# Difference between revisions of "Final topology"

en>Tobias Bergemann (Undid revision 598099240 by 137.204.30.43 (talk): No, it was right before?) |
(→Examples: Fixed the link to "quotient map") |
||

Line 11: | Line 11: | ||

is [[continuous function (topology)|continuous]]. | is [[continuous function (topology)|continuous]]. | ||

Explicitly, the final topology may be described as follows: a subset ''U'' of ''X'' is open [[if and only if]] <math>f_i^{-1}(U)</math> is open in ''Y''<sub>''i''</sub> for each ''i'' ∈ ''I''. | Explicitly, the final topology may be described as follows: a subset ''U'' of ''X'' is open [[if and only if]] <math>f_i^{-1}(U)</math> is open in ''Y''<sub>''i''</sub> for each ''i'' ∈ ''I''. | ||

== Examples == | == Examples == | ||

* The [[quotient topology]] is the final topology on the quotient space with respect to the [[quotient map]]. | * The [[quotient topology]] is the final topology on the quotient space with respect to the [[Quotient_space_(topology)#Quotient_map|quotient map]]. | ||

* The [[disjoint union (topology)|disjoint union]] is the final topology with respect to the family of [[canonical injection]]s. | * The [[disjoint union (topology)|disjoint union]] is the final topology with respect to the family of [[canonical injection]]s. | ||

* More generally, a topological space is [[coherent topology|coherent]] with a family of subspaces if it has the final topology coinduced by the inclusion maps. | * More generally, a topological space is [[coherent topology|coherent]] with a family of subspaces if it has the final topology coinduced by the inclusion maps. | ||

Line 41: | Line 41: | ||

==References== | ==References== | ||

* Stephen Willard | * {{cite book | first=Stephen | last=Willard | title=General Topology | year=1970 | publisher=Addison-Wesley | location=Reading, MA | zbl=0205.26601 | series=Addison-Wesley Series in Mathematics}}. ''(Provides a short, general introduction in section 9 and Exercise 9H)'' | ||

[[Category:General topology]] | [[Category:General topology]] |

## Latest revision as of 10:54, 22 May 2014

In general topology and related areas of mathematics, the **final topology** (or **strong topology** or **colimit topology** or **projective topology**) on a set , with respect to a family of functions into , is the finest topology on *X* which makes those functions continuous.

The dual notion is the initial topology.

## Definition

Given a set and a family of topological spaces with functions

the **final topology** on is the finest topology such that each

is continuous.

Explicitly, the final topology may be described as follows: a subset *U* of *X* is open if and only if is open in *Y*_{i} for each *i* ∈ *I*.

## Examples

- The quotient topology is the final topology on the quotient space with respect to the quotient map.
- The disjoint union is the final topology with respect to the family of canonical injections.
- More generally, a topological space is coherent with a family of subspaces if it has the final topology coinduced by the inclusion maps.
- The direct limit of any direct system of spaces and continuous maps is the set-theoretic direct limit together with the final topology determined by the canonical morphisms.
- Given a family of topologies {τ
_{i}} on a fixed set*X*the final topology on*X*with respect to the functions id_{X}: (*X*, τ_{i}) →*X*is the infimum (or meet) of the topologies {τ_{i}} in the lattice of topologies on*X*. That is, the final topology τ is the intersection of the topologies {τ_{i}}. - The etale space of a sheaf is topologized by a final topology.

## Properties

A subset of is closed/open if and only if its preimage under *f*_{i} is closed/open in for each *i* ∈ *I*.

The final topology on *X* can be characterized by the following universal property: a function from to some space is continuous if and only if is continuous for each *i* ∈ *I*.

By the universal property of the disjoint union topology we know that given any family of continuous maps *f*_{i} : *Y*_{i} → *X* there is a unique continuous map

If the family of maps *f*_{i} *covers* *X* (i.e. each *x* in *X* lies in the image of some *f*_{i}) then the map *f* will be a quotient map if and only if *X* has the final topology determined by the maps *f*_{i}.

## Categorical description

In the language of category theory, the final topology construction can be described as follows. Let *Y* be a functor from a discrete category *J* to the category of topological spaces **Top** which selects the spaces *Y*_{i} for *i* in *J*. Let Δ be the diagonal functor from **Top** to the functor category **Top**^{J} (this functor sends each space *X* to the constant functor to *X*). The comma category (*Y* ↓ Δ) is then the category of cones from *Y*, i.e. objects in (*Y* ↓ Δ) are pairs (*X*, *f*) where *f*_{i} : *Y*_{i} → *X* is a family of continuous maps to *X*. If *U* is the forgetful functor from **Top** to **Set** and Δ′ is the diagonal functor from **Set** to **Set**^{J} then the comma category (*UY* ↓ Δ′) is the category of all cones from *UY*. The final topology construction can then be described as a functor from (*UY* ↓ Δ′) to (*Y* ↓ Δ). This functor is left adjoint to the corresponding forgetful functor.

## See also

## References

- {{#invoke:citation/CS1|citation

|CitationClass=book
}}. *(Provides a short, general introduction in section 9 and Exercise 9H)*