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 A078056 #21 Mar 19 2025 09:51:36
%S A078056 1,-3,7,-16,36,-81,182,-409,919,-2065,4640,-10426,23427,-52640,118281,
%T A078056 -265775,597191,-1341876,3015168,-6775021,15223334,-34206521,76861355,
%U A078056 -172705897,388066628,-871977798,1959316327,-4402543824,9892426177,-22228079851,49946042055,-112227737784
%N A078056 Expansion of (1-x)/(1+2*x-x^2-x^3).
%C A078056 Absolute values of pairwise sums are in A052534.
%H A078056 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-2,1,1).
%F A078056 G.f.: (1-x)/(1+2*x-x^2-x^3).
%F A078056 a(n) = -2*a(n-1) + a(n-2) + a(n-3). - _Wesley Ivan Hurt_, Dec 29 2023
%F A078056 a(n) = (-1)^n*(A106805(n)+A106805(n+1)). -_R. J. Mathar_, Mar 19 2025
%t A078056 CoefficientList[Series[(1-x)/(1+2*x-x^2-x^3),{x,0,60}],x] (* _Harvey P. Dale_, Feb 20 2020 *)
%o A078056 (PARI) Vec((1-x)/(1+2*x-x^2-x^3)+O(x^99)) \\ _Charles R Greathouse IV_, Sep 27 2012
%o A078056 (Magma) R<x>:=PowerSeriesRing(Integers(), 40); Coefficients(R!((1-x)/(1+2*x-x^2-x^3))); // _Vincenzo Librandi_, Feb 21 2020
%Y A078056 Cf. A052534.
%K A078056 sign,easy
%O A078056 0,2
%A A078056 _N. J. A. Sloane_, Nov 17 2002