English

Bad oracles in higher computability and randomness

Logic 2019-12-03 v1

Abstract

Many constructions in computability theory rely on "time tricks". In the higher setting, relativising to some oracles shows the necessity of these. We construct an oracle~AA and a set~XX, higher Turing reducible to~XX, but for which Ψ(A)X\Psi(A)\ne X for any higher functional~Ψ\Psi which is consistent on all oracles. We construct an oracle~AA relative to which there is no universal higher ML-test. On the other hand, we show that badness has its limits: there are no higher self-PA oracles, and for no~AA can we construct a higher AA-c.e.\ set which is also higher AA-ML-random. We study various classes of bad oracles and differentiate between them using other familiar classes. For example, bad oracles for consistent reductions can be higher ML-random, whereas bad oracles for universal tests cannot.

Keywords

Cite

@article{arxiv.1912.00807,
  title  = {Bad oracles in higher computability and randomness},
  author = {Laurent Bienvenu and Noam Greenberg and Benoit Monin},
  journal= {arXiv preprint arXiv:1912.00807},
  year   = {2019}
}
R2 v1 2026-06-23T12:33:08.362Z