OFFSET
0,3
FORMULA
a(n) = (n-1)!*sum(k = 1..n, (4^(n-k)*binomial(n/2,n-k))/(k-1)!),n>0, a(0)=1.
a(n) ~ 2^((3*n-1)/2) * n^(n/2) / exp(n/2-1/4) if n is even, and a(n) ~ (-1)^((n+1)/2) * n^(n-1) * 2^n / exp(n+1/2) if n is odd. - Vaclav Kotesovec, Oct 31 2014
a(2*n+1) = 16^n*hypergeom([-2*n],[3/2-n],-1/4)*Gamma(3/2+n)/Gamma(3/2-n); a(2*n+1) ~ (-1)^(n+1)*2*(4*n/exp(1))^(2*n)/exp(1/2). - Peter Luschny, Oct 31 2014
D-finite with recurrence (-2*n+3)*a(n) +(-8*n^3+60*n^2-78*n-15)*a(n-2) +32*(n-2)*(n-3)*(n-4)*(2*n+1)*a(n-4)=0. - R. J. Mathar, Jul 27 2022
MAPLE
A249454 := proc(n)
if n = 0 then
1;
else
(n-1)!*add( 4^(n-k)*binomial(n/2, n-k)/(k-1)! , k=1..n) ;
end if;
end proc;
seq(A249454(n), n=0..40) ; # R. J. Mathar, Jul 27 2022
MATHEMATICA
CoefficientList[Series[E^(x*(Sqrt[4*x^2+1]+2*x)), {x, 0, 20}], x] * Range[0, 20]! (* Vaclav Kotesovec, Oct 31 2014 *)
PROG
(Maxima)
a(n):=if n=0 then 1 else (n-1)!*sum((4^(n-k)*binomial(n/2, n-k))/(k-1)!, k, 1, n);
CROSSREFS
KEYWORD
sign
AUTHOR
Vladimir Kruchinin, Oct 31 2014
STATUS
approved