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  1. New jump operators on equivalence relations.John D. Clemens & Samuel Coskey - 2022 - Journal of Mathematical Logic 22 (3).
    We introduce a new family of jump operators on Borel equivalence relations; specifically, for each countable group [Formula: see text] we introduce the [Formula: see text]-jump. We study the elementary properties of the [Formula: see text]-jumps and compare them with other previously studied jump operators. One of our main results is to establish that for many groups [Formula: see text], the [Formula: see text]-jump is proper in the sense that for any Borel equivalence relation [Formula: see text] the [Formula: see (...)
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  • The Borel complexity of von Neumann equivalence.Inessa Moroz & Asger Törnquist - 2021 - Annals of Pure and Applied Logic 172 (5):102913.
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  • On the classification of vertex-transitive structures.John Clemens, Samuel Coskey & Stephanie Potter - 2019 - Archive for Mathematical Logic 58 (5-6):565-574.
    We consider the classification problem for several classes of countable structures which are “vertex-transitive”, meaning that the automorphism group acts transitively on the elements. We show that the classification of countable vertex-transitive digraphs and partial orders are Borel complete. We identify the complexity of the classification of countable vertex-transitive linear orders. Finally we show that the classification of vertex-transitive countable tournaments is properly above \ in complexity.
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  • Borel equivalence relations induced by actions of the symmetric group.Greg Hjorth, Alexander S. Kechris & Alain Louveau - 1998 - Annals of Pure and Applied Logic 92 (1):63-112.
    We consider Borel equivalence relations E induced by actions of the infinite symmetric group, or equivalently the isomorphism relation on classes of countable models of bounded Scott rank. We relate the descriptive complexity of the equivalence relation to the nature of its complete invariants. A typical theorem is that E is potentially Π03 iff the invariants are countable sets of reals, it is potentially Π04 iff the invariants are countable sets of countable sets of reals, and so on. The proofs (...)
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  • Effective cardinals of boldface pointclasses.Alessandro Andretta, Greg Hjorth & Itay Neeman - 2007 - Journal of Mathematical Logic 7 (1):35-82.
    Assuming AD + DC, we characterize the self-dual boldface pointclasses which are strictly larger than the pointclasses contained in them: these are exactly the clopen sets, the collections of all sets of Wadge rank [Formula: see text], and those of Wadge rank [Formula: see text] when ξ is limit.
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  • The classification of countable models of set theory.John Clemens, Samuel Coskey & Samuel Dworetzky - 2020 - Mathematical Logic Quarterly 66 (2):182-189.
    We study the complexity of the classification problem for countable models of set theory (). We prove that the classification of arbitrary countable models of is Borel complete, meaning that it is as complex as it can conceivably be. We then give partial results concerning the classification of countable well‐founded models of.
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