A number of "stubbornly open" problems about countable Borel equivalence relations concern hyperfiniteness. For instance, the increasing union problem asks if the increasing union of a sequence of hyperfinite equivalence relations is still hyperfinite. In the past decade or so, the only progress on hyperfiniteness problems has been the proof of hyperfiniteness for orbit equivalence relations of countable abelian group actions (Gao and Jackson, "Countable abelian group actions and hyperfinite equivalence relations", Inventiones Mathematicae, 2015) and then the extension of this result to locally nilpotent groups (Schneider and Seward, "Locally nilpotent groups and hyperfinite equivalence relations", to appear).
The hyperfiniteness proofs are based on an elaborate theory of Borel marker structures with regularity properties. Now researchers have a good understanding of which regularity properties are possible and which are beyond hope. For the proofs of negative results two new concepts and methods have been playing a key role. One of them is the introduction and construction of hyperaperiodic elements with various additional properties. The other is the introduction of new forcing notions that are special cases of the so-called orbit forcing. The workshop will be roughly divided into four lectures:
Information about travel to Pittsburgh can be found here.
The CMU mathematics department is located in Wean Hall on the CMU campus. Wean Hall is building 33 on the campus map.
VERY IMPORTANT NOTE ABOUT LODGING: A block of rooms earmarked for attendees has been set aside at a local hotel (the Shadyside Inn). If we are covering your lodging expenses then we will need to make a reservation for you. Please don't make your own reservation if we have promised you support, this will cause confusion and may make it impossible for us to reimburse you.
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