Avoiding Coding Tricks by Hyperrobust Learning

The present work introduces and justifies the notion of hyperrobust learning where one fixed learner has to learn all functions in a given class plus their images under primitive recursive operators. The following is shown: This notion of learnability does not change if the class of primitive recursive operators is replaced by a larger enumerable class of operators. A class is hyperrobustly Ex-learnable iff it is a subclass of a recursively enumerable family of total functions. So, the notion of hyperrobust learning overcomes a problem of the traditional definitions of robustness which either do not preserve learning by enumeration or still permit topological coding tricks for the learning criterion Ex. Hyperrobust BC-learning as well as the hyperrobust version of Ex-learning by teams are more powerful than hyperrobust Ex-learning. The notion of bounded totally reliable BC-learning is properly between hyperrobust Ex-learning and hyperrobust BC-learning. Furthermore, the bounded totally reliably BC-learnable classes are characterized in terms of infinite branches of certain enumerable families of bounded recursive trees. A class of infinite branches of a further family of trees separates hyperrobust BC-learning from totally reliable BC-learning.