OFFSET
0,3
FORMULA
a(n) = n! * Sum_{k=0..floor(n/3)} binomial(n-2*k,n-3*k)/(6^k * k!).
PROG
(PARI) a(n) = n!*sum(k=0, n\3, binomial(n-2*k, n-3*k)/(6^k*k!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Jun 18 2024
STATUS
approved