public function getAdjoinMatrix() : Matrix {
        $matrix = new static();
        for ($i=0; $i<static::ROWS; $i++) {
            for ($j=0; $j<static::COLUMNS; $j++) {
                $subMatrix      = MatrixMath::getSubMatrix($this->toArray(), $i, $j);
                $matrix[$i][$j] = MatrixMath::calculateDeterminant($subMatrix);
            }
        }
        return $matrix;
    }