eigen/doc/snippets/SelfAdjointEigenSolver_eigenvectors.cpp

MatrixXd ones = MatrixXd::Ones(3,3);
SelfAdjointEigenSolver<MatrixXd> es(ones);
cout << "The first eigenvector of the 3x3 matrix of ones is:" 
     << endl << es.eigenvectors().col(0) << endl;