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A note on transitively D -spaces

Liang-Xue Peng (2011)

Czechoslovak Mathematical Journal

In this note, we show that if for any transitive neighborhood assignment φ for X there is a point-countable refinement such that for any non-closed subset A of X there is some V such that | V A | ω , then X is transitively D . As a corollary, if X is a sequential space and has a point-countable w c s * -network then X is transitively D , and hence if X is a Hausdorff k -space and has a point-countable k -network, then X is transitively D . We prove that if X is a countably compact sequential space and has a point-countable...

A relatively free topological group that is not varietal free

Vladimir Pestov, Dmitri Shakhmatov (1998)

Colloquium Mathematicae

Answering a 1982 question of Sidney A. Morris, we construct a topological group G and a subspace X such that (i) G is algebraically free over X, (ii) G is relatively free over X, that is, every continuous mapping from X to G extends to a unique continuous endomorphism of G, and (iii) G is not a varietal free topological group on X in any variety of topological groups.

A study of universal elements in classes of bases of topological spaces

Dimitris N. Georgiou, Athanasios C. Megaritis, Inderasan Naidoo, Fotini Sereti (2021)

Commentationes Mathematicae Universitatis Carolinae

The universality problem focuses on finding universal spaces in classes of topological spaces. Moreover, in “Universal spaces and mappings” by S. D. Iliadis (2005), an important method of constructing such universal elements in classes of spaces is introduced and explained in details. Simultaneously, in “A topological dimension greater than or equal to the classical covering dimension” by D. N. Georgiou, A. C. Megaritis and F. Sereti (2017), new topological dimension is introduced and studied, which...

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