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A countably cellular topological group all of whose countable subsets are closed need not be -factorizable

Mihail G. Tkachenko (2023)

Commentationes Mathematicae Universitatis Carolinae

We construct a Hausdorff topological group G such that 1 is a precalibre of G (hence, G has countable cellularity), all countable subsets of G are closed and C -embedded in G , but G is not -factorizable. This solves Problem 8.6.3 from the book “Topological Groups and Related Structures" (2008) in the negative.

A Dowker group

Klaas Pieter Hart, Heikki J. K. Junnila, Jan van Mill (1985)

Commentationes Mathematicae Universitatis Carolinae

A groupoid formulation of the Baire Category Theorem

Jonathan Brown, Lisa Orloff Clark (2014)

Fundamenta Mathematicae

We prove that the Baire Category Theorem is equivalent to the following: Let G be a topological groupoid such that the unit space is a complete metric space, and there is a countable cover of G by neighbourhood bisections. If G is effective, then G is topologically principal.

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