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Homeomorphic)

*This word should not be confused with homomorphism.*

In the mathematical field of topology a **homeomorphism** or **topological isomorphism** is a special isomorphism between topological spaces which respects topological properties. Two spaces with a homeomorphism between them are called **homeomorphic**. From a topological viewpoint they are the same.

Roughly speaking a topological space is a geometric object and the homeomorphism is a continuous stretching and bending of the object into a new shape. Thus a square and a circle are homeomorphic. The traditional joke is that the topologist can't tell the coffee cup she is drinking out of from the donut she is eating, since a sufficiently pliable donut could be reshaped to the form of a coffee cup by creating a dimple and progressively enlarging it, while shrinking the hole into a handle.

Intuitively a homeomorphism maps points in the first object that are "close together" to points in the second object that are close together, and points in the first object that are not close together to points in the second object that are not close together. Topology is the study of those properties of objects that do not change when homeomorphisms are applied.

## Definition

A function *f* between two topological spaces *X* and *Y* is called **homeomorphism** if it has the following properties

If such a function exists we say *X* and *Y* are **homeomorphic**. The homeomorphisms form an equivalence relation on the set of all topological spaces. The resulting equivalence classes are called **homeomorphism classes**

## Examples

- The unit 2-disc D
^{2} and the unit square in **R**^{2} are homeomorphic.
- The open interval (-1, 1) is homeomorphic to the real numbers
**R**.
- The product space S
^{1} × S^{1} and the two-dimensional torus are homeomorphic.
- Every uniform isomorphism is a homeomorphism

## Notes

The third requirement, that *f* ^{-1} be continuous, is essential. Consider for instance the function *f* : [0, 2π) → S^{1} defined by *f*(φ) = (cos(φ), sin(φ)). This function is bijective and continuous, but not a homeomorphism.

If two spaces are homeomorphic then they have exactly the same topological properties. For example, if one of them is compact, then the other is as well; if one of them is connected, then the other is as well; if one of them is Hausdorff, then the other is as well; their homology groups will coincide. Note however that this does not extend to properties defined via a metric; there are metric spaces which are homeomorphic even though one of them is complete and the other is not.

Homeomorphisms are the isomorphisms in the category of all topological spaces. As such, the composition of two homeomorphisms is again a homeomorphism, and the set of all homeomorphisms *X* → *X* forms a group.

## Informal discussion

The intuitive criterion of stretching, bending, cutting and gluing back together takes a certain amount of practice to apply correctly--it may not be obvious from the description above that deforming a line segment to a point is impermissible, for instance. It is thus important to realize that it is the formal definition given above that counts.

This characterization of a homeomorphism often leads to confusion with the concept of homotopy, which is actually *defined* as a continuous deformation, but from one *function* to another, rather than one space to another. In the case of a homeomorphism, envisioning a continuous deformation is a mental tool for keeping track of which points on space *X* correspond to which points on *Y* -- one just follows them as *X* deforms. In the case of homotopy, the continuous deformation from one map to the other is of the essence, and it is also less restrictive, since none of the maps involved need to be one-to-one or onto. Homotopy does lead to a relation on spaces: homotopy equivalence.

There is a name for the kind of deformation involved in visualizing a homeomorphism. It is (except when cutting and regluing are required) an isotopy between the identity map on *X* and the homeomorphism from *X* to *Y*.

## See also

Last updated: 02-08-2005 12:31:50

Last updated: 05-03-2005 17:50:55