the sphere is indecomposable as a topological space
Proposition. If for any topological spaces and the -dimensional sphere is homeomorphic to , then either has exactly one point or has exactly one point.
Proof. Recall that the homotopy group functor is additive, i.e. . Assume that is homeomorphic to . Now and thus we have:
Since is an indecomposable group, then either or .
Assume that . Consider the map such that . Since is homeomorphic to and , then is homotopic to some constant map. Let and be such that
Consider the map defined by the formula
Note that and and thus is a deformation retract of . But is a sphere and spheres do not have proper deformation retracts (please see this entry (http://planetmath.org/EveryMapIntoSphereWhichIsNotOntoIsNullhomotopic) for more details). Therefore , so has exactly one point.