# Some countable Borel equivalence relations

MIT Logic Seminar, Boston, March 2009

*Abstract*: Borel equivalence relations is an area of descriptive set theory which concerns the complexity of equivalence relations on a standard Borel space (i.e., a Polish space equipped just with its $\sigma$-algebra of Borel sets). There are interesting examples from within logic, such as the Turing equivalence relation $\equiv_T$. Moreover, many classification problems from other areas of mathematics can be regarded as equivalence relations on standard Borel spaces. For instance, the classification problem for torsion-free abelian groups of rank n corresponds to the isomorphism equivalence relation on a suitable subspace of $\mathcal P(\mathbb Q^n)$. All of these examples are instances of countable Borel equivalence relations, that is, equivalence relations that are Borel as subsets of the plane and which have the property that every equivalence class is countable. After giving the definitions, I’ll discuss what structure theory exists, paying close attention to the role of these special examples.