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In topology, a branch of mathematics, a retraction is a continuous mapping from a topological space into a subspace that preserves the position of all points in that subspace. [1] The subspace is then called a retract of the original space. A deformation retraction is a mapping that captures the idea of continuously shrinking a space into a ...
A subgroup is a retract if and only if it has a normal complement. [4] The normal complement, specifically, is the kernel of the retraction. Every direct factor is a retract. [1] Conversely, any retract which is a normal subgroup is a direct factor. [5] Every retract has the congruence extension property.
The concept of a retraction in category theory comes from the essentially similar notion of a retraction in topology: : where is a subspace of is a retraction in the topological sense, if it's a retraction of the inclusion map : in the category theory sense.
The FPP is also preserved by any retraction. According to the Brouwer fixed-point theorem, every compact and convex subset of a Euclidean space has the FPP. More generally, according to the Schauder-Tychonoff fixed point theorem every compact and convex subset of a locally convex topological vector space has the FPP. Compactness alone does not ...
Given a map :, the mapping cylinder is a space , together with a cofibration ~: and a surjective homotopy equivalence (indeed, Y is a deformation retract of ), such that the composition equals f. Thus the space Y gets replaced with a homotopy equivalent space M f {\displaystyle M_{f}} , and the map f with a lifted map f ~ {\displaystyle {\tilde ...
In mathematics, topology is a branch of geometry concerned with the study of topological spaces. The term topology is also used for a set of open sets used to define topological spaces. See the topology glossary for common terms and their definition. Properties of general topological spaces (as opposed to manifolds) are discussed in general ...
Retracted (phonetics), a sound pronounced to the back of the vocal tract, in linguistics; Retracted tongue root, a position of the tongue during the pronunciation of a vowel, in phonetics; Sternal retraction, a symptom of respiratory distress in humans; Retraction (kinesiology), an anatomical term of motion
Brouwer's fixed-point theorem is a fixed-point theorem in topology, named after L. E. J. (Bertus) Brouwer.It states that for any continuous function mapping a nonempty compact convex set to itself, there is a point such that () =.