epfl-archive/cs440-acg/ext/eigen/doc/snippets/SelfAdjointEigenSolver_eigenvalues.cpp

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