Borel and Baire reducibility
Tom 164 / 2000
Fundamenta Mathematicae 164 (2000), 61-69
DOI: 10.4064/fm-164-1-61-69
Streszczenie
We prove that a Borel equivalence relation is classifiable by countable structures if and only if it is Borel reducible to a countable level of the hereditarily countable sets. We also prove the following result which was originally claimed in [FS89]: the zero density ideal of sets of natural numbers is not classifiable by countable structures.
