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On the complexity of some σ -ideals of σ -P-porous sets

Luděk Zajíček, Miroslav Zelený (2003)

Commentationes Mathematicae Universitatis Carolinae

Let 𝐏 be a porosity-like relation on a separable locally compact metric space E . We show that the σ -ideal of compact σ - 𝐏 -porous subsets of E (under some general conditions on 𝐏 and E ) forms a Π 1 1 -complete set in the hyperspace of all compact subsets of E , in particular it is coanalytic and non-Borel. Our general results are applicable to most interesting types of porosity. It is shown in the cases of the σ -ideals of σ -porous sets, σ - g -porous sets, σ -strongly porous sets, σ -symmetrically porous sets...

On the complexity of sums of Dirichlet measures

Sylvain Kahane (1993)

Annales de l'institut Fourier

Let M be the set of all Dirichlet measures on the unit circle. We prove that M + M is a non Borel analytic set for the weak* topology and that M + M is not norm-closed. More precisely, we prove that there is no weak* Borel set which separates M + M from D (or even L 0 ) , the set of all measures singular with respect to every measure in M . This extends results of Kaufman, Kechris and Lyons about D and H and gives many examples of non Borel analytic sets.

On the conformal gauge of a compact metric space

Matias Carrasco Piaggio (2013)

Annales scientifiques de l'École Normale Supérieure

In this article we study the Ahlfors regular conformal gauge of a compact metric space ( X , d ) , and its conformal dimension dim A R ( X , d ) . Using a sequence of finite coverings of  ( X , d ) , we construct distances in its Ahlfors regular conformal gauge of controlled Hausdorff dimension. We obtain in this way a combinatorial description, up to bi-Lipschitz homeomorphisms, of all the metrics in the gauge. We show how to compute dim A R ( X , d ) using the critical exponent Q N associated to the combinatorial modulus.

On the control measures of vector measures.

Baltasar Rodríguez-Salinas (2003)

RACSAM

Si Σ es una σ-álgebra y X un espacio localmente convexo se estudian las condiciones para las cuales una medida vectorial σ-aditiva γ : Σ → χ tenga una medida de control μ. Si Σ es la σ-álgebra de Borel de un espacio métrico, se obtienen condiciones necesarias y suficientes usando la τ aditividad de γ. También se dan estos resultados para las polimedidas.

On the countable generator theorem

Michael Keane, Jacek Serafin (1998)

Fundamenta Mathematicae

Let T be a finite entropy, aperiodic automorphism of a nonatomic probability space. We give an elementary proof of the existence of a finite entropy, countable generating partition for T.

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