2 research outputs found
Immunity properties and strong positive reducibilities
We use certain strong Q-reducibilities, and their corresponding strong positive reducibilities, to characterize the hyperimmune sets and the hyperhyperimmune sets: if A is any infinite set then A is hyperimmune (respectively, hyperhyperimmune) if and only if for every infinite subset B of A, one has (respectively, ): here is the finite-branch version of s-reducibility, is the computably bounded version of , and is the complement of the halting set. Restriction to sets provides a similar characterization of the hyperhyperimmune sets in terms of s-reducibility. We also showthat no is hyperhyperimmune. As a consequence, is hyperhyperimmune-free, showing that the hyperhyperimmune s-degrees are not upwards closed