey P. Dale, <a href="/A133665/b133665.txt">Table of n, a(n) for n = 0..1000</a>
%H A133665 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1,-9).
%F A133665 G.f.: (1+2*x)/(1-x+9*x^2).
%F A133665 a(n) = Sum_{k=0..n} A133607(n,k)*3^k. - _Philippe Deléham_, Dec 29 2007
%t A133665 LinearRecurrence[{1,-9},{1,3},40] (* _Harvey P. Dale_, Apr 21 2019 *)
%o A133665 (PARI) Vec((1+2*x)/(1-x+9*x^2) + O(x^30)) \\ _Jinyuan Wang_, Apr 09 2020
%Y A133665 Cf. A133607.
%K A133665 sign,easy
%O A133665 0,2
%A A133665 _Philippe Deléham_, Dec 28 2007
%E A133665 Corrected and extended by _Harvey P. Dale_, Apr 21 2019