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 A097119 #13 Apr 04 2019 04:48:58
%S A097119 1,1,1,1,3,9,21,41,75,141,281,577,1179,2369,4709,9353,18659,37365,
%T A097119 74889,149937,299827,599289,1198101,2396137,4793051,9587421,19175449,
%U A097119 38349409,76695403,153388273,306778917,613566153,1227140787,2454279365
%N A097119 Expansion of (1-x)^2/((1-x)^3-2x^4).
%H A097119 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,1,2).
%F A097119 G.f.: (1-2x+x^2)/(1-3x+3x^2-x^3-2x^4).
%F A097119 a(n) = 3a(n-1)-3a(n-2)+a(n-3)+2a(n-4).
%F A097119 a(n) = Sum_{k=0..floor(n/3)} binomial(n-k,3k)*2^k.
%t A097119 CoefficientList[Series[(1-x)^2/((1-x)^3-2x^4),{x,0,50}],x] (* _Harvey P. Dale_, Feb 25 2011 *)
%K A097119 easy,nonn
%O A097119 0,5
%A A097119 _Paul Barry_, Jul 25 2004