Countably generated space

In mathematics, a topological space X is called countably generated if the topology of X is determined by the countable sets in a similar way as the topology of a sequential space (or a Fréchet space) by the convergent sequences.

The countable generated spaces are precisely the spaces having countable tightness - therefore the name countably tight is used as well.

Definition

A topological space $X$ is called countably generated if $V$ is closed in $X$ whenever for each countable subspace $U$ of $X$ the set $V\cap U$ is closed in $U$. Equivalently, $X$ is countably generated if and only if the closure of any $A\subset X$ equals the union of closures of all countable subsets of $A$.

Countable fan tightness

A topological space $X$ has countable fan tightness if for every point $x\in X$ and every sequence $A_{1},A_{2},\ldots$ of subsets of the space $X$ such that $x\in \bigcap _{n}{\overline {A_{n}}}$ , there are finite set $B_{1}\subset A_{1},B_{2}\subset A_{2},\ldots$ such that $x\in {\overline {\bigcup _{n}B_{n}}}$.

A topological space $X$ has countable strong fan tightness if for every point $x\in X$ and every sequence $A_{1},A_{2},\ldots$ of subsets of the space $X$ such that $x\in \bigcap _{n}{\overline {A_{n}}}$ , there are points $x_{1}\subset A_{1},x_{2}\subset A_{2},\ldots$ such that $x\in {\overline {\{x_{1},x_{2},\ldots \}}}$. Every strong Fréchet–Urysohn space has strong countable fan tightness.

Properties

A quotient of countably generated space is again countably generated. Similarly, a topological sum of countably generated spaces is countably generated. Therefore the countably generated spaces form a coreflective subcategory of the category of topological spaces. They are the coreflective hull of all countable spaces.

Any subspace of a countably generated space is again countably generated.

Examples

Every sequential space (in particular, every metrizable space) is countably generated.

An example of a space which is countably generated but not sequential can be obtained, for instance, as a subspace of Arens–Fort space.

See also

• The concept of finitely generated space is related to this notion.
• Tightness is a cardinal function related to countably generated spaces and their generalizations.

External links

• A Glossary of Definitions from General Topology [1]
• https://web.archive.org/web/20040917084107/http://thales.doa.fmph.uniba.sk/density/pages/slides/sleziak/paper.pdf

References

• Herrlich, Horst (1968). Topologische Reflexionen und Coreflexionen. Lecture Notes in Math. 78. Berlin: Springer.