Топологиялық кеңістіктердің қасиеттері

Mathematical property of a space
In topology and related areas of mathematics, a topological property or topological invariant is a property of a topological space that is invariant under homeomorphisms. Alternatively, a topological property is a proper class of topological spaces which is closed under homeomorphisms. That is, a property of spaces is a topological property if whenever a space X possesses that property every space homeomorphic to X possesses that property. Informally, a topological property is a property of the space that can be expressed using open sets. A common problem in topology is to decide whether two topological spaces are homeomorphic or not. To prove that two spaces are not homeomorphic, it is sufficient to find a topological property which is not shared by them.

The cardinality of the space The cardinality of the topology (the set of open subsets) of the space Weight , the least cardinality of a basis of the topology of the space Density , the least cardinality of a subset of whose closure is .

Some of these terms are defined differently in older mathematical literature; see history of the separation axioms. T0 or Kolmogorov. A space is Kolmogorov if for every pair of distinct points x and y in the space, there is at least either an open set containing x but not y, or an open set containing y but not x.
T1 or Fréchet. A space is Fréchet if for every pair of distinct points x and y in the space, there is an open set containing x but not y. (Compare with T0; here, we are allowed to specify which point will be contained in the open set.) Equivalently, a space is T1 if all its singletons are closed. T1 spaces are always T0. Sober. A space is sober if every irreducible closed set C has a unique generic point p. In other words, if C is not the (possibly nondisjoint) union of two smaller closed non empty subsets, then there is a p such that the closure of {p} equals C, and p is the only point with this property. T2 or Hausdorff. A space is Hausdorff if every two distinct points have disjoint neighbourhoods. T2 spaces are always T1. T2½ or Urysohn. A space is Urysohn if every two distinct points have disjoint closed neighbourhoods. T2½ spaces are always T2. Completely T2 or completely Hausdorff. A space is completely T2 if every two distinct points are separated by a function. Every completely Hausdorff space is Urysohn. Regular. A space is regular if whenever C is a closed set and p is a point not in C, then C and p have disjoint neighbourhoods. T3 or Regular Hausdorff. A space is regular Hausdorff if it is a regular T0 space. (A regular space is Hausdorff if and only if it is T0, so the terminology is consistent.) Completely regular. A space is completely regular if whenever C is a closed set and p is a point not in C, then C and {p} are separated by a function. T3½, Tychonoff, Completely regular Hausdorff or Completely T3. A Tychonoff space is a completely regular T0 space. (A completely regular space is Hausdorff if and only if it is T0, so the terminology is consistent.) Tychonoff spaces are always regular Hausdorff. Normal. A space is normal if any two disjoint closed sets have disjoint neighbourhoods. Normal spaces admit partitions of unity. T4 or Normal Hausdorff. A normal space is Hausdorff if and only if it is T1. Normal Hausdorff spaces are always Tychonoff. Completely normal. A space is completely normal if any two separated sets have disjoint neighbourhoods. T5 or Completely normal Hausdorff. A completely normal space is Hausdorff if and only if it is T1. Completely normal Hausdorff spaces are always normal Hausdorff. Perfectly normal. A space is perfectly normal if any two disjoint closed sets are precisely separated by a function. A perfectly normal space must also be completely normal. T6 or Perfectly normal Hausdorff, or perfectly T4. A space is perfectly normal Hausdorff, if it is both perfectly normal and T1. A perfectly normal Hausdorff space must also be completely normal Hausdorff. Discrete space. A space is discrete if all of its points are completely isolated, i. e. if any subset is open. Number of isolated points. The number of isolated points of a topological space.

Separable. A space is separable if it has a countable dense subset. First countable. A space is first countable if every point has a countable local base. Second countable. A space is second countable if it has a countable base for its topology. Second countable spaces are always separable, first countable and Lindelöf.

Connected. A space is connected if it is not the union of a pair of disjoint non empty open sets. Equivalently, a space is connected if the only clopen sets are the empty set and itself. Locally connected. A space is locally connected if every point has a local base consisting of connected sets. Totally disconnected. A space is totally disconnected if it has no connected subset with more than one point. Path connected. A space X is path connected if for every two points x, y in X, there is a path p from x to y, i. e., a continuous map p: [0,1] → X with p(0) = x and p(1) = y. Path connected spaces are always connected. Locally path connected. A space is locally path connected if every point has a local base consisting of path connected sets. A locally path connected space is connected if and only if it is path connected. Arc connected. A space X is arc connected if for every two points x, y in X, there is an arc f from x to y, i. e., an injective continuous map with and Arc connected spaces are path connected. Simply connected. A space X is simply connected if it is path connected and every continuous map is homotopic to a constant map. Locally simply connected. A space X is locally simply connected if every point x in X has a local base of neighborhoods U that is simply connected. Semi locally simply connected. A space X is semi locally simply connected if every point has a local base of neighborhoods U such that every loop in U is contractible in X. Semi local simple connectivity, a strictly weaker condition than local simple connectivity, is a necessary condition for the existence of a universal cover. Contractible. A space X is contractible if the identity map on X is homotopic to a constant map. Contractible spaces are always simply connected. Hyperconnected. A space is hyperconnected if no two non empty open sets are disjoint. Every hyperconnected space is connected. Ultraconnected. A space is ultraconnected if no two non empty closed sets are disjoint. Every ultraconnected space is path connected. Indiscrete or trivial. A space is indiscrete if the only open sets are the empty set and itself. Such a space is said to have the trivial topology.

Compact. A space is compact if every open cover has a finite subcover. Some authors call these spaces quasicompact and reserve compact for Hausdorff spaces where every open cover has finite subcover. Compact spaces are always Lindelöf and paracompact. Compact Hausdorff spaces are therefore normal. Sequentially compact. A space is sequentially compact if every sequence has a convergent subsequence. Countably compact. A space is countably compact if every countable open cover has a finite subcover. Pseudocompact. A space is pseudocompact if every continuous real valued function on the space is bounded. σ compact. A space is σ compact if it is the union of countably many compact subsets. Lindelöf. A space is Lindelöf if every open cover has a countable subcover. Paracompact. A space is paracompact if every open cover has an open locally finite refinement. Paracompact Hausdorff spaces are normal. Locally compact. A space is locally compact if every point has a local base consisting of compact neighbourhoods. Slightly different definitions are also used. Locally compact Hausdorff spaces are always Tychonoff. Ultraconnected compact. In an ultra connected compact space X every open cover must contain X itself. Non empty ultra connected compact spaces have a largest proper open subset called a monolith.

Metrizable. A space is metrizable if it is homeomorphic to a metric space. Metrizable spaces are always Hausdorff and paracompact (and hence normal and Tychonoff), and first countable. Moreover, a topological space is said to be metrizable if there exists a metric for such that the metric topology is identical with the topology
Polish. A space is called Polish if it is metrizable with a separable and complete metric. Locally metrizable. A space is locally metrizable if every point has a metrizable neighbourhood.

There are many examples of properties of metric spaces, etc, which are not topological properties. To show a property is not topological, it is sufficient to find two homeomorphic topological spaces such that has , but does not have
For example, the metric space properties of boundedness and completeness are not topological properties. Let and be metric spaces with the standard metric. Then, via the homeomorphism However, is complete but not bounded, while is bounded but not complete.