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 A077974 #18 Sep 08 2022 08:45:08
%S A077974 1,-1,1,-3,5,-7,13,-23,37,-63,109,-183,309,-527,893,-1511,2565,-4351,
%T A077974 7373,-12503,21205,-35951,60957,-103367,175269,-297183,503917,-854455,
%U A077974 1448821,-2456655,4165565,-7063207,11976517,-20307647,34434061,-58387095,99002389,-167870511,284644701
%N A077974 Expansion of 1/(1+x+2*x^3).
%H A077974 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,0,-2).
%F A077974 a(0)=1, a(1)=-1, a(2)=1, a(n)=a(n-1)-2*a(n-3). - _Harvey P. Dale_, Aug 29 2012
%t A077974 CoefficientList[Series[1/(1+x+2*x^3),{x,0,45}],x] (* or *) LinearRecurrence[ {-1,0,-2},{1,-1,1},45] (* _Harvey P. Dale_, Aug 29 2012 *)
%o A077974 (PARI) Vec(1/(1+x+2*x^3)+O(x^45)) \\ _Charles R Greathouse IV_, Sep 26 2012
%o A077974 (Magma) R<x>:=PowerSeriesRing(Integers(), 45); Coefficients(R!( 1/(1+x+2*x^3) )); // _G. C. Greubel_, Jun 25 2019
%o A077974 (Sage) (1/(1+x+2*x^3)).series(x, 45).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 25 2019
%o A077974 (GAP) a:=[1,-1,1];; for n in [4..45] do a[n]:=-a[n-1]-2*a[n-3]; od; a; # _G. C. Greubel_, Jun 25 2019
%Y A077974 Signed version of A077949.
%K A077974 sign,easy
%O A077974 0,4
%A A077974 _N. J. A. Sloane_, Nov 17 2002