public function rho($float_min, $float_max)
    {
        self::checkOrder($float_min, $float_max);

        if ($float_min < 0 || $float_max < 0) {
            throw new \InvalidArgumentException('Rho value must be a positive number!');
        }

        if ($this->r || $this->i) {
            throw new \RuntimeException('You cannot set rho value, because algebraic form is in use.');
        }

        $this->rho = new \stdClass();
        $this->rho->min = $float_min;
        $this->rho->max = $float_max;

        return $this;
    }