OFFSET
0,2
COMMENTS
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..40
FORMULA
a(n) = (1/n) * Sum_{k=1..n} C(2*k^2,k^2) * a(n-k) for n>0 with a(0)=1.
a(n) ~ 4^(n^2) / (sqrt(Pi)*n^2). - Vaclav Kotesovec, Mar 06 2014
EXAMPLE
G.f.: A(x) = 1 + 2*x + 37*x^2 + 16278*x^3 + 150303194*x^4 +...
where
log(A(x)) = 2*x + 70*x^2/2 + 48620*x^3/3 + 601080390*x^4/4 +...+ C(2*n^2,n^2)*x^n/n +...
MATHEMATICA
nmax = 10; b = ConstantArray[0, nmax+1]; b[[1]] = 1; Do[b[[n+1]] = 1/n*Sum[Binomial[2*k^2, k^2]*b[[n-k+1]], {k, 1, n}], {n, 1, nmax}]; b (* Vaclav Kotesovec, Mar 06 2014 *)
PROG
(PARI) {a(n)=polcoeff(exp(sum(m=1, n, binomial(2*m^2, m^2)*x^m/m)+x*O(x^n)), n)}
(PARI) {a(n)=if(n==0, 1, (1/n)*sum(k=1, n, binomial(2*k^2, k^2)*a(n-k)))}
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Dec 02 2011
STATUS
approved