Polyhedron/external/eigen-3.4.0/doc/snippets/EigenSolver_eigenvalues.cpp

MatrixXd ones = MatrixXd::Ones(3,3);
EigenSolver<MatrixXd> es(ones, false);
cout << "The eigenvalues of the 3x3 matrix of ones are:" 
     << endl << es.eigenvalues() << endl;