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 A271945 #17 Sep 08 2022 08:46:16
%S A271945 0,0,4,4,40,72,428,1036,4816,13712,56020,174612,665080,2180568,
%T A271945 7991676,26951708,96696224,331269920,1174584228,4059317284,
%U A271945 14299305416,49658576744,174293008204,606920893484,2125899390576,7413894423728,25940068045428,90539218468404
%N A271945 Expansion of 4*x^2/(1 - x - 9*x^2 + x^3).
%H A271945 Roman Witula, Damian Slota and Adam Warzynski, <a href="http://www.cs.uwaterloo.ca/journals/JIS/VOL9/Slota/slota57.html">Quasi-Fibonacci Numbers of the Seventh Order</a>, J. Integer Seq., 9 (2006), Article 06.4.3 (page 26, table 5).
%H A271945 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,9,-1).
%F A271945 G.f.: 4*x^2/(1 - x - 9*x^2 + x^3).
%F A271945 a(n) = a(n-1) + 9*a(n-2) - a(n-3).
%t A271945 CoefficientList[Series[4 x^2 /(1 - x - 9 x^2 + x^3), {x, 0, 30}], x]
%t A271945 LinearRecurrence[{1,9,-1},{0,0,4},30] (* _Harvey P. Dale_, Jul 18 2021 *)
%o A271945 (Magma) [n le 2 select 2*n*(n-1) else Self(n)+9*Self(n-1)-Self(n-2): n in [0..30]];
%o A271945 (PARI) x='x+O('x^99); concat([0, 0], Vec(4*x^2/(1-x-9*x^2+x^3))) \\ _Altug Alkan_, Apr 18 2016
%o A271945 (Sage) gf = 4*x^2/(1 - x - 9*x^2 + x^3); taylor(gf, x, 0, 30).list() # _Bruno Berselli_, Apr 18 2016
%Y A271945 Cf. A121442, A121458, A215484, A215492, A271944.
%K A271945 nonn,easy
%O A271945 0,3
%A A271945 _Vincenzo Librandi_, Apr 18 2016