OFFSET
0,5
FORMULA
a(n) = n! * Sum_{k=0..floor(n/4)} (-1)^k * binomial(n-3*k,k)/(n-3*k)!.
a(n) = a(n-1) - 4! * binomial(n-1,3) * a(n-4) for n > 3.
PROG
(PARI) my(N=40, x='x+O('x^N)); Vec(serlaplace(exp(x*(1-x^3))))
(PARI) a(n) = n!*sum(k=0, n\4, (-1)^k*binomial(n-3*k, k)/(n-3*k)!);
(PARI) a(n) = if(n<4, 1, a(n-1)-4!*binomial(n-1, 3)*a(n-4));
CROSSREFS
KEYWORD
sign
AUTHOR
Seiichi Manyama, Feb 25 2022
STATUS
approved