private function getFactors($number)
    {
        $factors = [];

        for ($i = 2; $number / $i >= $i; ++$i) {
            while (0 === $number % $i) {
                $factors[] = $i;
                $number /= $i;
            }
        }

        if (1 < $number) {
            $factors[] = $number;
        }

        return $factors;
    }