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Fundamenta Mathematicae
Article . 2011 . Peer-reviewed
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Borel extensions of Baire measures in ZFC

Authors: Kojman, Menachem; Michalewski, Henryk;

Borel extensions of Baire measures in ZFC

Abstract

Given a topological space, its Baire sets form the smallest \(\sigma\)-algebra that makes all continuous real-valued functions measurable, its Borel sets form the \(\sigma\)-algebra generated by its topology. In [Czech. Math. J. 7(82), 248--253 (1957; Zbl 0091.05501)] \textit{J. Mařík} proved that in a normal and countably paracompact space every Baire measure admits a unique regular Borel extension. Thus, Dowker spaces (normal but \textit{not} countably paracompact) are spaces where such an extension theorem may fail. \textit{P.~Simon} [Commentat. Math. Univ. Carol. 12, 825--834 (1971; Zbl 0232.54026)] proved that \textit{M.~E. Rudin}'s Dowker space [Fundam. Math. 73, 179-186 (1971; Zbl 0224.54019)] does not have Mařík's extension property. Rudin's space is a subspace of the box product \(\prod_{n<\omega}\omega_{n+2}\), where each factor carries its order topology. The authors investigate a class of spaces, which they call Rudin spaces, these are closed subspaces of Rudin's space~\(X\) that are cofinal in \(X\cap\prod_{n<\omega}g(n)\) with respect to the product order, for some ordinal valued function~\(g\) (that varies with the subspace). One such example is \textit{M. Kojman} and \textit{S. Shelah}'s Dowker subspace of~\(X\) of cardinality~\(\aleph_{\omega+1}\) from [Proc. Am. Math. Soc. 126, No.8, 2459-2465 (1998; Zbl 0895.54022)]. They prove that no Rudin space satisfies Mařík's theorem but that in many Rudin spaces of cardinality~\(\aleph_{\omega+1}\) every Baire measure has some Borel extension. If the latter fails for at least one Rudin space then the continuum must be real-valued measurable.

Keywords

Other connections with logic and set theory, Large cardinals, Higher separation axioms (completely regular, normal, perfectly or collectionwise normal, etc.), Classes of sets (Borel fields, \(\sigma\)-rings, etc.), measurable sets, Suslin sets, analytic sets, Measures on Boolean rings, measure algebras, Dowker space, Borel measure, Ordered sets and their cofinalities; pcf theory, Ordinal and cardinal numbers, Counterexamples in general topology, Baire measure, Consistency and independence results, \(P\)-spaces

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
0
Average
Average
Average
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