Structurable equivalence relations and $\mathcal{L}_{ω_1ω}$ interpretations

Abstract: We show that the category of countable Borel equivalence relations (CBERs) is dually equivalent to the category of countable $\mathcal{L}{\omega_1\omega}$ theories which admit a one-sorted interpretation of a particular theory we call $\mathcal{T}\mathsf{LN} \sqcup \mathcal{T}\mathsf{sep}$ that witnesses embeddability into $2^\mathbb{N}$ and the Lusin--Novikov uniformization theorem. This allows problems about Borel combinatorial structures on CBERs to be translated into syntactic definability problems in $\mathcal{L}{\omega_1\omega}$, modulo the extra structure provided by $\mathcal{T}\mathsf{LN} \sqcup \mathcal{T}\mathsf{sep}$, thereby formalizing a folklore intuition in locally countable Borel combinatorics. We illustrate this with a catalogue of the precise interpretability relations between several standard classes of structures commonly used in Borel combinatorics, such as Feldman--Moore $\omega$-colorings and the Slaman--Steel marker lemma. We also generalize this correspondence to locally countable Borel groupoids and theories interpreting $\mathcal{T}_\mathsf{LN}$, which admit a characterization analogous to that of Hjorth--Kechris for essentially countable isomorphism relations.