Initial topology
In general topology and related areas of mathematics, the initial topology (or induced topology[1][2] or weak topology or limit topology or projective topology) on a set , with respect to a family of functions on , is the coarsest topology on X that makes those functions continuous.
The subspace topology and product topology constructions are both special cases of initial topologies. Indeed, the initial topology construction can be viewed as a generalization of these.
The dual notion is the final topology, which for a given family of functions mapping to a set is the finest topology on that makes those functions continuous.
Definition[]
Given a set X and an indexed family (Yi)i∈I of topological spaces with functions
the initial topology on is the coarsest topology on X such that each
is continuous.
Explicitly, the initial topology is the collection of open sets generated by all sets of the form , where is an open set in for some i ∈ I, under finite intersections and arbitrary unions. The sets are often called cylinder sets. If I contains exactly one element, all the open sets of are cylinder sets.
Examples[]
Several topological constructions can be regarded as special cases of the initial topology.
- The subspace topology is the initial topology on the subspace with respect to the inclusion map.
- The product topology is the initial topology with respect to the family of projection maps.
- The inverse limit of any inverse system of spaces and continuous maps is the set-theoretic inverse limit together with the initial topology determined by the canonical morphisms.
- The weak topology on a locally convex space is the initial topology with respect to the continuous linear forms of its dual space.
- Given a family of topologies {τi} on a fixed set X the initial topology on X with respect to the functions idi : X → (X, τi) is the supremum (or join) of the topologies {τi} in the lattice of topologies on X. That is, the initial topology τ is the topology generated by the union of the topologies {τi}.
- A topological space is completely regular if and only if it has the initial topology with respect to its family of (bounded) real-valued continuous functions.
- Every topological space X has the initial topology with respect to the family of continuous functions from X to the Sierpiński space.
Properties[]
Characteristic property[]
The initial topology on X can be characterized by the following characteristic property:
A function from some space to is continuous if and only if is continuous for each i ∈ I.
Note that, despite looking quite similar, this is not a universal property. A categorical description is given below.
Evaluation[]
By the universal property of the product topology, we know that any family of continuous maps determines a unique continuous map
This map is known as the evaluation map.
A family of maps is said to separate points in X if for all in X there exists some i such that . Clearly, the family separates points if and only if the associated evaluation map f is injective.
The evaluation map f will be a topological embedding if and only if X has the initial topology determined by the maps and this family of maps separates points in X.
Separating points from closed sets[]
If a space X comes equipped with a topology, it is often useful to know whether or not the topology on X is the initial topology induced by some family of maps on X. This section gives a sufficient (but not necessary) condition.
A family of maps {fi: X → Yi} separates points from closed sets in X if for all closed sets A in X and all x not in A, there exists some i such that
where cl denotes the closure operator.
- Theorem. A family of continuous maps {fi: X → Yi} separates points from closed sets if and only if the cylinder sets , for U open in Yi, form a base for the topology on X.
It follows that whenever {fi} separates points from closed sets, the space X has the initial topology induced by the maps {fi}. The converse fails, since generally the cylinder sets will only form a subbase (and not a base) for the initial topology.
If the space X is a T0 space, then any collection of maps {fi} that separates points from closed sets in X must also separate points. In this case, the evaluation map will be an embedding.
Categorical description[]
In the language of category theory, the initial topology construction can be described as follows. Let be the functor from a discrete category to the category of topological spaces which maps . Let be the usual forgetful functor from to . The maps can then be thought of as a cone from to . That is, is an object of —the category of cones to . More precisely, this cone defines a -structured cosink in .
The forgetful functor induces a functor . The characteristic property of the initial topology is equivalent to the statement that there exists a universal morphism from to , i.e.: a terminal object in the category .
Explicitly, this consists of an object in together with a morphism such that for any object in and morphism there exists a unique morphism such that the following diagram commutes:
The assignment placing the initial topology on extends to a functor which is right adjoint to the forgetful functor . In fact, is a right-inverse to ; since is the identity functor on .
See also[]
- Final topology
- Induced topology
References[]
- ^ Rudin, Walter (1991). Functional Analysis. International Series in Pure and Applied Mathematics. 8 (Second ed.). New York, NY: McGraw-Hill Science/Engineering/Math. ISBN 978-0-07-054236-5. OCLC 21163277.
- ^ Adamson, Iain T. (1996). "Induced and Coinduced Topologies". A General Topology Workbook. Birkhäuser, Boston, MA: 23–30. doi:10.1007/978-0-8176-8126-5_3. ISBN 978-0-8176-3844-3. Retrieved July 21, 2020.
... the topology induced on E by the family of mappings ...
Sources[]
- Willard, Stephen (1970). General Topology. Reading, Massachusetts: Addison-Wesley. ISBN 0-486-43479-6.
- "Initial topology". PlanetMath.
- "Product topology and subspace topology". PlanetMath.
- General topology