COSC-4P80-Assignment-2/lib/eigen-3.4.0/doc/snippets/SelfAdjointView_operatorNorm.cpp

MatrixXd ones = MatrixXd::Ones(3,3);
cout << "The operator norm of the 3x3 matrix of ones is "
     << ones.selfadjointView<Lower>().operatorNorm() << endl;