This article defines a property over pairs of a topological space and a subspace, or equivalently, properties over subspace embeddings (viz, subsets) in topological spaces
A subspace of a topological space is said to be a retract if it satisfies the following equivalent conditions:
- There is a continuous map on the whole topological space that maps everything to within the subspace, and that is the identity map when restricted to the subspace.
- There is a continuous idempotent map whose image-cum-fixed-point space is precisely the given subspace.
- The inclusion of the subspace in the whole space has a left inverse.
Such a map (satisfying any of the three equivalent conditions) is termed a retraction.
Definition with symbols
Fill this in later
Clearly the whole space is a retract of itself (the identity map being a retraction) and every one-point subspace is also a retract (the constant map to that one point being the retraction).
Relation with other properties
- Homotopically injective subspace
- Homologically injective subspace
- Weak retract
- Neighbourhood retract
- In a Hausdorff space, any retract is a closed subset
- Many properties of topological spaces are preserved on taking retracts. Examples are properties like being simply connected, weakly contractible, contractible, or having the fixed-point property. For a full list of such properties, refer: Category:Retract-hereditary properties of topological spaces
- Topology (2nd edition) by James R. MunkresMore info, Page 223, Exercise 4 (definition introduced in exercise)
- An Introduction to Algebraic Topology (Graduate Texts in Mathematics) by Joseph J. RotmanMore info, Page 4 (formal definition)
- Algebraic Topology by Edwin H. SpanierMore info, Page 28 (formal definition)