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 A077984 #14 Sep 08 2022 08:45:08
%S A077984 1,-2,6,-18,52,-152,444,-1296,3784,-11048,32256,-94176,274960,-802784,
%T A077984 2343840,-6843168,19979584,-58333184,170311872,-497249280,1451788672,
%U A077984 -4238699648,12375475200,-36131927040,105492203776,-307999212032,899246685696,-2625476203008,7665444201472
%N A077984 Expansion of 1/(1+2*x-2*x^2+2*x^3).
%H A077984 G. C. Greubel, <a href="/A077984/b077984.txt">Table of n, a(n) for n = 0..1000</a>
%H A077984 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-2, 2, -2).
%F A077984 a(n) = -2*a(n-1) + 2*a(n-2) - 2*a(n-3).
%F A077984 a(n) is the nearest integer to c*d^n where c=0.7166689603... satisfies 67*c^3 - 67*c^2 + 15*c - 1 = 0 and d=-2.9196395658... satisfies d^3 + 2*d^2 - 2*d + 2 = 0.
%F A077984 a(n) = (-1)^n * A077835(n). - _R. J. Mathar_, Aug 07 2015
%t A077984 CoefficientList[Series[1/(1+2x-2x^2+2x^3),{x,0,30}],x] (* or *) LinearRecurrence[{-2,2,-2},{1,-2,6},30] (* _Harvey P. Dale_, Jun 17 2014 *)
%o A077984 (PARI) my(x='x+O('x^30)); Vec(1/(1+2*x-2*x^2+2*x^3)) \\ _G. C. Greubel_, Jun 25 2019
%o A077984 (Magma) R<x>:=PowerSeriesRing(Integers(), 30); Coefficients(R!( 1/(1+2*x-2*x^2+2*x^3) )); // _G. C. Greubel_, Jun 25 2019
%o A077984 (Sage) (1/(1+2*x-2*x^2+2*x^3)).series(x, 30).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 25 2019
%o A077984 (GAP) a:=[1,-2,6];; for n in [4..30] do a[n]:=-2*a[n-1]+2*a[n-2] - 2*a[n-3]; od; a; # _G. C. Greubel_, Jun 25 2019
%Y A077984 Cf. A077835.
%K A077984 sign,easy
%O A077984 0,2
%A A077984 _N. J. A. Sloane_, Nov 17 2002