OFFSET
0,4
FORMULA
a(n) = n! * Sum_{k=0..floor(n/2)} binomial(3*k,n-2*k)/(2^k * k!).
a(n) = (n-1)/2 * (2*a(n-2) + 9*(n-2)*a(n-3) + 12*(n-2)*(n-3)*a(n-4) + 5*(n-2)*(n-3)*(n-4)*a(n-5)).
PROG
(PARI) a(n) = n!*sum(k=0, n\2, binomial(3*k, n-2*k)/(2^k*k!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jun 16 2024
STATUS
approved