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Cleanup for S22 Fortissimo space on the reals (#1207)
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prabau authored Jan 26, 2025
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8 changes: 4 additions & 4 deletions spaces/S000022/README.md
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---
uid: S000022
name: Fortissimo Space on the Real Numbers
name: Fortissimo space on the real numbers
aliases:
- Fortissimo Space
- Fortissimo space
- One-point Lindelöfication of uncountable discrete space
counterexamples_id: 25
refs:
- doi: 10.1007/978-1-4612-6290-9
name: Counterexamples in Topology
- wikipedia: Fort_space
name: Fort space
name: Fort space on Wikipedia
---

Let $X=\mathbb{R}\cup\{\infty\}$.
Define $U \subseteq X$ to be open if it does not contain $\infty$ or its complement is countable.
Every point of $\mathbb R$ is isolated and the open neighborhoods of the point $\infty$ are the cocountable subsets of $X$ containing that point.

This space is the one-point Lindelöfication of an uncountable discrete space.

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---
space: S000022
property: P000008
property: P000002
value: true
refs:
- doi: 10.1007/978-1-4612-6290-9_6
name: Counterexamples in Topology
---

See item #1 for space #25 in {{doi:10.1007/978-1-4612-6290-9_6}}.
By inspection, every singleton is closed.
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