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 A276229 #20 Sep 08 2022 08:46:17
%S A276229 0,0,1,-1,-1,4,-3,-6,16,-7,-31,61,-6,-147,220,68,-655,739,639,-2772,
%T A276229 2233,3950,-11188,5521,20805,-43035,6946,99929,-156856,-36056,449697,
%U A276229 -534441,-401009,1919588,-1652011,-2588174,7811784,-4287447,-13924295,30310973
%N A276229 a(n+3) = -a(n+2) - 2*a(n+1) + a(n) with a(0)=0, a(1)=0, a(2)=1.
%C A276229 Essentially the same as A077978. - _Georg Fischer_, Oct 02 2018
%H A276229 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,-2,1).
%F A276229 G.f.: x^2/(1 + x + 2*x^2 - x^3).
%F A276229 Let P = (b-c)*(b-d), Q = (c-b)*(b-d), R = (d-b)*(d-c), (b, c, d) be the three roots of x^3 = 2*x^2 + x + 1, then a(n) = P^(-1)*b^(1-n) + Q^(-1)*c^(1-n) + R^(-1)*d^(1-n).
%F A276229 a(2*n) = -3*a(2*n-2) - 6*a(2*n-4) + a(2*n-6).
%t A276229 LinearRecurrence[{-1, -2, 1}, {0, 0, 1}, 50]
%t A276229 CoefficientList[Series[x^2/(1 + x + 2 x^2 - x^3), {x, 0, 39}], x] (* _Michael De Vlieger_, Aug 25 2016 *)
%o A276229 (Magma) I:=[0,0,1]; [n le 3 select I[n] else -Self(n-1)- 2*Self(n-2)+Self(n-3): n in [1..40]]; // _Vincenzo Librandi_, Aug 25 2016
%o A276229 (PARI) concat([0, 0], Vec(x^2/(1+x+2*x^2-x^3) + O(x^99))) \\ _Altug Alkan_, Aug 25 2016
%Y A276229 Cf. A077939, A276228.
%K A276229 sign,easy
%O A276229 0,6
%A A276229 _G. C. Greubel_, Aug 24 2016