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Weakly mixing transformations and the Carathéodory definition of measurable sets

Amos Koeller, Rodney Nillsen, Graham Williams (2007)

Colloquium Mathematicae

Let 𝕋 denote the set of complex numbers of modulus 1. Let v ∈ 𝕋, v not a root of unity, and let T: 𝕋 → 𝕋 be the transformation on 𝕋 given by T(z) = vz. It is known that the problem of calculating the outer measure of a T-invariant set leads to a condition which formally has a close resemblance to Carathéodory's definition of a measurable set. In ergodic theory terms, T is not weakly mixing. Now there is an example, due to Kakutani, of a transformation ψ̃ which is weakly mixing but not strongly...

Weakly α-favourable measure spaces

David Fremlin (2000)

Fundamenta Mathematicae

I discuss the properties of α-favourable and weakly α-favourable measure spaces, with remarks on their relations with other classes.

Weak-star continuous homomorphisms and a decomposition of orthogonal measures

B. J. Cole, Theodore W. Gamelin (1985)

Annales de l'institut Fourier

We consider the set S ( μ ) of complex-valued homomorphisms of a uniform algebra A which are weak-star continuous with respect to a fixed measure μ . The μ -parts of S ( μ ) are defined, and a decomposition theorem for measures in A L 1 ( μ ) is obtained, in which constituent summands are mutually absolutely continuous with respect to representing measures. The set S ( μ ) is studied for T -invariant algebras on compact subsets of the complex plane and also for the infinite polydisc algebra.

What’s the price of a nonmeasurable set?

Mirko Sardella, Guido Ziliotti (2002)

Mathematica Bohemica

In this note, we prove that the countable compactness of { 0 , 1 } together with the Countable Axiom of Choice yields the existence of a nonmeasurable subset of . This is done by providing a family of nonmeasurable subsets of whose intersection with every non-negligible Lebesgue measurable set is still not Lebesgue measurable. We develop this note in three sections: the first presents the main result, the second recalls known results concerning non-Lebesgue measurability and its relations with the Axiom...

When ℵ₁ many sets are contained in a countably generated σ-field

R. Drabiński, E. Grzegorek (2009)

Colloquium Mathematicae

We discuss the problem when ℵ₁ sets are contained in a σ-generated σ-field on some set X. This is related to a problem raised by K. P. S. Bhaskara Rao and Rae Michael Shortt [Dissertationes Math. 372 (1998)] which we answer. We also briefly discuss generating the family of all subsets from rectangles.

When is the union of an increasing family of null sets?

Juan González-Hernández, Fernando Hernández-Hernández, César E. Villarreal (2007)

Commentationes Mathematicae Universitatis Carolinae

We study the problem in the title and show that it is equivalent to the fact that every set of reals is an increasing union of measurable sets. We also show the relationship of it with Sierpi'nski sets.

Where are typical C 1 functions one-to-one?

Zoltán Buczolich, András Máthé (2006)

Mathematica Bohemica

Suppose F [ 0 , 1 ] is closed. Is it true that the typical (in the sense of Baire category) function in C 1 [ 0 , 1 ] is one-to-one on F ? If dim ̲ B F < 1 / 2 we show that the answer to this question is yes, though we construct an F with dim B F = 1 / 2 for which the answer is no. If C α is the middle- α Cantor set we prove that the answer is yes if and only if dim ( C α ) 1 / 2 . There are F ’s with Hausdorff dimension one for which the answer is still yes. Some other related results are also presented.

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