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Equivalence of codes for countable sets of reals

Part of: Set theory

Published online by Cambridge University Press:  20 August 2020

William Chan*
Affiliation:
Department of Mathematics, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA15213

Abstract

A set $U \subseteq {\mathbb {R}} \times {\mathbb {R}}$ is universal for countable subsets of ${\mathbb {R}}$ if and only if for all $x \in {\mathbb {R}}$ , the section $U_x = \{y \in {\mathbb {R}} : U(x,y)\}$ is countable and for all countable sets $A \subseteq {\mathbb {R}}$ , there is an $x \in {\mathbb {R}}$ so that $U_x = A$ . Define the equivalence relation $E_U$ on ${\mathbb {R}}$ by $x_0 \ E_U \ x_1$ if and only if $U_{x_0} = U_{x_1}$ , which is the equivalence of codes for countable sets of reals according to U. The Friedman–Stanley jump, $=^+$ , of the equality relation takes the form $E_{U^*}$ where $U^*$ is the most natural Borel set that is universal for countable sets. The main result is that $=^+$ and $E_U$ for any U that is Borel and universal for countable sets are equivalent up to Borel bireducibility. For all U that are Borel and universal for countable sets, $E_U$ is Borel bireducible to $=^+$ . If one assumes a particular instance of $\mathbf {\Sigma }_3^1$ -generic absoluteness, then for all $U \subseteq {\mathbb {R}} \times {\mathbb {R}}$ that are $\mathbf {\Sigma }_1^1$ (continuous images of Borel sets) and universal for countable sets, there is a Borel reduction of $=^+$ into $E_U$ .

Type
Article
Copyright
© Canadian Mathematical Society 2020

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Footnotes

The author was supported by NSF grant DMS-1703708.

References

Chan, W., Equivalence relations which are Borel somewhere . J. Symb. Log. 82(2017), no. 3, 893930. https://doi.org/10.1017/jsl.2017.22 CrossRefGoogle Scholar
Chan, W., Ordinal definability and combinatorics of equivalence relations . J. Math. Log. 19(2019), no. 2, 1950009, 24. https://doi.org/10.1142/S0219061319500090 CrossRefGoogle Scholar
Chan, W. and Magidor, M., When an equivalence relation with all Borel classes will be Borel somewhere? Preprint, 2020. aXiv:1608.04913v1 CrossRefGoogle Scholar
Ding, L. and Yu, P., Reductions on equivalence relations generated by universal sets . MLQ Math. Log. Q. 65(2019), no. 1, 813. https://doi.org/10.1002/malq.201700066 CrossRefGoogle Scholar
Friedman, H. and Stanley, L., A Borel reducibility theory for classes of countable structures . J. Symb. Log. 54(1989), no. 3, 894914. https://doi.org/10.2307/2274750 CrossRefGoogle Scholar
Gao, S., Invariant descriptive set theory. Pure and Applied Mathematics, 293, CRC Press, Boca Raton, FL, 2009.Google Scholar
Kanovei, V., Borel equivalence relations. Structure and classification . University Lecture Series, 44, American Mathematical Society, Providence, RI, 2008. https://doi.org/10.1090/ulect/044 Google Scholar
Kanovei, V., Sabok, M., and Zapletal, J., Canonical Ramsey theory on Polish spaces . Cambridge Tracts in Mathematics, 202, Cambridge University Press, Cambridge, UK, 2013. https://doi.org/10.1017/CBC09781139208666 Google Scholar
Kechris, A. S., Classical descriptive set theory . Graduate Texts in Mathematics, 156, Springer-Verlag, New York, 1995. https://doi.org/10.1007/978-1-4612-4190-4 Google Scholar
Larson, P. B. and Zapletal, J., Geometric set theory . Mathematical Surveys and Monographs, 248, American Mathematical Society, Providence, RI, 2020.Google Scholar
Moschovakis, Y. N., Descriptive set theory . 2nd ed., Mathematical Surveys and Monographs, 155, American Mathematical Society, Providence, RI, 2009.Google Scholar
Zapletal, J., Forcing idealized . Cambridge Tracts in Mathematics, 174, Cambridge University Press, Cambridge, UK, 2008.Google Scholar