OFFSET
0,5
FORMULA
a(n) = n! * Sum_{k=0..floor(n/3)} (n-3*k)^k * binomial(3*k+1,n-3*k)/( (3*k+1)*k! ).
PROG
(PARI) a(n) = n!*sum(k=0, n\3, (n-3*k)^k*binomial(3*k+1, n-3*k)/((3*k+1)*k!));
CROSSREFS
KEYWORD
nonn
AUTHOR
Seiichi Manyama, Mar 09 2024
STATUS
approved