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La classe des constructibles de la géométrie algébrique est close par projection. La théorie des modèles exprime ce fait en disant que les corps algébriquement clos éliminent les quantificateurs dans le langage des anneaux. De façon analogue, les corps algébriquement clos non trivialement valués éliminent les quantificateurs dans le langage des anneaux enrichi de la relation dite de divisibilité . Cela implique en particulier la « -minimalité » : une partie définissable d’un corps algébriquement...
Corrección del artículo del autor "Anàlisi formalment recursiva", publicado en Publicacions de la Secció de Matemàtiques de la UAB, 30 (2-3), p. 35-75 (1986).
The discrete algebras over a commutative ring which can be realized as the full endomorphism algebra of a torsion-free -module have been investigated by Dugas and Göbel under the additional set-theoretic axiom of constructibility, . Many interesting results have been obtained for cotorsion-free algebras but the proofs involve rather elaborate calculations in linear algebra. Here these results are rederived in a more natural topological setting and substantial generalizations to topological...
We show that:
(1) It is provable in ZF (i.e., Zermelo-Fraenkel set theory minus the Axiom of Choice AC) that every compact scattered T₂ topological space is zero-dimensional.
(2) If every countable union of countable sets of reals is countable, then a countable compact T₂ space is scattered iff it is metrizable.
(3) If the real line ℝ can be expressed as a well-ordered union of well-orderable sets, then every countable compact zero-dimensional T₂ space...
We show that all sufficiently nice λ-sets are countable dense homogeneous (𝖢𝖣𝖧). From this fact we conclude that for every uncountable cardinal κ ≤ 𝔟 there is a countable dense homogeneous metric space of size κ. Moreover, the existence of a meager in itself countable dense homogeneous metric space of size κ is equivalent to the existence of a λ-set of size κ. On the other hand, it is consistent with the continuum arbitrarily large that every 𝖢𝖣𝖧 metric space has size either ω₁ or 𝔠. An...
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