Definition:T5 Space/Definition 2

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Definition

Let $T = \left({S, \tau}\right)$ be a topological space.


$\left({S, \tau}\right)$ is a $T_5$ space if and only if:

every subset $Y \subseteq S$ contains a closed neighborhood of each $A \subseteq Y^\circ$ for which $A^- \subseteq Y$.

In the above, $Y^\circ$ denotes the interior of $Y$ and $A^-$ denotes the closure of $A$.


Variants of Name

From about 1970, treatments of this subject started to refer to this as a completely normal space, and what is defined on $\mathsf{Pr} \infty \mathsf{fWiki}$ as a completely normal space as a $T_5$ space.

However, the names are to a fair extent arbitrary and a matter of taste, as there appears to be no completely satisfactory system for naming all these various Tychonoff separation axioms.


The system as used here broadly follows 1970: Lynn Arthur Steen and J. Arthur Seebach, Jr.: Counterexamples in Topology.

The system used on the Separation axiom page at Wikipedia differs from this.


Also see

  • Results about $T_5$ spaces can be found here.


Sources