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 A078015 #12 Sep 08 2022 08:45:08
%S A078015 1,0,-1,1,2,-1,-1,4,3,-3,2,11,3,-4,15,25,2,7,55,52,11,69,162,115,91,
%T A078015 300,439,321,482,1039,1199,1124,2003,3277,3522,4251,7283,10076,11295,
%U A078015 15785,24642,31447,38375,56212,80731,101269,132962,193155,262731,335500,459079,649041
%N A078015 Expansion of (1-x)/(1-x+x^2-2*x^3).
%H A078015 G. C. Greubel, <a href="/A078015/b078015.txt">Table of n, a(n) for n = 0..1000</a>
%H A078015 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,-1,2).
%F A078015 G.f.: (1-x)/(1-x+x^2-2*x^3).
%F A078015 a(n) = A077951(n) - A077951(n-1). - _G. C. Greubel_, Jun 29 2019
%t A078015 LinearRecurrence[{1,-1,2}, {1,0,-1}, 60] (* or *) CoefficientList[Series[ (1-x)/(1-x+x^2-2*x^3), {x,0,60}], x] (* _G. C. Greubel_, Jun 29 2019 *)
%o A078015 (PARI) my(x='x+O('x^60)); Vec((1-x)/(1-x+x^2-2*x^3)) \\ _G. C. Greubel_, Jun 29 2019
%o A078015 (Magma) R<x>:=PowerSeriesRing(Integers(), 60); Coefficients(R!( (1-x)/(1-x+x^2-2*x^3) )); // _G. C. Greubel_, Jun 29 2019
%o A078015 (Sage) ((1-x)/(1-x+x^2-2*x^3)).series(x, 60).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 29 2019
%o A078015 (GAP) a:=[1,0,-1];; for n in [4..60] do a[n]:=a[n-1]-a[n-2]+2*a[n-3]; od; a; # _G. C. Greubel_, Jun 29 2019
%Y A078015 Cf. A077951.
%K A078015 sign
%O A078015 0,5
%A A078015 _N. J. A. Sloane_, Nov 17 2002