OFFSET
0,3
FORMULA
a(n) = n! * Sum_{k=0..n} 2^(n-k) * (n-k-2)^k / k!.
PROG
(PARI) a(n) = n!*sum(k=0, n, 2^(n-k)*(n-k-2)^k/k!);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Seiichi Manyama, Dec 19 2023
STATUS
approved