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 A113312 #7 Jul 31 2015 18:10:02
%S A113312 1,2,3,3,4,3,5,2,7,-1,12,-9,25,-30,59,-85,148,-229,381,-606,991,-1593,
%T A113312 2588,-4177,6769,-10942,17715,-28653,46372,-75021,121397,-196414,
%U A113312 317815,-514225,832044,-1346265,2178313,-3524574,5702891,-9227461,14930356,-24157813,39088173,-63245982,102334159
%N A113312 Expansion of (1+x)^2/(1-2x^2+x^3).
%C A113312 Diagonal sums of A113310.
%H A113312 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (0, 2, -1).
%F A113312 a(n)=2a(n-2)-a(n-3); a(n)=sum{k=0..floor(n/2), sum{i=0..n-2k, (-1)^i*C(i+k-2, i)}}.
%t A113312 LinearRecurrence[{0,2,-1},{1,2,3},50] (* _Harvey P. Dale_, May 19 2014 *)
%K A113312 easy,sign
%O A113312 0,2
%A A113312 _Paul Barry_, Oct 25 2005