eigenvalue of matrix c 2b 2b using eigen

1  #include <eigen3/Eigen/Eigenvalues> // header file
2  #include <iostream>
3
4  int main(){
5  Eigen::Matrix<double, 2, 2> A; // declare a real (double) 2x2 matrix
6  A << 0, 2, 1, 0; // defined the matrix A
7
8  Eigen::EigenSolver<Eigen::Matrix<double, 2,2> > s(A); // the instance s(A) includes the eigensystem
9  std::cout << A << std::endl;
10  std::cout << "eigenvalues:" << std::endl;
11  std::cout << s.eigenvalues()(0) << std::endl;
12  std::cout << "eigenvectors=" << std::endl;
13  std::cout << s.eigenvectors() << std::endl;
14
15  return(0);
16}