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 A124217 #7 Jun 14 2016 16:33:22
%S A124217 1,1,2,5,11,26,61,143,336,789,1853,4352,10221,24005,56378,132409,
%T A124217 310975,730354,1715305,4028555,9461440,22221081,52188297,122569120,
%U A124217 287865097,676078233,1587833266,3729175645,8758319459,20569736330
%N A124217 Expansion of (1-x-x^2)/(1-2x-x^2+x^4).
%C A124217 Diagonal sums of generalized Pascal triangle A124216.
%H A124217 Harvey P. Dale, <a href="/A124217/b124217.txt">Table of n, a(n) for n = 0..1000</a>
%H A124217 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,0,-1).
%F A124217 a(n)=sum{k=0..floor(n/2), sum{j=0..n-k, C(n-k,j)C(j,2(j-k))2^(j-k)}};
%t A124217 CoefficientList[Series[(1-x-x^2)/(1-2x-x^2+x^4),{x,0,40}],x] (* or *) LinearRecurrence[{2,1,0,-1},{1,1,2,5},40] (* _Harvey P. Dale_, Jun 14 2016 *)
%K A124217 easy,nonn
%O A124217 0,3
%A A124217 _Paul Barry_, Oct 19 2006