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 A078020 #19 Feb 06 2022 12:57:23
%S A078020 1,0,-2,-2,2,6,2,-10,-14,6,34,22,-46,-90,2,182,178,-186,-542,-170,914,
%T A078020 1254,-574,-3082,-1934,4230,8098,-362,-16558,-15834,17282,48950,14386,
%U A078020 -83514,-112286,54742,279314,169830,-388798,-728458,49138,1506054,1407778,-1604330,-4419886,-1211226,7628546
%N A078020 Expansion of (1-x)/(1-x+2*x^2).
%C A078020 Equals the INVERT transform of [1, -1, -1, 1, 1, -1, -1, 1, 1, ...], i.e., 1 followed by repeats of (-1, -1, 1, 1, ...). - _Gary W. Adamson_, Sep 16 2008
%C A078020 Pisano period lengths: 1, 1, 8, 1, 24, 8, 21, 2, 24, 24, 10, 8, 168, 21, 24, 2, 144, 24, 360, 24, ... - _R. J. Mathar_, Aug 10 2012
%H A078020 G. C. Greubel, <a href="/A078020/b078020.txt">Table of n, a(n) for n = 0..1000</a>
%H A078020 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (1,-2).
%F A078020 a(n) = A107920(n+1) - A107920(n). - _R. J. Mathar_, Mar 14 2011
%F A078020 a(n) = (-1)^n*(A001607(n) + A001607(n-1)). - _G. C. Greubel_, Jun 29 2019
%t A078020 LinearRecurrence[{1,-2}, {1,0}, 50] (* or *) CoefficientList[Series[(1 - x)/(1-x+2*x^2), {x, 0, 50}], x] (* _G. C. Greubel_, Jun 29 2019 *)
%o A078020 (PARI) Vec((1-x)/(1-x+2*x^2)+O(x^50)) \\ _Charles R Greathouse IV_, Sep 25 2012
%o A078020 (Magma) R<x>:=PowerSeriesRing(Integers(), 50); Coefficients(R!( (1-x)/(1-x+2*x^2) )); // _G. C. Greubel_, Jun 29 2019
%o A078020 (Sage) ((1-x)/(1-x+2*x^2)).series(x, 50).coefficients(x, sparse=False) # _G. C. Greubel_, Jun 29 2019
%o A078020 (GAP) a:=[1,0];; for n in [2..50] do a[n]:=a[n-1]-2*a[n-2]; od; a; # _G. C. Greubel_, Jun 29 2019
%Y A078020 Cf. A001607, A107920.
%K A078020 sign,easy
%O A078020 0,3
%A A078020 _N. J. A. Sloane_, Nov 17 2002