| Separation Axioms in Topological Spaces |
| Kolmogorov (T0) version |
|---|
| T0 | T1 | T2 | T2½ | completely T2 T3 | T3½ | T4 | T5 | T6 |
In topology and related branches of mathematics, a normal space is a topological space X that satisfies Axiom T4: every two disjoint closed sets of X have disjoint open neighborhoods. A normal Hausdorff space is also called a T4 space. These conditions are examples of separation axioms and their further strengthenings define completely normal Hausdorff spaces, or T5 spaces, and perfectly normal Hausdorff spaces, or T6 spaces.
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A topological space X is a normal space if, given any disjoint closed sets E and F, there are open neighbourhoods U of E and V of F that are also disjoint. More intuitively, this condition says that E and F can be separated by neighbourhoods.
A T4 space is a T1 space X that is normal; this is equivalent to X being Hausdorff and normal.
A completely normal space or a hereditarily normal space is a topological space X such that every subspace of X with subspace topology is a normal space. It turns out that X is completely normal if and only if every two separated sets can be separated by neighbourhoods.
A completely T4 space, or T5 space is a completely normal Hausdorff topological space X; equivalently, every subspace of X must be a T4 space.
A perfectly normal space is a topological space X in which every two disjoint non-empty closed sets E and F can be precisely separated by a continuous function f from X to the real line R: the preimages of {0} and {1} under f are, respectively, E and F. (In this definition, the real line can be replaced with the unit interval [0,1].)
It turns out that X is perfectly normal if and only if X is normal and every closed set is a Gδ set. Equivalently, X is perfectly normal if and only if every closed set is a zero set. Every perfectly normal space is automatically completely normal.
A Hausdorff perfectly normal space X is a T6 space, or perfectly T4 space.
Note that the terms "normal space" and "T4" and derived concepts occasionally have a different meaning. (Nonetheless, "T5" always means the same as "completely T4", whatever that may be.) The definitions given here are the ones usually used today, and the ones used in Wikipedia articles on separation axioms. For more on this issue, see History of the separation axioms.
Terms like "normal regular space" and "normal Hausdorff space" also turn up in the literature — they simply mean that the space both is normal and satisfies the other condition mentioned. In particular, a normal Hausdorff space is the same thing as a T4 space. Given the historical confusion of the meaning of the terms, we prefer verbal descriptions when applicable, that is, "normal Hausdorff" instead of "T4", or "completely normal Hausdorff" instead of "T5".
Fully normal spaces and fully T4 spaces are discussed elsewhere; they are related to paracompactness.
A locally normal space is a topological space where every point has an open neighbourhood that is normal. Every normal space is locally normal, but the converse is not true. A classical example of a completely regular locally normal space that is not normal is the Nemytskii plane.
Most spaces encountered in mathematical analysis are normal Hausdorff spaces, or at least normal regular spaces:
Also, all fully normal spaces are normal (even if not regular). Sierpinski space is an example of a normal space that is not regular.
An important example of a non-normal topology is given by the Zariski topology on an algebraic variety or on the spectrum of a ring, which is used in algebraic geometry.
A non-normal space of some relevance to analysis is the topological vector space of all functions from the real line R to itself, with the topology of pointwise convergence. More generally, a theorem of A. H. Stone states that the product of uncountably many non-compact Hausdorff spaces is never normal.
The main significance of normal spaces lies in the fact that they admit "enough" continuous real-valued functions, as expressed by the following theorems valid for any normal space X.
Urysohn's lemma: If A and B are two disjoint closed subsets of X, then there exists a continuous function f from X to the real line R such that f(x) = 0 for all x in A and f(x) = 1 for all x in B. In fact, we can take the values of f to be entirely within the unit interval [0,1]. (In fancier terms, disjoint closed sets are not only separated by neighbourhoods, but also separated by a function.)
More generally, the Tietze extension theorem: If A is a closed subset of X and f is a continuous function from A to R, then there exists a continuous function F: X → R which extends f in the sense that F(x) = f(x) for all x in A.
If U is a locally finite open cover of a normal space X, then there is a partition of unity precisely subordinate to U. (This shows the relationship of normal spaces to paracompactness.)
In fact, any space that satisfies any one of these conditions must be normal.
A product of normal spaces is not necessarily normal. This fact was first proved by Robert Sorgenfrey. An example of this phenomenon is the Sorgenfrey plane. Also, a subset of a normal space need not be normal (i.e. not every normal Hausdorff space is a completely normal Hausdorff space), since every Tychonoff space is a subset of its Stone-Cech compactification (which is normal Hausdorff). A more explicit example is the Tychonoff plank.
If a normal space is R0, then it is in fact completely regular. Thus, anything from "normal R0" to "normal completely regular" is the same as what we normally call normal regular. Taking Kolmogorov quotients, we see that all normal T1 spaces are Tychonoff. These are what we normally call normal Hausdorff spaces.
Counterexamples to some variations on these statements can be found in the lists above. Specifically, Sierpinski space is normal but not regular, while the space of functions from R to itself is Tychonoff but not normal.