OFFSET
0,5
FORMULA
a(n) = n! * Sum_{k=0..floor(n/3)} binomial(2*k,n-3*k)/(6^k * k!).
a(n) = (n-1)*(n-2)/6 * (3*a(n-3) + 8*(n-3)*a(n-4) + 5*(n-3)*(n-4)*a(n-5)).
PROG
(PARI) a(n) = n!*sum(k=0, n\3, binomial(2*k, n-3*k)/(6^k*k!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jun 16 2024
STATUS
approved