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 A077971 #18 Sep 08 2022 08:45:08
%S A077971 1,-1,2,-1,1,2,-3,7,-6,7,1,-6,21,-25,34,-17,1,50,-83,135,-118,87,65,
%T A077971 -214,453,-537,562,-193,-319,1250,-1955,2567,-2022,679,2433,-5798,
%U A077971 9589,-10521,8514,143,-12671,29842,-42227,46727,-29270,-8457,72641,-139638,195365,-189721,105810,95199,-368831
%N A077971 Expansion of 1/(1+x-x^2-2*x^3).
%H A077971 G. C. Greubel, <a href="/A077971/b077971.txt">Table of n, a(n) for n = 0..1000</a>
%H A077971 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,1,2).
%F A077971 a(n) = (-1)^n * A077948(n). - _G. C. Greubel_, Jun 24 2019
%t A077971 LinearRecurrence[{-1,1,2}, {1,-1,2}, 60] (* or *) CoefficientList[Series[ 1/(1 +x-x^2-2*x^3), {x,0,60}], x] (* _G. C. Greubel_, Jun 24 2019 *)
%o A077971 (PARI) Vec(1/(1+x-x^2-2*x^3)+O(x^60)) \\ _Charles R Greathouse IV_, Sep 26 2012
%o A077971 (Magma) R<x>:=PowerSeriesRing(Integers(), 60); Coefficients(R!( 1/(1+x-x^2-2*x^3) )); // _G. C. Greubel_, Jun 24 2019
%o A077971 (Sage) (1/(1+x-x^2-2*x^3)).series(x, 60).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 24 2019
%o A077971 (GAP) a:=[1,-1,2];; for n in [4..60] do a[n]:=-a[n-1]+a[n-2]+2*a[n-3]; od; a; # _G. C. Greubel_, Jun 24 2019
%Y A077971 Cf. A077948.
%Y A077971 First differences of A077901.
%K A077971 sign,easy
%O A077971 0,3
%A A077971 _N. J. A. Sloane_, Nov 17 2002