Completely Hausdorff space

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Kolmogorov (T₀) version
Topological spaces in separation axiom
T₀ \| T₁ \| T₂ \| T_2½ \| completely T₂ T₃ \| T_3½ \| T₄ \| T₅ \| T₆

In topology, completely Hausdorff spaces and Urysohn spaces are types of topological spaces satisfying slightly stronger separation axioms than the more familiar Hausdorff space.

1 Definitions
2 Naming conventions
3 Relation to other separation axioms
4 Examples
5 References

[edit] Definitions

Suppose that X is a topological space. Let x and y be points in X.

We say that x and y can be separated by closed neighborhoods if there exists a closed neighborhood U of x and a closed neighborhood V of y such that U and V are disjoint (U ∩ V = ∅).
We say that x and y can be separated by a function if there exists a continuous function f : X → [0,1] (the unit interval) with f(x) = 0 and f(y) = 1.

A Urysohn space, or T_2½ space, is a space in which any two distinct points can be separated by closed neighborhoods.

A completely Hausdorff space, or functionally Hausdorff space, is a space in which any two distinct points can be separated by a function.

[edit] Naming conventions

The study of separation axioms is notorious for conflicts with naming conventions used. The definitions used in this article are those given by Willard (1970) and are the more modern definitions. Steen and Seebach (1970) and various other authors reverse the definition of completely Hausdorff spaces and Urysohn spaces. Readers of textbooks in topology must be sure to check the definitions used by the author. See History of the separation axioms for more on this issue.

[edit] Relation to other separation axioms

It is an easy exercise to show that any two points which can be separated by a function can be separated by closed neighborhoods. If they can be separated by closed neighborhoods then clearly they can be separated by neighborhoods. It follows that every completely Hausdorff space is Urysohn and every Urysohn space is Hausdorff.

One can also show that every regular Hausdorff space is Urysohn and every Tychonoff space (=completely regular Hausdorff space) is completely Hausdorff. In summary we have the following implications:

Tychonoff (T_3½)	$\Rightarrow$	regular Hausdorff (T₃)
$\Downarrow$		$\Downarrow$
completely Hausdorff	$\Rightarrow$	Urysohn (T_2½)	$\Rightarrow$	Hausdorff (T₂)	$\Rightarrow$	T₁

One can find counterexamples showing that none of these implications reverse^[1].

[edit] Examples

The cocountable extension topology is the topology on the real line generated by the union of the usual Euclidean topology and the cocountable topology. Sets are open in this topology if and only if they are of the form U \ A where U is open in the Euclidean topology and A is countable. This space is completely Hausdorff and Urysohn, but not regular (and thus not Tychonoff).

There are obscure examples of spaces which are Hausdorff but not Urysohn, and spaces which are Urysohn but not completely Hausdorff or regular Hausdorff. For details see Steen and Seebach.

[edit] References

^ Hausdorff space not completely Hausdorff on PlanetMath

Lynn Arthur Steen and J. Arthur Seebach, Jr., Counterexamples in Topology. Springer-Verlag, New York, 1978. Reprinted by Dover Publications, New York, 1995. ISBN 0-486-68735-X (Dover edition).
Stephen Willard, General Topology, Addison-Wesley, 1970. Reprinted by Dover Publications, New York, 2004. ISBN 0-486-43479-6 (Dover edition).