General topology Exotic spaces and counterexamples Counterexample
Fort space
Builds on Compact spaceHausdorff spaceFirst-countable spaceMetrizable spaceOne-point compactificationUncountable set
Counterexample
Let be an infinite set and let . Declare open if , or if and is finite. This is the Fort space on with point . It is compact and Hausdorff. If is uncountable, it is not first-countable, and so not metrizable.
Remarks
Compactness: in an open cover, one member contains , and the finitely many points outside need finitely many more members. Hausdorff: two points are separated by and , and is separated from by and . Let be uncountable and let be open sets containing . Their intersection has countable complement, so it contains a point . Then is an open set containing that contains no , so has no countable neighbourhood base. The Fort space is the one-point compactification of the discrete space . If is countably infinite, it is homeomorphic to with the usual topology, which is metrizable.
Reading path
To understand this page from the ground up, read these 7 pages in order. The foundations come first.
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- no evidence level
- Compact space concept
- First-countable space concept
- Hausdorff space concept
- Metrizable space concept
- One-point compactification concept
- Uncountable set concept
- Fort space counterexample
Further reading
About this page
| Tag | 7FYKA5F3S6 |
|---|---|
| URI | m2://7FYKA5F3S6 |
| Kind | counterexample |
| Writing | W0 stub |
| MSC | 54G20, 54D30 |
| Uses | Compact space, Hausdorff space, First-countable space, Metrizable space, One-point compactification, Uncountable set |
| Changed | 2 November 2026 |
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