| public static subfactorial ( integer $n ) : integer | ||
| $n | integer | |
| Результат | integer | number of permutations of n |
public static function subfactorial(int $n)
{
if ($n < 0) {
throw new Exception\OutOfBoundsException('Cannot compute subfactorial of a negative number.');
}
$n！ = self::factorial($n);
$∑ = 0;
for ($i = 0; $i <= $n; $i++) {
$i！ = self::factorial($i);
$∑ += (-1) ** $i / $i！;
}
return $n！ * $∑;
}