The conjecture that essentially countable subrelations of abelian Polish-group orbit relations are essentially hyperfinite

Let GG be an abelian Polish group, and let FF be an orbit equivalence relation induced by a Borel action of GG on a Polish space. Let EE be a Borel equivalence relation, and write EBFE\leq_B F when EE is Borel reducible to FF. An equivalence relation is essentially countable if it is Borel reducible to a countable Borel equivalence relation, and essentially hyperfinite if it is Borel reducible to a hyperfinite Borel equivalence relation.

Essential hyperfiniteness conjecture. If

EBFE\leq_B F

and EE is essentially countable, then EE is essentially hyperfinite.

The statement generalizes the paper's theorem that this conclusion holds in a special case. Its general validity is presented as an open conjecture.

Sources & referencesView supporting material

Primary source

Longyun Ding and Su Gao, “Non-Archimedean Abelian Polish Groups and Their Actions”, arXiv:1512.07677 (2015).

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