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

MatrixXd ones = MatrixXd::Ones(3,3);
VectorXd eivals = ones.selfadjointView<Lower>().eigenvalues();
cout << "The eigenvalues of the 3x3 matrix of ones are:" << endl << eivals << endl;