Contractible

A set in which can be reduced to one of its points, say , by a continuous deformation, is said to be contractible. The transformation is such that each point of the set is driven to through a path with the properties that

1. Each path runs entirely inside the set.

2. Nearby points move on "neighboring" paths.

Condition (1) implies that a disconnected set, i.e., a set consisting of separate parts, cannot be contractible.

Condition (2) implies that the circumference of a circle is not contractible. The latter follows by considering two near points and lying on different sides of a point . The paths connecting and with are either opposite each other or have different lengths. A similar argument shows that, in general, for all , the -sphere (i.e., the boundary of the -dimensional ball) is not contractible.

A gap or a hole in a set can be an obstruction to contractibility. There are, however, examples of contractible sets with holes, for example, the "house with two rooms." In a case like this, it is not evident how to construct a transformation of the type described above. However, its existence is assured by the formal definition of contractibility of a set , namely that is homotopic to one of its points . This means that there is a continuous map such that is the identity map and is the constant map sending each point to . Thus, describes a continuous path from to as varies from 0 to 1, and (1) is fulfilled. Moreover, since the map is also continuous with respect to the second component, the path starting at varies continuously with respect to , as required by (2).