Unfriendly partitions of locally finite Borel graphs

Abstract

We answer in the negative the question of Thomas, recorded by Conley, Conley--Marks--Unger, and Conley--Tamuz, of whether every locally finite Borel graph admits a Borel unfriendly partition. Our counterexample has unbounded degree and is closed on a zero-dimensional Polish space; its connectedness relation is hyperfinite, and its components are bipartite and one-ended. Every unfriendly colouring is proper. Together with a parity obstruction, this rigidity rules out Baire measurable colourings that are unfriendly on a comeager set, and measurable colourings that are unfriendly almost everywhere for a quasi-invariant probability of finite average degree. In the positive direction, a Borel graph of maximum degree at most four admits a Borel unfriendly colouring whenever each component contains a cycle or a vertex of degree at most two. This reduces the Borel problem in maximum degree three to cubic forests and, with a theorem of Conley--Marks--Unger, gives Baire measurable unfriendly colourings for all Borel graphs of maximum degree at most four.