Urysohn and completely Hausdorff spaces

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Separation axioms
in topological spaces
Kolmogorov classification
T0  (Kolmogorov)
T1   
T2  (Hausdorff)
T2½ (Urysohn)
completely T2  (completely Hausdorff)
T3  (regular Hausdorff)
T (Tychonoff)
T4  (normal Hausdorff)
T5  (completely normal
 Hausdorff)
T6  (perfectly normal
 Hausdorff)
History

In topology, a discipline within mathematics, an Urysohn space, or T space, is a topological space in which any two distinct points can be separated by closed neighborhoods. A completely Hausdorff space, or functionally Hausdorff space, is a topological space in which any two distinct points can be separated by a continuous function. These conditions are separation axioms that are somewhat stronger than the more familiar Hausdorff axiom T2.

Definitions[edit]

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

A Urysohn space, or T 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 continuous function.

Naming conventions[edit]

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.

Relation to other separation axioms[edit]

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)  \Rightarrow  regular Hausdorff (T3)
\Downarrow \Downarrow
completely Hausdorff  \Rightarrow  Urysohn (T)  \Rightarrow  Hausdorff (T2)  \Rightarrow  T1

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

Examples[edit]

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.

Notes[edit]

  1. ^ Hausdorff space not completely Hausdorff at PlanetMath.org.

References[edit]