“…In deep learning, there is an increasing interest in studying and improving the robustness and stability of the trained neural networks; see, for example, [18,24,25], where it has been reported that a simple modification in the input data might fool a well-trained neural network, returning a wrong output. For instance, a picture that is previously well classified by a trained neural network could be incorrectly classified once we perturb one or more pixels in it.…”