Function subfactorial 

pub fn subfactorial(n: u64) -> u128

Subfactorial (derangement number): !n = n! * sum(k=0..n) (-1)^k / k!.

Examples

use mathverse_core::integer::subfactorial;

assert_eq!(subfactorial(0), 1);
assert_eq!(subfactorial(1), 0);
assert_eq!(subfactorial(4), 9);