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Normal space
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This article is about a basic definition in topology. The article text may, however, contain more than just the basic definition. Rate its utility as a basic definition article on the talk page
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This article defines a property of topological space that is pivotal (viz important) among currently studied properties of topological spaces
In the T family (properties of topological spaces related to separation axioms), this is called: T4
For survey articles related to this, refer: Category:Survey articles related to normality
Definition
Symbol-free definition
A topological space is said to be normal if it satisfies the following equivalent conditions:
- All points in it are closed sets, and given any two disjoint closed subsets in the topological space, there are disjoint open sets containing them.
- All points in it are closed sets, and given any two disjoint closed subsets, there is a continuous function taking the value 0 at one closed set and 1 at the other
- All points are closed, and every point-finite open cover possesses a shrinking
Definition with symbols
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Relation with other properties
This property is a pivotal (important) member of its property space. Its variations, opposites, and other properties related to it and defined using it are often studied
Stronger properties
- Compact Hausdorff space
- Hereditarily normal space
- Paracompact Hausdorff space
- Regular Lindelof space
- Perfectly normal space
- Metrizable space
- CW-space
- Linearly orderable space
- Collectionwise normal space
Weaker properties
- Completely regular space: For proof of the implication, refer normal implies completely regular and for proof of its strictness (i.e. the reverse implication being false) refer completely regular not implies normal
- Regular space: For proof of the implication, refer normal implies regular and for proof of its strictness (i.e. the reverse implication being false) refer regular not implies normal
- Hausdorff space
- T1 space
- Kolmogorov space
Metaproperties
A direct product of normal spaces need not be normal. For full proof, refer: Normality is not product-closed
Weak hereditariness
This property of topological spaces is weakly hereditary or closed subspace-closed; in other words, any closed subspace of a space with the property, also has the property
View other weakly hereditary properties of topological spaces OR view all hereditary properties of topological spaces
Any subspace of a normal space need not be normal. However, any closed subset of a normal space is normal, under the subspace topology.
- Further information: Normality is weakly hereditary
Facts
- Any connected normal space having at least two points (and more generally, any connected Urysohn space having at least two points) is uncountable. For full proof, refer: connected Urysohn implies uncountable
Effect of property operators
The subspace operator
Applying the subspace operator to this property gives: completely regular space
A topological space can be realized as a subspace of a normal space iff it is completely regular. Necessity follows from the fact that normal spaces are completely regular, and any subspace of a completely regular space is completely regular. Sufficiency follows from the Stone-Cech compactification.
The hereditarily operator
Applying the hereditarily operator to this property gives: hereditarily normal space
A topological space in which every subspace is normal is termed hereditarily normal (some people call it completely normal). Note that metrizable spaces are hereditarily normal.
The locally operator
Applying the locally operator to this property gives: locally normal space
References
Textbook references
- Topology (2nd edition) by James R. MunkresMore info, Page 195 (formal definition)
- Lecture Notes on Elementary Topology and Geometry (Undergraduate Texts in Mathematics) by I. M. Singer and J. A. ThorpeMore info, Page 28 (formal definition)
External links
Definition links
Categories: Basic definitions in topology | Standard terminology | Pivotal properties of topological spaces | Properties of topological spaces | Properties of topological spaces in the T family | Separation-related properties of topological spaces | Weakly hereditary properties of topological spaces
