public function getS()
    {
        for ($i = 0; $i < $this->n; ++$i) {
            for ($j = 0; $j < $this->n; ++$j) {
                $S[$i][$j] = 0.0;
            }
            $S[$i][$i] = $this->s[$i];
        }
        return new Matrix($S);
    }