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math.LO1996
Orbit cardinalities
G. Hjorth
This paper considers "definable cardinalities" arising from Polish group actions. The first part of the paper answers a question of Becker-Kechris by showing that under suitable de…
math.LO1996
Sharper changes in topologies
Greg Hjorth
Becker and Kechris showed that if a Polish group G acts continuously on a Polish space X, then for any invariant Borel set B we can change the topology on X so that B becomes open,…