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 A138240 #8 May 25 2013 10:02:07
%S A138240 0,1,6,40,296,2400,20928,192768,1848960,18277888,184890368,1904259072,
%T A138240 19898765312,210424545280,2247494172672,24209586782208,
%U A138240 262696649785344,2868744309571584,31504024885002240,347697247933169664
%N A138240 Expansion of (1/4)(1-sqrt(1-12x)/sqrt(1-4x)).
%C A138240 Hankel transform of a(n) is -4^comb(n,2)*A099156(n)=-4^comb(n,2)*[x^n](x/(1-8x+4x^2)).
%C A138240 Hankel transform of a(n+1) is 4^comb(n+1,2)=A053763(n+1).
%C A138240 Hankel transform of a(n+2) is 4^comb(n+1,2)*A102591(n+1)=4^comb(n+1,2)*[x^n](6-4x)/(1-8x+4x^2).
%H A138240 Vincenzo Librandi, <a href="/A138240/b138240.txt">Table of n, a(n) for n = 0..200</a>
%F A138240 Recurrence: n*a(n) = 4*(4*n-5)*a(n-1) - 48*(n-2)*a(n-2) . - _Vaclav Kotesovec_, Oct 20 2012
%F A138240 a(n) ~ 2^(2*n-7/2)*3^(n+1/2)/(sqrt(Pi)*n^(3/2)) . - _Vaclav Kotesovec_, Oct 20 2012
%t A138240 CoefficientList[Series[1/4*(1-Sqrt[1-12*x]/Sqrt[1-4*x]), {x, 0, 20}], x] (* _Vaclav Kotesovec_, Oct 20 2012 *)
%Y A138240 Cf. A104498.
%K A138240 easy,nonn
%O A138240 0,3
%A A138240 _Paul Barry_, Mar 07 2008