Hausdorff space

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

Definition

===Equivalent definitions in tabular format: A topological space is said to be Hausdorff if it satisfies the following equivalent conditions:

No.	Shorthand	A topological space is termed Hausdorff if ...	A topological space $X$ is termed Hausdorff if ...
1	Separation axiom	given any two distinct points in the topological space, there are disjoint open sets containing the two points respectively.	given any two points $x\ne y\in X$, there exist open subsets $U\ni x$ and $V\ni y$ such that $U\cap V$ is empty
2	Diagonal in square	the diagonal is closed in the product of the space with itself	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	Ultrafilter convergence	every ultrafilter of subsets converges to at most one point	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.

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

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

property	quick description	proof of implication	proof of strictness (reverse implication failure)	intermediate notions
Normal space	$T_1$ and any two disjoint closed subsets are separated by disjoint open subsets	normal implies Hausdorff	Hausdorff not implies normal	Completely regular space|FULL LIST, MORE INFO
Collectionwise normal space	$T_1$ and any discrete collection of closed subsets is separated by disjoint open subsets	(via normal)	(via normal)	Normal Hausdorff space|FULL LIST, MORE INFO
Regular space	$T_1$ and any point and closed subset not containing it are separated by disjoint open subsets	regular implies Hausdorff	Hausdorff not implies regular	|FULL LIST, MORE INFO
Completely regular space	$T_1$ and any point and disjoint closed subset are separated by a continuous function to $[0,1]$	(via regular)	(via regular)	|FULL LIST, MORE INFO
Collectionwise Hausdorff space	any discrete set of points can be separated by disjoint open subsets	collectionwise Hausdorff implies Hausdorff	Hausdorff not implies collectionwise Hausdorff	|FULL LIST, MORE INFO
Metrizable space	underlying topology of a metric space	metrizable implies Hausdorff	Hausdorff not implies metrizable	Collectionwise Hausdorff space, Completely regular space, Functionally Hausdorff space, Monotonically normal space, Moore space, Normal Hausdorff space, Regular Hausdorff space, Submetrizable space, Tychonoff space|FULL LIST, MORE INFO
CW-space	underlying topological space of a CW-complex	CW implies Hausdorff	Hausdorff not implies CW	Completely regular space, Normal Hausdorff space, Regular Hausdorff space|FULL LIST, MORE INFO
Manifold				Metrizable space, Monotonically normal space, Submetrizable space|FULL LIST, MORE INFO

Weaker properties

property	quick description	proof of implication	proof of strictness (reverse implication failure)	intermediate notions
Locally Hausdorff space	every point is contained in an open subset that's Hausdorff	Hausdorff implies locally Hausdorff	locally Hausdorff not implies Hausdorff	|FULL LIST, MORE INFO
KC-space	every compact subset is closed	Hausdorff implies KC	KC not implies Hausdorff	Weakly Hausdorff space|FULL LIST, MORE INFO
US-space	every convergent sequence has a unique limit	Hausdorff implies US	US not implies Hausdorff	KC-space, Weakly Hausdorff space|FULL LIST, MORE INFO
Sober space	every irreducible subset is the closure of a single point	Hausdorff implies sober	sober not implies Hausdorff	Sober T0 space, Sober T1 space|FULL LIST, MORE INFO
T1 space	every point is closed	Hausdorff implies T1	T1 not implies Hausdorff	KC-space, Locally Hausdorff space, Sober T1 space, Weakly Hausdorff space|FULL LIST, MORE INFO
Kolmogorov space (also called $T_0$)				KC-space, Locally Hausdorff space, Sober T0 space|FULL LIST, MORE INFO

Metaproperties

Metaproperty name	Satisfied?	Proof	Statement with symbols
product-closed property of topological spaces	Yes	Hausdorffness is product-closed	If $X_i,i\in I$ is a (finite or infinite) collection of Hausdorff topological spaces, the product of all the $X_i$s, equipped with the product topology, is also Hausdorff.
box product-closed property of topological spaces	Yes	Hausdorffness is box product-closed	If $X_i,i\in I$ is a (finite or infinite) collection of Hausdorff topological spaces, the product of all the $X_i$s, equipped with the box topology, is also Hausdorff.
subspace-hereditary property of topological spaces	Yes	Hausdorffness is hereditary	Suppose $X$ is a Hausdorff space and $A$ is a subset of $X$. Under the subspace topology, $A$ is also Hausdorff.
refining-preserved property of topological spaces	Yes	Hausdorffness is refining-preserved	Suppose ${\tau }_1$ and ${\tau }_2$ are two topologies on a set $X$, such that ${\tau }_1\subseteq {\tau }_2$, i.e., every subset of $X$ open with respect to $T_1$ is also open with respect to ${\tau }_2$. Then, if $X$ is Hausdorff with respect to ${\tau }_1$, it is also Hausdorff with respect to ${\tau }_2$.

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)