Final topology

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

Contents 1 Definition 2 Examples 3 Properties 4 Categorical description 5 See also 6 References

[edit] Definition

Given a set X and a family of topological spaces Y_i with functions

the final topology τ on X 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.

[edit] 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}.

[edit] Properties

A subset of X is closed if and only if its preimage under f_i is closed in Y_i for each i ∈ I.

The final topology on X can be characterized by the following universal property: a function g from X to some space Z 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.

[edit] 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.

[edit] See also

• Initial topology

[edit] References

• Stephen Willard, General Topology, (1970) Addison-Wesley Publishing Company, Reading Massachusetts. (Provides a short, general introduction)