OFFSET
0,2
FORMULA
a(n) = 2^(-n-1)*Gamma(2*n + 4)/3.
a(n) = (2*(n-1)^2 + 9*n + 1)*a(n-1) for n > 0.
a(n) / n! = A000457(n).
MAPLE
a:= n-> combinat[multinomial](2*n+3, 3, 2$n):
seq(a(n), n=0..17); # Alois P. Heinz, Sep 07 2019
PROG
(SageMath)
def a(n):
return multinomial([3] + [2] * n)
[a(n) for n in range(20)]
CROSSREFS
KEYWORD
nonn
AUTHOR
Peter Luschny, Sep 07 2019
STATUS
approved