OFFSET
0,2
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..200
FORMULA
a(n) = ( Sum_{k=0..[n/2]} 3^(n-2*k)/2^k * n!/((n-2*k)!*k!) )^2.
a(n) ~ n^n*exp(6*sqrt(n)-9/2-n)/2 * (1+15/(4*sqrt(n))). - Vaclav Kotesovec, May 23 2013
D-finite with recurrence: a(n) = (n+8)*a(n-1) + (n-1)*(n+8)*a(n-2) - (n-1)*(n-2)^2*a(n-3). - Vaclav Kotesovec, May 23 2013
EXAMPLE
E.g.f.: A(x) = 1 + 9*x + 100*x^2/2! + 1296*x^3/3! + 19044*x^4/4! + ...
where A(x) = 1 + 3^2*x + 10^2*x^2/2! + 36^2*x^3/3! + 138^2*x^4/4! + ... + A202834(n)^2*x^n/n! + ...
MATHEMATICA
With[{nn=20}, CoefficientList[Series[Exp[((9x)/(1-x))]/Sqrt[1-x^2], {x, 0, nn}], x]Range[0, nn]!] (* Harvey P. Dale, Apr 23 2012 *)
PROG
(PARI) {a(n)=n!*polcoeff(exp(9*x/(1-x)+x*O(x^n))/sqrt(1-x^2+x*O(x^n)), n)}
(PARI) {a(n)=n!^2*polcoeff(exp(3*x+x^2/2+x*O(x^n)), n)^2}
(Magma) R<x>:=PowerSeriesRing(Rationals(), 40); Coefficients(R!(Laplace( Exp(9*x/(1-x))/Sqrt(1-x^2) ))); // G. C. Greubel, Jun 22 2022
(SageMath)
def A202833_list(prec):
P.<x> = PowerSeriesRing(QQ, prec)
return P( exp(9*x/(1-x))/sqrt(1-x^2) ).egf_to_ogf().list()
A202833_list(40) # G. C. Greubel, Jun 22 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Dec 25 2011
STATUS
approved