Hjorth–Kechris dichotomy conjecture for Borel equivalence relations
Hjorth–Kechris dichotomy conjecture for Borel equivalence relations
Let be a Borel equivalence relation, and let denote eventual agreement on countable sequences of reals. For equivalence relations and , write when is Borel reducible to , and say that they are Borel bireducible when each is Borel reducible to the other. An orbit equivalence relation is the equivalence relation induced by a Borel action of a Polish group. Hjorth–Kechris conjecture. Either or is Borel bireducible with an orbit equivalence relation. This is a proposed dichotomy for Borel equivalence relations, refining the obstruction given by . The analogous Kechris–Louveau dichotomy is known in restricted settings but is false in general; the Hjorth–Kechris version is presented as a conjecture and its status is not resolved in the supplied text.
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Primary source
Filippo Calderoni and Luca Motto Ros, “Structural results on idealistic equivalence relations”, arXiv:2506.08217 (2025).
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