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For example, any Star Domain of a Euclidean Space is contractible. On the other hand, Spheres of any finite dimension are not contractible.

Since a contractible space is homotopy equivalent to a point, all the Homotopy Group s of a contractible space are Trivial . Therefore any space with a nontrivial homotopy group cannot be contractible. Similarly, since Singular Homology is a homotopy invariant, the Reduced Homology Groups of a contractible space are all trivial.

For a topological space ''X'' the following are all equivalent (here ''Y'' is an arbitrary topological space):
  • ''X'' is contractible (i.e. the identity map is null-homotopic).

  • ''X'' is homotopy equivalent to a one-point space.

  • Any two maps ''f'',''g'' : ''Y'' → ''X'' are homotopic.

  • Any map ''f'' : ''Y'' → ''X'' is null-homotopic.


Any space which Deformation Retract s onto a point is clearly contractible. The converse, however, is false. There are examples of contractible spaces which do not deformation retract onto any point.

The Cone on a space ''X'' is always contractible. Therefore any space can be embedded in a contractible one.

Furthermore, ''X'' is contractible if and only if there exists a Retraction from the cone of ''X'' to ''X''.