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 A109220 #6 Jun 13 2015 00:51:51
%S A109220 1,3,7,20,53,143,385,1036,2789,7507,20207,54392,146409,394095,1060801,
%T A109220 2855400,7685993,20688691,55688567,149899116,403489373,1086088287,
%U A109220 2923466753,7869210964,21181866061,57016065763,153472396895
%N A109220 Expansion of (1+x-x^2)/(1-2x-2x^2+x^4).
%C A109220 Transform of 2^n under matrix A108756.
%H A109220 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,2,0,-1).
%F A109220 a(n)=2a(n-1)+2a(n-2)-a(n-4); a(n)=a(n)=sum{k=0..n, binomial(floor((n+k+1)/2)+k, floor((n+k)/2)-k)*2^k}.
%K A109220 easy,nonn
%O A109220 0,2
%A A109220 _Paul Barry_, Jun 22 2005