Math 2.0

General topology Exotic spaces and counterexamples Counterexample

Fort space

Builds on Compact spaceHausdorff spaceFirst-countable spaceMetrizable spaceOne-point compactificationUncountable set

Counterexample

Let XX be an infinite set and let p∈Xp \in X. Declare U⊆XU \subseteq X open if p∉Up \notin U, or if p∈Up \in U and X∖UX \setminus U is finite. This is the Fort space on XX with point pp. It is compact and Hausdorff. If XX is uncountable, it is not first-countable, and so not metrizable.

Remarks

Compactness: in an open cover, one member UU contains pp, and the finitely many points outside UU need finitely many more members. Hausdorff: two points x,y≠px, y \ne p are separated by {x}\set{x} and {y}\set{y}, and x≠px \ne p is separated from pp by {x}\set{x} and X∖{x}X \setminus \set{x}. Let XX be uncountable and let U1,U2,…U_1, U_2, \dots be open sets containing pp. Their intersection has countable complement, so it contains a point x≠px \ne p. Then X∖{x}X \setminus \set{x} is an open set containing pp that contains no UnU_n, so pp has no countable neighbourhood base. The Fort space is the one-point compactification of the discrete space X∖{p}X \setminus \set{p}. If XX is countably infinite, it is homeomorphic to {0}∪{1/n:n≥1}\set{0} \cup \set{1/n : n \ge 1} with the usual topology, which is metrizable.

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To understand this page from the ground up, read these 7 pages in order. The foundations come first.

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  1. Compact space concept
  2. First-countable space concept
  3. Hausdorff space concept
  4. Metrizable space concept
  5. One-point compactification concept
  6. Uncountable set concept
  7. Fort space counterexample

Further reading

  1. Web https://en.wikipedia.org/wiki/Fort_space
About this page
Tag7FYKA5F3S6
URIm2://7FYKA5F3S6
Kindcounterexample
WritingW0 stub
MSC54G20, 54D30
UsesCompact space, Hausdorff space, First-countable space, Metrizable space, One-point compactification, Uncountable set
Changed2 November 2026

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