OFFSET
0,3
FORMULA
a(n) = Sum_{k=0..floor(n/3)} (n-2*k)! * binomial(n-2*k,k).
PROG
(PARI) my(N=30, x='x+O('x^N)); Vec(sum(k=0, N, k!*(x*(1+x^2))^k))
(PARI) a(n) = sum(k=0, n\3, (n-2*k)!*binomial(n-2*k, k));
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Feb 20 2024
STATUS
approved