Orbit cardinals: On the effective cardinalities arising as quotient spaces of the formX/G whereG acts on a Polish spaceX
We prove an Ulm-type classification theorem for actions inL(ℝ), thereby answering a question of Becker and Kechris, and investigate the effective cardinalities which can be induced by various classes of Polish groups.
KeywordsEquivalence Relation Polish Group Polish Space Generic Extension Force Notion
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