Tree-like graphings of countable Borel equivalence relations

Abstract

We present a streamlined exposition of a construction by R. Chen, A. Poulin, R. Tao, and A. Tserunyan, which proves the treeability of equivalence relations generated by any locally-finite Borel graph such that each component is a quasi-tree. More generally, we show that if each component of a locally-finite Borel graph admits a finitely-separating Borel family of cuts, then we may 'canonically' construct a forest of special ultrafilters; moreover, if the cuts are dense towards ends, then this forest is a Borel treeing.