MatrixXd X = MatrixXd::Random(5, 5);
MatrixXd A = X * X.transpose();
X = MatrixXd::Random(5, 5);
MatrixXd B = X * X.transpose();

GeneralizedSelfAdjointEigenSolver<MatrixXd> es(A, B, EigenvaluesOnly);
cout << "The eigenvalues of the pencil (A,B) are:" << endl << es.eigenvalues() << endl;
es.compute(B, A, false);
cout << "The eigenvalues of the pencil (B,A) are:" << endl << es.eigenvalues() << endl;