OFFSET
0,4
FORMULA
a(n) = n! * Sum_{k=0..floor(n/3)} binomial(k,n-3*k)/k!.
a(n) = (n-1)*(n-2) * (3*a(n-3) + 4*(n-3)*a(n-4)).
a(n) ~ 2^(n/2 - 1) * exp(-27/1024 + 45*2^(-19/2)*n^(1/4) - 9*n^(1/2)/64 + 2^(-3/2)*n^(3/4) - 3*n/4) * n^(3*n/4) * (1 + 264471/(5*2^(37/2)*n^(1/4))). - Vaclav Kotesovec, Sep 26 2024
PROG
(PARI) a(n) = n!*sum(k=0, n\3, binomial(k, n-3*k)/k!);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Sep 25 2024
STATUS
approved