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New Dichotomies for Borel Equivalence Relations

  • Greg Hjorth (a1) and Alexander S. Kechris (a2)

Extract

We announce two new dichotomy theorems for Borel equivalence relations, and present the results in context by giving an overview of related recent developments.

§1. Introduction. For X a Polish (i.e., separable, completely metrizable) space and E a Borel equivalence relation on X, a (complete) classification of X up to E-equivalence consists of finding a set of invariants I and a map c : XI such that xEyc(x) = c(y). To be of any value we would expect I and c to be “explicit” or “definable”. The theory of Borel equivalence relations investigates the nature of possible invariants and provides a hierarchy of notions of classification.

The following partial (pre-)ordering is fundamental in organizing this study. Given equivalence relations E and F on X and Y, resp., we say that E can be Borel reduced to F, in symbols

if there is a Borel map f : XY with xEyf(x)Ff(y). Then if is an embedding of X/E into Y/F, which is “Borel” (in the sense that it has a Borel lifting).

Intuitively, E B F might be interpreted in any one of the following ways:

(i) The classi.cation problem for E is simpler than (or can be reduced to) that of F: any invariants for F work as well for E (after composing by an f as above).

(ii) One can classify E by using as invariants F-equivalence classes.

(iii) The quotient space X/E has “Borel cardinality” less than or equal to that of Y/F, in the sense that there is a “Borel” embedding of X/E into Y/F.

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References

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New Dichotomies for Borel Equivalence Relations

  • Greg Hjorth (a1) and Alexander S. Kechris (a2)

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