An Effective Tietze-Urysohn Theorem for QCB-Spaces

An Analysis of the Lemmas of Urysohn and Urysohn-Tietze According to Effective Borel Measurability

About extension of upper semicontinuous multi-valued maps and applications

We formulate a multi-valued version of the Tietze-Urysohn extension theorem. Precisely, we prove that any upper semicontinuous multi-valued map with nonempty closed convex values defined on a closed subset (resp. closed perfectly normal subset) of a completely normal (resp. of a normal) space X into the unit interval [0, 1] can be extended to the whole space X. The extension is upper semicontin...

A rich hierarchy of functionals of finite types

We are considering typed hierarchies of total, continuous functionals using complete, separable metric spaces at the base types. We pay special attention to the socalled Urysohn space constructed by P. Urysohn. One of the properties of the Urysohn space is that every other separable metric space can be isometrically embedded into it. We discuss why the Urysohn space may be considered as the uni...

The Intersection of Topological and Metric Spaces

In this paper we will prove the Urysohn Meterization Theorem, which gives sufficient conditions for a topological space to be metrizable. In the process of proving this theorem, we will the discuss separation and countability axiom of topological spaces and prove the Urysohn Lemma.

Topological predomains and qcb spaces are not closed under sobrification

In (Simpson 2003) A. Simpson introduced the category PreDom of topological predomains as a framework for denotational semantics containing also most classical spaces, namely all countably based T0 spaces. Countably based T0 spaces are isomorphic to subspaces of Pω where the latter is endowed with the Scott topology. A qcb space (as introduced in (Menni and Simpson 2002)) is a T0 space which app...