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 A077903 #15 Sep 03 2019 02:29:26
%S A077903 1,0,2,-3,6,-12,25,-48,98,-195,390,-780,1561,-3120,6242,-12483,24966,
%T A077903 -49932,99865,-199728,399458,-798915,1597830,-3195660,6391321,
%U A077903 -12782640,25565282,-51130563,102261126,-204522252,409044505,-818089008,1636178018,-3272356035,6544712070
%N A077903 Expansion of (1-x)^(-1)/(1 + x - x^2 + 2*x^3).
%C A077903 Convolution of A010892(n) and (-1)^n*A001045(n+1). The positive sequence has g.f. 1/((1-x-2x^2)*(1+x+x^2)). This is the convolution of A001045(n+1) and A049347(n). - _Paul Barry_, May 19 2004
%H A077903 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,-3,2)
%F A077903 G.f.: 1/((1+x-2x^2)*(1-x+x^2));
%F A077903 a(n) = Sum_{k=0..n} (2*(-2)^k/3 + 1/3)*2*sin(Pi*(n-k)/3 + Pi/3)/sqrt(3);
%F A077903 a(n) = 2^(n+3)*cos(Pi*n)/21 + 8*sqrt(3)*cos(Pi*n/3 + Pi/6)/63 + 4*sqrt(3)*sin(Pi*n/3 + Pi/3)/63 + 2*sqrt(3)*sin(Pi*n/3)/9 + 1/3. - _Paul Barry_, May 19 2004
%F A077903 a(n) = 1/3 + (-1)^n*2^(n+3)/21 - A117373(n+1)/7. - _R. J. Mathar_, Sep 27 2012
%t A077903 CoefficientList[Series[(1-x)^(-1)/(1+x-x^2+2x^3),{x,0,40}],x] (* or *) LinearRecurrence[{0,2,-3,2},{1,0,2,-3},40] (* _Harvey P. Dale_, Apr 25 2016 *)
%K A077903 sign
%O A077903 0,3
%A A077903 _N. J. A. Sloane_, Nov 17 2002