Note that the third coefficient 1 − λ 1 − λ 2 must be positive, which is the reason why the curve decreases sharply when λ 2 is fixed and λ 1 > 0.65.
figure 3 illustrates that the robustness of our method is very good, which means if the hyperparameters λ 1 , λ 2 are larger or smaller, it will not affect the result very much.