Normal Hausdorff space

This article is about a basic definition in topology.
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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:

1. 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.
2. 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
3. All points are closed, and every point-finite open cover possesses a shrinking.

Some versions of the definition omit the condition that points are closed subsets. This is a different, and weaker, notion, and is covered here as normal-minus-Hausdorff space.

Definition with symbols

A topological space $X$ is said to be normal if it satisfies the following equivalent conditions:

1. For all $x\in X$, the set $\{x\}$ is closed, and for any two closed subsets $A,B\subseteq X$, there exist open subsets $U,V\subseteq X$ such that $A\subseteq U,B\subseteq V$, and $U\cap V=\varnothing$.
2. For all $x\in X$, the set $\{x\}$ is closed, and for any two closed subsets $A,B\subseteq X$, there exists a continuous map $f:X\to [0,1]$ such that $f(x)=0\mathrm{\forall }x\in A$ and $f(x)=1\mathrm{\forall }x\in A$.
3. For all $x\in X$, the set $\{x\}$ is closed, and for any point-finite open cover $U_i,i\in I$ of $X$, there exists a shrinking $V_i,i\in I$: the $V_i$ form an open cover and $\overline{V_i}\subseteq U_i$.

Some versions of the definition omit the condition that points are closed subsets. This is a different, and weaker, notion, and is covered here as normal-minus-Hausdorff space.

Equivalence of definitions

The direction (2) implies (1) is easy: if there is a continuous function $f:X\to [0,1]$ such that $A\subseteq f^{-1}(\{0\})$ and $B\subseteq f^{-1}(\{1\})$, then we can take the open sets $f^{-1}((0,1/2))$ and $f^{-1}((1/2,1))$.

The direction (1) implies (2) follows from Urysohn's lemma.

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

• Category:Variations of normality

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
• Monotonically 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

Products

NO: This property of topological spaces is not a product-closed property of topological spaces: a product of topological spaces, each satisfying the property, when equipped with the product topology, does not necessarily satisfy the property.
View other properties that are not product-closed

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 subset (equipped with the subspace topology) of a space with the property, also has the property.
View all weakly hereditary properties of topological spaces | View all subspace-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. Munkres^{More info}, Page 195,Chapter 4, Section 31 (formal definition, along with definition of regular space)
• Lecture Notes on Elementary Topology and Geometry (Undergraduate Texts in Mathematics) by I. M. Singer and J. A. Thorpe^{More info}, Page 28 (formal definition)

External links

Definition links

• Wikipedia page