A Additional related work

Our focus in this work is on perturbation attacks, where we aim to provide learners that remain robust even when the test data points are perturbed. A distribution shift refers to the phenomenon where the training distribution differs from the test distribution which often leads to a degradation in the learner's performance. Let H be any hypothesis class. Let H be any hypothesis class. Note that the supremum exists here since a union of open sets is also open.