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 A077993 #12 Sep 08 2022 08:45:08
%S A077993 1,-2,2,-2,4,-8,12,-16,24,-40,64,-96,144,-224,352,-544,832,-1280,1984,
%T A077993 -3072,4736,-7296,11264,-17408,26880,-41472,64000,-98816,152576,
%U A077993 -235520,363520,-561152,866304,-1337344,2064384,-3186688,4919296,-7593984,11722752,-18096128,27934720,-43122688
%N A077993 Expansion of 1/(1+2*x+2*x^2+2*x^3).
%H A077993 G. C. Greubel, <a href="/A077993/b077993.txt">Table of n, a(n) for n = 0..1000</a>
%H A077993 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-2,-2,-2).
%F A077993 a(n) = (-1)^n * A077943(n). - _R. J. Mathar_, Aug 04 2008
%t A077993 LinearRecurrence[{-2,-2,-2}, {1,-2,2}, 50] (* or *) CoefficientList[ Series[1/(1+2*x+2*x^2+2*x^3), {x,0,50}], x] (* _G. C. Greubel_, Jun 27 2019 *)
%o A077993 (PARI) my(x='x+O('x^50)); Vec(1/(1+2*x+2*x^2+2*x^3)) \\ _G. C. Greubel_, Jun 27 2019
%o A077993 (Magma) R<x>:=PowerSeriesRing(Integers(), 50); Coefficients(R!( 1/(1+2*x+2*x^2+2*x^3) )); // _G. C. Greubel_, Jun 27 2019
%o A077993 (Sage) (1/(1+2*x+2*x^2+2*x^3)).series(x, 50).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 27 2019
%o A077993 (GAP) a:=[1,-2,2];; for n in [4..50] do a[n]:=-2*(a[n-1]+a[n-2]+a[n-3]); od; a; # _G. C. Greubel_, Jun 27 2019
%Y A077993 Cf. A077943.
%K A077993 sign
%O A077993 0,2
%A A077993 _N. J. A. Sloane_, Nov 17 2002