public static function lahNumber(int $n, int $k)
{
if ($n < 1 || $k < 1) {
throw new Exception\OutOfBoundsException("n and k must be < 1 for Lah Numbers");
}
if ($n < $k) {
throw new Exception\OutOfBoundsException("n must be >= k for Lah Numbers");
}
$nCk = self::combinations($n - 1, $k - 1);
$n! = self::factorial($n);
$k! = self::factorial($k);
return $nCk * ($n! / $k!);
}