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Wednesday, November 28, 2018

**Abstract:** We will continue to discuss Gaboriau's 2002 paper "Invariants $\ell^2$ de relations d'équivalence et de groupes" from a descriptive set-theoretic point of view. In this second talk, we will discuss Borel fields of sets and structures (in particular, simplicial complexes and Hilbert spaces) over a countable Borel equivalence relation, and how to measure their "Borel cardinality" using Tarski's theory of cardinal algebras.