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Tuesday, May 28, 2019

**Abstract:** Introduced by Levitt and extensively developed by Gaboriau, *cost* is a very useful real-valued invariant for probability measure preserving countable equivalence relations that measures the infimum amount of edges needed to connect a.e. equivalence class in a uniformly Borel fashion. We discuss Gaboriau's original proof of his Fundamental Theorem of Cost, which states that the cost of a treeable equivalence relation is achieved by any Borel treeing of it.