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3-manifolds
== Distinct from Homeomorphism ==
== Distinct from Homeomorphism ==
Two topological spaces are said to be homeomorphic if there exists a homeomorphism between them. In this case, the two spaces are topologically the same. Similarly, two smooth manifolds (which carry the structure of topological spaces) are said to be diffeomorphic if there exists a diffeomorphism between them. Since a diffeomorphism is a special kind of homeomorphism, any two smooth manifolds which are diffeomorphic are in fact homeomorphic. One might expect that if two smooth manifolds are homeomorphic (i.e., equivalent as topological spaces) then they are diffeomorphic (i.e., equivalent as manifolds). However, this is not the case! It is possible to give the 7-dimensional sphere <math>S^7</math> the structure of a smooth manifold in such a way that it is not diffeomorphic to <math>S^7</math> with the standard smooth structure, though it is homeomorphic! In fact, there are exactly 28 such exotic structures on the 7-sphere up to diffeomorphism. Such examples are called '''exotic spheres'''. While the exotic spheres were the first known example of this phenomenon, it was later shown that even 4-dimension Euclidean space may be endowed with any of an uncountable number of non-diffeomorphic smooth structures.
Two topological spaces are said to be homeomorphic if there exists a homeomorphism between them. In this case, the two spaces are topologically the same. Similarly, two smooth manifolds (which carry the structure of topological spaces) are said to be diffeomorphic if there exists a diffeomorphism between them. Since a diffeomorphism is a special kind of homeomorphism, any two smooth manifolds which are diffeomorphic are in fact homeomorphic. One might expect that if two smooth manifolds are homeomorphic (i.e., equivalent as topological spaces) then they are diffeomorphic (i.e., equivalent as manifolds). However, this is not the case! It is possible to give the 7-dimensional sphere <math>S^7</math> the structure of a smooth manifold in such a way that it is not diffeomorphic to <math>S^7</math> with the standard smooth structure, though it is homeomorphic! In fact, there are exactly 28 such exotic structures on the 7-sphere up to diffeomorphism. Such examples are called '''exotic spheres'''. While the exotic spheres were the first known example of this phenomenon, it was later shown that even 4-dimension Euclidean space may be endowed with any of an uncountable number of non-diffeomorphic smooth structures.
In some important cases, however, the notions of diffeomorphism and homeomorphism coincide. It is a fundamental theorem of low-dimensional topology that two smooth 3-manifolds are diffeomorphic if and only if they are homeomorphic.