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 A077921 #23 Apr 15 2016 13:55:16
%S A077921 1,-1,4,-8,21,-49,120,-288,697,-1681,4060,-9800,23661,-57121,137904,
%T A077921 -332928,803761,-1940449,4684660,-11309768,27304197,-65918161,
%U A077921 159140520,-384199200,927538921,-2239277041,5406093004,-13051463048,31509019101,-76069501249,183648021600
%N A077921 Expansion of (1-x)^(-1)/(1+2*x-x^2).
%C A077921 Partial sums of sequence of signed Pell numbers (-1)^n*A000129(n). - _Paul Barry_, May 09 2003
%H A077921 Colin Barker, <a href="/A077921/b077921.txt">Table of n, a(n) for n = 0..1000</a>
%H A077921 <a href="/index/Tu#2wis">Index entries for two-way infinite sequences</a>
%H A077921 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,3,-1).
%F A077921 G.f.: 1/((1-x)*(1+2*x-x^2)).
%F A077921 From _Colin Barker_, Apr 15 2016: (Start)
%F A077921 a(n) = -a(n-1)+3*a(n-2)-a(n-3) for n>2.
%F A077921 a(n) = (2-(-1-sqrt(2))^(1+n)-(-1+sqrt(2))^(1+n))/4.
%F A077921 (End)
%F A077921 E.g.f.: (1/4)*(2*exp(x) + (1 + sqrt(2))*exp((-1-sqrt(2))*x) - (sqrt(2) - 1)*exp((sqrt(2)-1)*x)). - _Ilya Gutkovskiy_, Apr 15 2016
%t A077921 CoefficientList[Series[(1/(1-x))/(1+2x-x^2), {x,0,50}], x] (* _Harvey P. Dale_, Mar 20 2011 *)
%o A077921 (PARI) Vec((1-x)^(-1)/(1+2*x-x^2)+O(x^99)) \\ _Charles R Greathouse IV_, Sep 26 2012
%Y A077921 -a(-3-n) = A048739(n).
%K A077921 sign,easy
%O A077921 0,3
%A A077921 _N. J. A. Sloane_, Nov 17 2002