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 A077976 #21 Sep 08 2022 08:45:08
%S A077976 1,-1,0,-1,3,-2,1,-5,8,-5,7,-18,21,-17,32,-57,59,-66,121,-173,184,
%T A077976 -253,415,-530,621,-921,1360,-1681,2163,-3202,4401,-5525,7528,-10805,
%U A077976 14327,-18578,25861,-35937,47232,-63017,87659,-119106,157481,-213693,294424,-395693,528655,-721810,984541,-1320041
%N A077976 Expansion of 1/(1+x+x^2+2*x^3).
%H A077976 G. C. Greubel, <a href="/A077976/b077976.txt">Table of n, a(n) for n = 0..1000</a>
%H A077976 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,-1,-2).
%t A077976 LinearRecurrence[{-1, -1, -2}, {1, -1, 0}, 50] (* _Vladimir Joseph Stephan Orlovsky_, Feb 24 2012 *)
%o A077976 (PARI) my(x='x+O('x^50)); Vec(1/(1+x+x^2+2*x^3)) \\ _G. C. Greubel_, Jun 25 2019
%o A077976 (Magma) R<x>:=PowerSeriesRing(Integers(), 50); Coefficients(R!( 1/(1+x+x^2+2*x^3) )); // _G. C. Greubel_, Jun 25 2019
%o A077976 (Sage) (1/(1+x+x^2+2*x^3)).series(x, 50).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 25 2019
%o A077976 (GAP) a:=[1,-1,0];; for n in [4..50] do a[n]:=-a[n-1]-a[n-2]-2*a[n-3]; od; a; # _G. C. Greubel_, Jun 25 2019
%Y A077976 Partial sums give: A077909.
%K A077976 sign,easy
%O A077976 0,5
%A A077976 _N. J. A. Sloane_, Nov 17 2002