Aopt                    Calculate the A-optimal design under the
                        second-order Least squares estimator
Dopt                    Calculate the D-optimal design under the SLSE
calc_phiA               Calculate the loss function of the A-optimal
                        design
calc_phiD               Calculate the loss function of the D-optimal
                        design
plot_direction_Aopt     Verify the optimality condition for the
                        A-optimal design
plot_direction_Dopt     Verify the optimality condition for the
                        D-optimal design
plot_weight             Plot the weight distribution of the optimal
                        design for univaraite regression model
