Separation axiom


In topology and related fields of mathematics, there are several restrictions that one often makes on the kinds of topological spaces that one wishes to consider. Some of these restrictions are given by the separation axioms. These are sometimes called Tychonoff separation axioms, after Andrey Tychonoff.
The separation axioms are axioms only in the sense that, when defining the notion of topological space, one could add these conditions as extra axioms to get a more restricted notion of what a topological space is. The modern approach is to fix once and for all the axiomatization of topological space and then speak of kinds of topological spaces.
However, the term "separation axiom" has stuck. The separation axioms are denoted with the letter "T" after the German Trennungsaxiom, which means "separation axiom."
The precise meanings of the terms associated with the separation axioms has varied over time, as explained in History of the separation axioms. It is important to understand the authors' definition of each condition mentioned to know exactly what they mean, especially when reading older literature.

Preliminary definitions

Before we define the separation axioms themselves, we give concrete meaning to the concept of separated sets in topological spaces.
The separation axioms are about the use of topological means to distinguish disjoint sets and distinct points. It's not enough for elements of a topological space to be distinct ; we may want them to be topologically distinguishable. Similarly, it's not enough for subsets of a topological space to be disjoint; we may want them to be separated. The separation axioms all say, in one way or another, that points or sets that are distinguishable or separated in some weak sense must also be distinguishable or separated in some stronger sense.
Let X be a topological space. Then two points x and y in X are topologically distinguishable if they do not have exactly the same neighbourhoods ; that is, at least one of them has a neighbourhood that is not a neighbourhood of the other.
Two points x and y are separated if each of them has a neighbourhood that is not a neighbourhood of the other; that is, neither belongs to the other's closure. More generally, two subsets A and B of X are separated if each is disjoint from the other's closure. All of the remaining conditions for separation of sets may also be applied to points by using singleton sets. Points x and y will be considered separated, by neighbourhoods, by closed neighbourhoods, by a continuous function, precisely by a function, if and only if their singleton sets and are separated according to the corresponding criterion.
Subsets A and B are separated by neighbourhoods if they have disjoint neighbourhoods. They are separated by closed neighbourhoods if they have disjoint closed neighbourhoods. They are separated by a continuous function if there exists a continuous function f from the space X to the real line R such that the image f equals and f equals. Finally, they are precisely separated by a continuous function if there exists a continuous function f from X to R such that the preimage f−1 equals A and f−1 equals B.
These conditions are given in order of increasing strength: Any two topologically distinguishable points must be distinct, and any two separated points must be topologically distinguishable. Any two separated sets must be disjoint, any two sets separated by neighbourhoods must be separated, and so on.
For more on these conditions, see the articles Separated sets and Topological distinguishability.

Main definitions

These definitions all use essentially the [|preliminary definitions] above.
Many of these names have alternative meanings in some of mathematical literature, as explained on History of the separation axioms; for example, the meanings of "normal" and "T4" are sometimes interchanged, similarly "regular" and "T3", etc. Many of the concepts also have several names; however, the one listed first is always least likely to be ambiguous.
Most of these axioms have alternative definitions with the same meaning; the definitions given here fall into a consistent pattern that relates the various notions of separation defined in the previous section. Other possible definitions can be found in the individual articles.
In all of the following definitions, X is again a topological space.
The following table summarizes the separation axioms as well as the implications between them: cells which are merged represent equivalent properties, each axiom implies the ones in the cells to its left, and if we assume the T1 axiom, then each axiom also implies the ones in the cells above it.

Relationships between the axioms

The T0 axiom is special in that it can not only be added to a property but also be subtracted from a property, in a fairly precise sense; see Kolmogorov quotient for more information. When applied to the separation axioms, this leads to the relationships in the table to the left below. In this table, you go from the right side to the left side by adding the requirement of T0, and you go from the left side to the right side by removing that requirement, using the Kolmogorov quotient operation.
T0 versionNon-T0 version
T0
T1R0
Hausdorff R1
T2½
Completely Hausdorff
Regular Hausdorff Regular
Tychonoff Completely regular
Normal T0Normal
Normal Hausdorff Normal regular
Completely normal T0Completely normal
Completely normal Hausdorff Completely normal regular
Perfectly normal T0Perfectly normal
Perfectly normal Hausdorff Perfectly normal regular

Other than the inclusion or exclusion of T0, the relationships between the separation axioms are indicated in the diagram to the right. In this diagram, the non-T0 version of a condition is on the left side of the slash, and the T0 version is on the right side. Letters are used for abbreviation as follows:
"P" = "perfectly", "C" = "completely", "N" = "normal", and "R" = "regular". A bullet indicates that there is no special name for a space at that spot. The dash at the bottom indicates no condition.
You can combine two properties using this diagram by following the diagram upwards until both branches meet. For example, if a space is both completely normal and completely Hausdorff, then following both branches up, you find the spot "•/T5".
Since completely Hausdorff spaces are T0, you take the T0 side of the slash, so a completely normal completely Hausdorff space is the same as a T5 space.
As you can see from the diagram, normal and R0 together imply a host of other properties, since combining the two properties leads you to follow a path through the many nodes on the rightside branch. Since regularity is the most well known of these, spaces that are both normal and R0 are typically called "normal regular spaces". In a somewhat similar fashion, spaces that are both normal and T1 are often called "normal Hausdorff spaces" by people that wish to avoid the ambiguous "T" notation. These conventions can be generalised to other regular spaces and Hausdorff spaces.

Other separation axioms

There are some other conditions on topological spaces that are sometimes classified with the separation axioms, but these don't fit in with the usual separation axioms as completely. Other than their definitions, they aren't discussed here; see their individual articles.