Nonamenable subforests of multi‐ended quasi‐pmp graphs

Abstract

We prove the a.e. nonamenability of locally finite quasi‐pmp Borel graphs whose every component admits at least three nonvanishing ends with respect to the underlying Radon–Nikodym cocycle. We witness their nonamenability by constructing Borel subforests with at least three nonvanishing ends per component, and then applying Tserunyan and Tucker‐Drob's recent characterization of amenability for acyclic quasi‐pmp Borel graphs. Our main technique is a weighted cycle‐cutting algorithm, which yields a weight‐maximal spanning forest. We also introduce a random version of this forest, which generalizes the Free Minimal Spanning Forest, to capture nonunimodularity in the context of percolation theory.