Retract

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

Definition

A subspace of a topological space is said to be a retract if there is a continuous map on the whole topological space that maps everything to within the subspace, and that is identity on the retract. In other words, there is a continuous idempotent map whose image-cum-fixed-point space is precisely the given subspace. Such a map is termed a retraction.

Facts

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

Stronger properties

• Homotopy retract
• Weak deformation retract
• Deformation retract

Weaker properties

• Homotopically injective subspace
• Homologically injective subspace
• Weak retract
• Neighbourhood retract

Facts

• 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