#includeIn the example above, we create a 3x3 matrix `mat` and access the second column and third row using the `col()` and `row()` functions, respectively. Then we multiply two matrices `mat1` and `mat2` and take the transpose of `mat`. Eigen is a free and open-source library and can be downloaded from the Eigen website or installed using package managers such as apt, Homebrew, or RPM.int main() { // Creating a matrix Eigen::Matrix3d mat; mat << 1, 2, 3, 4, 5, 6, 7, 8, 9; // Accessing rows and columns Eigen::VectorXd col2 = mat.col(1); Eigen::RowVectorXd row3 = mat.row(2); // Matrix multiplication Eigen::MatrixXd mat1(2,3); Eigen::MatrixXd mat2(3,4); Eigen::MatrixXd result = mat1 * mat2; // Transpose Eigen::MatrixXd mat_T = mat.transpose(); }