Abstract
We give solutions to two of the questions in a paper by Brendle, Brooke-Taylor, Ng and Nies. Our examples derive from a 2014 construction by Khan and Miller as well as new direct constructions using martingales.
At the same time, we introduce the concept of i.o. subuniformity and relate this concept to recursive measure theory. We prove that there are classes closed downwards under Turing reducibility that have recursive measure zero and that are not i.o. subuniform. This shows that there are examples of classes that cannot be covered with methods other than probabilistic ones. It is easily seen that every set of hyperimmune degree can cover the recursive sets. We prove that there are both examples of hyperimmune-free degree that can and that cannot compute such a cover.
This work was partially supported by a grant from the Simons Foundation (#315188 to Bjørn Kjos-Hanssen) and by a grant from the NUS (R146-000-181-112 to F. Stephan). A substantial part of the work was performed while the first and third authors were supported by the Institute for Mathematical Sciences of the National University of Singapore during the workshop on Algorithmic Randomness during 2–30 June 2014.
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Kjos-Hanssen, B., Stephan, F., Terwijn, S.A. (2015). Covering the Recursive Sets. In: Beckmann, A., Mitrana, V., Soskova, M. (eds) Evolving Computability. CiE 2015. Lecture Notes in Computer Science(), vol 9136. Springer, Cham. https://doi.org/10.1007/978-3-319-20028-6_5
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