OFFSET
0,2
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 0..200
FORMULA
a(n) = ( Sum_{k=0..[n/2]} 2^(n-3*k)*n!/((n-2*k)!*k!) )^2. [From formula by Huajun Huang in A005425]
a(n) ~ n^n*exp(4*sqrt(n)-2-n)/2 * (1+5/(3*sqrt(n))). - Vaclav Kotesovec, May 23 2013
D-finite with recurrence: a(n) = (n+3)*a(n-1) +(n-1)*(n+3)*a(n-2) - (n-1)*(n-2)^2*a(n-3). - Vaclav Kotesovec, May 23 2013
EXAMPLE
E.g.f.: A(x) = 1 + 4*x + 25*x^2/2! + 196*x^3/3! + 1849*x^4/4! +...
where A(x) = 1 + 2^2*x + 5^2*x^2/2! + 14^2*x^3/3! + 43^2*x^4/4! +...+ A005425(n)^2*x^n/n! +...
MATHEMATICA
With[{nn=20}, CoefficientList[Series[Exp[(4x)/(1-x)]/Sqrt[1-x^2], {x, 0, nn}], x]Range[0, nn]!] (* Harvey P. Dale, Dec 31 2011 *)
PROG
(PARI) {a(n)=n!*polcoeff(exp(4*x/(1-x)+x*O(x^n))/sqrt(1-x^2+x*O(x^n)), n)}
(PARI) {a(n)=sum(k=0, n\2, 2^(n-3*k)*n!/((n-2*k)!*k!))^2}
(Magma) R<x>:=PowerSeriesRing(Rationals(), 30); Coefficients(R!(Laplace( Exp(4*x/(1-x))/Sqrt(1-x^2) ))); // G. C. Greubel, Jun 21 2022
(SageMath)
def A202827_list(prec):
P.<x> = PowerSeriesRing(QQ, prec)
return P( exp(4*x/(1-x))/sqrt(1-x^2) ).egf_to_ogf().list()
A202827_list(40) # G. C. Greubel, Jun 21 2022
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Dec 25 2011
STATUS
approved