The typical Turing degree

George Barmpalias, Adam R. Day, Andrew E. M. Lewis

Published 2011-11-04Version 1

The Turing degree of a real measures the computational difficulty of producing its binary expansion. Since Turing degrees are tailsets, it follows from Kolmogorov's 0-1 law that for any property which may or may not be satisfied by any given Turing degree, the satisfying class will either be of Lebesgue measure 0 or 1, so long as it is measurable. So either the \emph{typical} degree satisfies the property, or else the typical degree satisfies its negation. Further, there is then some level of randomness sufficient to ensure typicality in this regard. A similar analysis can be made in terms of Baire category, where a standard form of genericity now plays the role that randomness plays in the context of measure. We describe and prove a number of results in a programme of research which aims to establish the properties of the typical Turing degree, where typicality is gauged either in terms of Lebesgue measure or Baire category.