# T1 space

In topology and related branches of mathematics, a T$_1$ space is a topological space in which, for every pair of distinct points, each has a neighborhood not containing the other point. An R$_0$ space is one in which this holds for every pair of topologically distinguishable points. The properties T$_1$ and R$_0$ are examples of separation axioms.

## Definitions

Let X be a topological space and let x and y be points in X. We say that x and y are separated if each lies in a neighbourhood that does not contain the other point.
* X is called a T$_1$ space if any two distinct points in X are separated.
* X is called an R$_0$ space if any two topologically distinguishable points in X are separated.
A T$_1$ space is also called an accessible space or a space with Fréchet topology and an R$_0$ space is also called a symmetric space. (The term Fréchet space also has an entirely different meaning in functional analysis. For this reason, the term T$_1$ space is preferred. There is also a notion of a Fréchet–Urysohn space as a type of sequential space. The term symmetric space also has another meaning.)
A topological space is a T$_1$ space if and only if it is both an R$_0$ space and a Kolmogorov (or T$_0$) space (i.e., a space in which distinct points are topologically distinguishable). A topological space is an R$_0$ space if and only if its Kolmogorov quotient is a T$_1$ space.

## Properties

If $X$ is a topological space then the following conditions are equivalent:
* $X$ is a T$_1$ space.
* $X$ is a T$_0$ space and an R$_0$ space.
* Points are closed in $X$; that is, for every point $x\in X,$ the singleton set $\{x\}$ is a closed subset of $X.$
* Every subset of $X$ is the intersection of all the open sets containing it.
* Every finite set is closed.
* Every cofinite set of $X$ is open.
* For every $x\in X,$ the fixed ultrafilter at $x$ converges only to $x.$
* For every subset $S$ of $X$ and every point $x\in X,$ $x$ is a limit point of $S$ if and only if every open neighbourhood of $x$ contains infinitely many points of $S.$
* Each map from the Sierpiński space to $X$ is trivial.
* The map from the Sierpiński space to the single point has the lifting property with respect to the map from $X$ to the single point.
If $X$ is a topological space then the following conditions are equivalent: (where $\operatorname {cl} \{x\}$ denotes the closure of $\{x\}$)
* $X$ is an R$_0$ space.
* Given any $x\in X,$ the closure of $\{x\}$ contains only the points that are topologically indistinguishable from $x.$
* The Kolmogorov quotient of $X$ is T$_1$.
* For any $x,y\in X,$ $x$ is in the closure of $\{y\}$ if and only if $y$ is in the closure of $\{x\}.$
* The specialization preorder on $X$ is symmetric (and therefore an equivalence relation).
* The sets $\operatorname {cl} \{x\}$ for $x\in X$ form a partition of $X$ (that is, any two such sets are either identical or disjoint).
* If $F$ is a closed set and $x$ is a point not in $F$, then $F\cap \operatorname {cl} \{x\}=\emptyset .$
* Every neighbourhood of a point $x\in X$ contains $\operatorname {cl} \{x\}.$
* Every open set is a union of closed sets.
* For every $x\in X,$ the fixed ultrafilter at $x$ converges only to the points that are topologically indistinguishable from $x.$
In any topological space we have, as properties of any two points, the following implications
    separated $\implies$ topologically distinguishable $\implies$ distinct
If the first arrow can be reversed the space is R$_0$. If the second arrow can be reversed the space is T$_0$. If the composite arrow can be reversed the space is T$_1$. A space is T$_1$ if and only if it is both R$_0$ and T$_0$.
A finite T$_1$ space is necessarily discrete (since every set is closed).
A space that is locally T$_1$, in the sense that each point has a T$_1$ neighbourhood (when given the subspace topology), is also T$_1$. Similarly, a space that is locally R$_0$ is also R$_0$. In contrast, the corresponding statement does not hold for T$_2$ spaces. For example, the line with two origins is not a Hausdorff space but is locally Hausdorff.

## Examples

* Sierpiński space is a simple example of a topology that is T$_0$ but is not T$_1$, and hence also not R$_0$.
* The overlapping interval topology is a simple example of a topology that is T$_0$ but is not T$_1$.
* Every weakly Hausdorff space is T$_1$ but the converse is not true in general.
* The cofinite topology on an infinite set is a simple example of a topology that is T$_1$ but is not Hausdorff (T$_2$). This follows since no two nonempty open sets of the cofinite topology are disjoint. Specifically, let $X$ be the set of integers, and define the open sets $O_{A}$ to be those subsets of $X$ that contain all but a finite subset $A$ of $X.$ Then given distinct integers $x$ and $y$:
   * the open set $O_{\{x\}}$ contains $y$ but not $x,$ and the open set $O_{\{y\}}$ contains $x$ and not $y$;
   * equivalently, every singleton set $\{x\}$ is the complement of the open set $O_{\{x\}},$ so it is a closed set;
    so the resulting space is T$_1$ by each of the definitions above. This space is not T$_2$, because the intersection of any two open sets $O_{A}$ and $O_{B}$ is $O_{A}\cap O_{B}=O_{A\cup B},$ which is never empty. Alternatively, the set of even integers is compact but not closed, which would be impossible in a Hausdorff space.
* The above example can be modified slightly to create the double-pointed cofinite topology, which is an example of an R$_0$ space that is neither T$_1$ nor R$_1$. Let $X$ be the set of integers again, and using the definition of $O_{A}$ from the previous example, define a subbase of open sets $G_{x}$ for any integer $x$ to be $G_{x}=O_{\{x,x+1\}}$ if $x$ is an even number, and $G_{x}=O_{\{x-1,x\}}$ if $x$ is odd. Then the basis of the topology are given by finite intersections of the subbasic sets: given a finite set $A,$the open sets of $X$ are
       $U_{A}:=\bigcap _{x\in A}G_{x}.$
    The resulting space is not T$_0$ (and hence not T$_1$), because the points $x$ and $x+1$ (for $x$ even) are topologically indistinguishable; but otherwise it is essentially equivalent to the previous example.
* The Zariski topology on an algebraic variety (over an algebraically closed field) is T$_1$. To see this, note that the singleton containing a point with local coordinates $\left(c_{1},\ldots ,c_{n}\right)$ is the zero set of the polynomials $x_{1}-c_{1},\ldots ,x_{n}-c_{n}.$ Thus, the point is closed. However, this example is well known as a space that is not Hausdorff (T$_2$). The Zariski topology is essentially an example of a cofinite topology.
* The Zariski topology on a commutative ring (that is, the prime spectrum of a ring) is T$_0$ but not, in general, T$_1$. To see this, note that the closure of a one-point set is the set of all prime ideals that contain the point (and thus the topology is T$_0$). However, this closure is a maximal ideal, and the only closed points are the maximal ideals, and are thus not contained in any of the open sets of the topology, and thus the space does not satisfy axiom T$_1$. To be clear about this example: the Zariski topology for a commutative ring $A$ is given as follows: the topological space is the set $X$ of all prime ideals of $A.$ The base of the topology is given by the open sets $O_{a}$ of prime ideals that do not contain $a\in A.$ It is straightforward to verify that this indeed forms the basis: so $O_{a}\cap O_{b}=O_{ab}$ and $O_{0}=\varnothing$ and $O_{1}=X.$ The closed sets of the Zariski topology are the sets of prime ideals that do contain $a.$ Notice how this example differs subtly from the cofinite topology example, above: the points in the topology are not closed, in general, whereas in a T$_1$ space, points are always closed.
* Every totally disconnected space is T$_1$, since every point is a connected component and therefore closed.

## Generalisations to other kinds of spaces

The terms "T$_1$", "R$_0$", and their synonyms can also be applied to such variations of topological spaces as uniform spaces, Cauchy spaces, and convergence spaces. The characteristic that unites the concept in all of these examples is that limits of fixed ultrafilters (or constant nets) are unique (for T$_1$ spaces) or unique up to topological indistinguishability (for R$_0$ spaces).
As it turns out, uniform spaces, and more generally Cauchy spaces, are always R$_0$, so the T$_1$ condition in these cases reduces to the T$_0$ condition. But R$_0$ alone can be an interesting condition on other sorts of convergence spaces, such as pretopological spaces.