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 A124370 #7 Jul 13 2015 21:54:26
%S A124370 1,0,2,3,5,12,21,42,83,159,313,609,1186,2316,4512,8799,17158,33450,
%T A124370 65225,127173,247958,483471,942660,1837989,3583691,6987429,13624009,
%U A124370 26563920,51793996,100987296,196903761,383920500
%N A124370 Expansion of 1/(1-2x^2-3x^3-x^4).
%C A124370 Diagonal sums of number triangle A124369.
%H A124370 Harvey P. Dale, <a href="/A124370/b124370.txt">Table of n, a(n) for n = 0..1000</a>
%H A124370 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0,2,3,1).
%F A124370 a(n)=sum{k=0..floor(n/2), sum{j=0..n-k, C(j,n-k-j)*C((j+k)/2,(j-k)/2)*(1+(-1)^(j-k))/2}}
%F A124370 a(0)=1, a(1)=0, a(2)=2, a(3)=3, a(n)=2*a(n-2)+3*a(n-3)+a(n-4). - _Harvey P. Dale_, Feb 11 2015
%t A124370 CoefficientList[Series[1/(1-2x^2-3x^3-x^4),{x,0,40}],x] (* or *) LinearRecurrence[{0,2,3,1},{1,0,2,3},40] (* _Harvey P. Dale_, Feb 11 2015 *)
%K A124370 easy,nonn
%O A124370 0,3
%A A124370 _Paul Barry_, Oct 27 2006