Jan Grebik




Monday, April 1, 2024 - 4:00pm to 5:30pm



RH 440 R

Abstract: The notions of measure hyperfiniteness and measure reducibility of countable Borel equivalence relations are variants of the usual notions of hyperfiniteness and Borel reducibility. Conley and Miller proved that every basis for the countable Borel equivalence relations strictly above E_0 under measure reducibility is uncountable and asked whether there is a "measure successor of E_0"—i.e. a countable Borel equivalence relation E such that E is not measure reducible to E_0 and any F which is measure reducible to E is either equivalent to E or measure reducible to E_0. In an ongoing work with Patrick Lutz, we have isolated a combinatorial condition on a Borel group action (a strong form of expansion that we call "lossless expansion" after a similar property which is studied in computer science and finite combinatorics) which implies that the associated orbit equivalence relation is a measure successor of E_0. We have also found several examples of group actions which are plausible candidates for satisfying this condition. In this talk, I will explain the context for Conley and Miller's question, the condition that we have isolated and discuss some of the candidate examples we have identified.

All of this is joint work with Patrick Lutz.