There is a fascinating interplay and overlap between recursion theory and
descriptive set theory. A particularly beautiful source of such interaction has
been Martin's conjecture on Turing invariant functions. This longstanding open
problem in recursion theory has connected to many problems in descriptive set
theory, particularly in the theory of countable Borel equivalence relations.
In this paper, we shall give an overview of some work that has been done on
Martin's conjecture, and applications that it has had in descriptive set
theory. We will present a long unpublished result of Slaman and Steel that
arithmetic equivalence is a universal countable Borel equivalence relation.
This theorem has interesting corollaries for the theory of universal countable
Borel equivalence relations in general. We end with some open problems, and
directions for future research.Comment: Corrected typo
