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CONSTRUCTING WADGE CLASSES

Published online by Cambridge University Press:  26 January 2022

RAPHAËL CARROY
Affiliation:
DIPARTIMENTO DI MATEMATICA “GIUSEPPE PEANO” PALAZZO CAMPANA, UNIVERSITÁ DI TORINO VIA CARLO ALBERTO 10 10123TURIN, ITALYE-mail: raphael.carroy@unito.it
ANDREA MEDINI
Affiliation:
INSTITUT FÜR DISKRETE MATHEMATIK UND GEOMETRIE TECHNISCHE UNIVERSITÄT WIEN WIEDNER HAUPTSTRASSE 8-10/104 1040VIENNA, AUSTRIAE-mail: andrea.medini@tuwien.ac.atE-mail: sandra.mueller@tuwien.ac.atURL: http://muellersandra.github.io
SANDRA MÜLLER
Affiliation:
INSTITUT FÜR DISKRETE MATHEMATIK UND GEOMETRIE TECHNISCHE UNIVERSITÄT WIEN WIEDNER HAUPTSTRASSE 8-10/104 1040VIENNA, AUSTRIAE-mail: andrea.medini@tuwien.ac.atE-mail: sandra.mueller@tuwien.ac.atURL: http://muellersandra.github.io

Abstract

We show that, assuming the Axiom of Determinacy, every non-selfdual Wadge class can be constructed by starting with those of level $\omega _1$ (that is, the ones that are closed under Borel preimages) and iteratively applying the operations of expansion and separated differences. The proof is essentially due to Louveau, and it yields at the same time a new proof of a theorem of Van Wesep (namely, that every non-selfdual Wadge class can be expressed as the result of a Hausdorff operation applied to the open sets). The exposition is self-contained, except for facts from classical descriptive set theory.

Type
Articles
Copyright
© The Author(s), 2022. Published by Cambridge University Press on behalf of The Association for Symbolic Logic

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