Hausdorff space

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: T2

This article is about a basic definition in topology.
VIEW: Definitions built on this | Facts about this | Survey articles about this
View a complete list of basic definitions in topology

For survey articles related to this, refer: Category:Survey articles related to Hausdorffness

Please also read the Topospaces Convention page: Convention:Hausdorffness assumption

Definition

Symbol-free definition

A topological space is said to be Hausdorff if it satisfies the following equivalent conditions:

1. Given any two points in the topological space, there are disjoint open sets containing the two points respectively.
2. The diagonal is closed in the product of the space with itself
3. Every ultrafilter of subsets converges to at most one point

Definition with symbols

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

1. Given any two points $x\neq y\in X$, there exist disjoint open subsets $U\ni x$ and $V\ni y$.
2. In the product space $X\times X$, endowed with the product topology, the diagonal, viz., the subset given by $\{(x,x)\mid x\in X\}$ is a closed subset
3. If $S_{\alpha }$ is an ultrafilter of subsets of $X$, then there is at most one $x\in X$ for which $S_{\alpha }\to x$

Examples

Extreme examples

• The empty space is considered a Hausdorff space. For this space, the Hausdorffness condition is vacuously satisfied.
• The one-point space is considered a Hausdorff space. For this space, the Hausdorffness condition is vacuously satisfied.
• Any discrete space (i.e., a topological space with the discrete topology) is considered a Hausdorff space.

Typical examples

• Euclidean space, and more generally, any manifold, closed subset of Euclidean space, and any subset of Euclidean space is Hausdorff.
• Any metric space is Hausdorff in the induced topology, i.e., any metrizable space is Hausdorff.

Non-examples

• The spectrum of a commutative unital ring is generally not Hausdorff under the Zariski topology.
• The etale space of continuous functions, and more general etale spaces, are usually not Hausdorff.

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 Hausdorffness

Stronger properties

• Normal space
  • Completely regular space
  • Regular space
• Collectionwise normal space
• Collectionwise Hausdorff space
• Metrizable space
• CW-space
• Manifold

Weaker properties

• Locally Hausdorff space
• KC-space: This is based on the fact that any compact subset of a Hausdorff space is closed. For full proof, refer: Hausdorff implies KC
• US-space: This is based on the fact that any convergent sequence in a Hausdorff space has a unique limit. For full proof, refer: Hausdorff implies US
• Sober space
• T1 space
• T0 space

Metaproperties

Metaproperty name	Satisfied?	Proof	Section in this article
product-closed property of topological spaces	Yes	Hausdorffness is product-closed	#Products
subspace-hereditary property of topological spaces	Yes	Hausdorffness is hereditary	#Hereditariness
refining-preserved property of topological spaces	Yes	Hausdorffness is refining-preserved	#Refining

Products

This property of topological spaces is closed under taking arbitrary products
View all properties of topological spaces closed under products

An arbitrary (finite or infinite) product of Hausdorff spaces is Hausdorff. For full proof, refer: Hausdorffness is product-closed

Hereditariness

This property of topological spaces is hereditary, or subspace-closed. In other words, any subspace (subset with the subspace topology) of a topological space with this property also has this property.
View other subspace-hereditary properties of topological spaces

Any subspace of a Hausdorff space is Hausdorff. For full proof, refer: Hausdorffness is hereditary

Refining

This property of topological spaces is preserved under refining, viz, if a set with a given topology has the property, the same set with a finer topology also has the property
View all refining-preserved properties of topological spaces OR View all coarsening-preserved properties of topological spaces

Moving to a finer topology increases the number of possible open sets to choose from, and hence, preserves the property of Hausdorffness. For full proof, refer: Hausdorffness is refining-preserved

References

Textbook references

• Topology (2nd edition) by James R. Munkres^{More info}, Page 98, Chapter 2, Section 17 (formal definition)
• Lecture Notes on Elementary Topology and Geometry (Undergraduate Texts in Mathematics) by I. M. Singer and J. A. Thorpe^{More info}, Page 26 (formal definition)

External links

Definition links

• Wikipedia page