Measurable chromatic numbers

Abstract

We show that if add(null) = 𝔠, then the globally Baire and universally measurable chromatic numbers of the graph of any Borel function on a Polish space are equal and at most three. In particular, this holds for the graph of the unilateral shift on [ℕ]^ℕ, although its Borel chromatic number is ℵ₀. We also show that if add(null) = 𝔠, then the universally measurable chromatic number of every treeing of a measure amenable equivalence relation is at most three. In particular, this holds for “the” minimum analytic graph 𝔊₀ with uncountable Borel (and Baire measurable) chromatic number. In contrast, we show that for all κ ∈ { 2, 3, …, ℵ_0, 𝔠 }, there is a treeing of E₀ with Borel and Baire measurable chromatic number κ. Finally, we use a Glimm—Effros style dichotomy theorem to show that every basis for a non-empty initial segment of the class of graphs of Borel functions of Borel chromatic number at least three contains a copy of (ℝ^<ℕ, ⊇).