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 A112520 #5 Nov 28 2016 10:36:05
%S A112520 1,1,1,3,6,21,54,195,573,2086,6646,24289,81540,299136,1040262,3832923,
%T A112520 13660698,50572983,183450064,682447378,2507959846,9374078232,
%U A112520 34791088014,130624080993,488542321116,1841890385196,6931198308924
%N A112520 Expansion of 2/(3-sqrt(3-2*sqrt(1-4x))).
%C A112520 Row sums of number triangle A112519.
%F A112520 a(n)=sum{k=0..n, sum{j=0..n, (k/n)(-1)^(j-k)C(2n-j-1, n-j)C(2j-k-1, j-k)}}.
%t A112520 CoefficientList[Series[2/(3-Sqrt[3-2*Sqrt[1-4x]]),{x,0,30}],x] (* _Harvey P. Dale_, Nov 28 2016 *)
%K A112520 easy,nonn
%O A112520 0,4
%A A112520 _Paul Barry_, Sep 09 2005