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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 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[edit]

Given a set and an indexed family of topological spaces with functions the initial topology on is the coarsest topology on such that each is continuous.


Definition in terms of open sets


If is a family of topologies indexed by then the least upper bound topology of these topologies is the coarsest topology on that is finer than each This topology always exists and it is equal to the topology generated by [3]


If for every denotes the topology on then is a topology on , and the initial topology of the by the mappings is the least upper bound topology of the -indexed family of topologies (for ).[3] 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 under finite intersections and arbitrary unions.


Sets of the form are often called cylinder sets. If contains exactly one element, then all the open sets of the initial topology are cylinder sets.

The is the initial topology on the subspace with respect to the inclusion map.

subspace topology

The is the initial topology with respect to the family of projection maps.

product topology

The 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.

inverse limit

The on a locally convex space is the initial topology with respect to the continuous linear forms of its dual space.

weak topology

Given a of topologies on a fixed set the initial topology on with respect to the functions is the supremum (or join) of the topologies in the lattice of topologies on That is, the initial topology is the topology generated by the union of the topologies

family

A topological space is if and only if it has the initial topology with respect to its family of (bounded) real-valued continuous functions.

completely regular

Every topological space has the initial topology with respect to the family of continuous functions from to the .

Sierpiński space

Several topological constructions can be regarded as special cases of the initial topology.

Properties[edit]

Characteristic property[edit]

The initial topology on can be characterized by the following characteristic property:
A function from some space to is continuous if and only if is continuous for each [4]

 – Finest topology making some functions continuous

Final topology

 – Topology on Cartesian products of topological spaces

Product topology

 – Topological space construction

Quotient space (topology)

 – Inherited topology

Subspace topology

(1989) [1966]. General Topology: Chapters 1–4 [Topologie Générale]. Éléments de mathématique. Berlin New York: Springer Science & Business Media. ISBN 978-3-540-64241-1. OCLC 18588129.

Bourbaki, Nicolas

(1989) [1967]. General Topology 2: Chapters 5–10 [Topologie Générale]. Éléments de mathématique. Vol. 4. Berlin New York: Springer Science & Business Media. ISBN 978-3-540-64563-4. OCLC 246032063.

Bourbaki, Nicolas

(1966). Topology. Boston: Allyn and Bacon. ISBN 978-0-697-06889-7. OCLC 395340485.

Dugundji, James

(1973). Topological Vector Spaces. Translated by Chaljub, Orlando. New York: Gordon and Breach Science Publishers. ISBN 978-0-677-30020-7. OCLC 886098.

Grothendieck, Alexander

Willard, Stephen (2004) [1970]. . Mineola, N.Y.: Dover Publications. ISBN 978-0-486-43479-7. OCLC 115240.

General Topology

Willard, Stephen (1970). . Reading, Massachusetts: Addison-Wesley. ISBN 0-486-43479-6.

General Topology

at PlanetMath.

Initial topology

at PlanetMath.

Product topology and subspace topology