Homeomorphism

A continuous deformation between a coffee mug and a donut (torus) illustrating that they are homeomorphic. But there need not be a continuous deformation for two spaces to be homeomorphic — only a continuous mapping with a continuous inverse function.

In the mathematical field of topology, a homeomorphism or topological isomorphism or bi continuous function is a continuous function between topological spaces that has a continuous inverse function. Homeomorphisms are the isomorphisms in the category of topological spaces—that is, they are the mappings that preserve all the topological properties of a given space. Two spaces with a homeomorphism between them are called homeomorphic, and from a topological viewpoint they are the same. The word homeomorphism comes from the Greek words ὅμοιος (homoios) = similar and μορφή (morphē) = shape, form.^[1]

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 to each other, but a sphere and a torus are not. An often-repeated mathematical joke is that topologists can't tell the difference between a coffee cup and a donut,^[2] since a sufficiently pliable donut could be reshaped to the form of a coffee cup by creating a dimple and progressively enlarging it, while preserving the donut hole in a cup's handle.

Definition

A function f: X → Y between two topological spaces (X, T_X) and (Y, T_Y) is called a homeomorphism if it has the following properties:

• f is a bijection (one-to-one and onto),
• f is continuous,
• the inverse function f⁻¹ is continuous (f is an open mapping).

A function with these three properties is sometimes called bicontinuous. If such a function exists, we say X and Y are homeomorphic. A self-homeomorphism is a homeomorphism of a topological space and itself. The homeomorphisms form an equivalence relation on the class of all topological spaces. The resulting equivalence classes are called homeomorphism classes.

Examples

A trefoil knot is homeomorphic to a circle, but not isotopic. Continuous mappings are not always realizable as deformations. Here the knot has been thickened to make the image understandable.

• The unit 2-disc D² and the unit square in R² are homeomorphic.
• The open interval (a, b) is homeomorphic to the real numbers R for any a < b. (In this case, a bicontinuous forward mapping is given by f(x) = 1/(a − x) + 1/(b − x) while other such mappings are given by scaled and translated versions of the tan or arg tanh functions).
• The product space S¹ × S¹ and the two-dimensional torus are homeomorphic . ("To a topologist, there is no difference between" a square whose opposite sides are identified in direct-parallel (equipollent) fashion, and the surface of a torus.)
• Every uniform isomorphism and isometric isomorphism is a homeomorphism.
• The stereographic projection is a homeomorphism between the unit sphere in R³ with a single point removed and the set of all points in R² (a 2-dimensional plane).
• If A is a commutative ring with unity, and S is a multiplicative subset of A, then the spectrum Spec(A_S) is homeomorphic to {p ∈ Spec(A) : p ∩ S = ∅}.
• If G is a topological group, its inversion map $x \mapsto x^{-1}$ is a homeomorphism. Also, for any $x \in G$, the left translation $y \mapsto xy$, the right translation $y \mapsto yx$, and the inner automorphism $y \mapsto xyx^{-1}$ are homeomorphisms.

Non-examples

• R^m and Rⁿ are not homeomorphic for m ≠ n.
• The Euclidean real line is not homeomorphic to the unit circle as a subspace of R², since the unit circle is compact as a subspace of Euclidean R² but the real line is not compact.

Notes

The third requirement, that f ⁻¹ be continuous, is essential. Consider for instance the function f: [0, 2π) → S¹ (the unit circle in $\mathbb{R}^2$) defined by f(φ) = (cos(φ), sin(φ)). This function is bijective and continuous, but not a homeomorphism (S¹ is compact but [0, 2π) is not). The function f ⁻¹ is not continuous at the point (1, 0), because although f ⁻¹ maps (1, 0) to 0, any neighbourhood of this point also includes points that the function maps close to 2π, but the points it maps to numbers in between lie outside the neighbourhood.^[3]

Homeomorphisms are the isomorphisms in the category of topological spaces. As such, the composition of two homeomorphisms is again a homeomorphism, and the set of all self-homeomorphisms X → X forms a group, called the homeomorphism group of X, often denoted Homeo(X); this group can be given a topology, such as the compact-open topology, making it a topological group.

For some purposes, the homeomorphism group happens to be too big, but by means of the isotopy relation, one can reduce this group to the mapping class group.

Similarly, as usual in category theory, given two spaces that are homeomorphic, the space of homeomorphisms between them, Homeo(X, Y), is a torsor for the homeomorphism groups Homeo(X) and Homeo(Y), and given a specific homeomorphism between X and Y, all three sets are identified.

Properties

• Two homeomorphic spaces share 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 homotopy & homology groups will coincide. Note however that this does not extend to properties defined via a metric; there are metric spaces that are homeomorphic even though one of them is complete and the other is not.
• A homeomorphism is simultaneously an open mapping and a closed mapping; that is, it maps open sets to open sets and closed sets to closed sets.
• Every self-homeomorphism in $S^1$ can be extended to a self-homeomorphism of the whole disk $D^2$ (Alexander's trick).

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

• Local homeomorphism
• Diffeomorphism
• Uniform isomorphism is an isomorphism between uniform spaces
• Isometric isomorphism is an isomorphism between metric spaces
• Dehn twist
• Homeomorphism (graph theory) (closely related to graph subdivision)
• Homotopy#Isotopy
• Mapping class group
• Poincaré conjecture

References

1. Gamelin, T. W., & Greene, R. E. (1999). Introduction to topology. Courier Corporation.
2. Hubbard, John H.; West, Beverly H. (1995). Differential Equations: A Dynamical Systems Approach. Part II: Higher-Dimensional Systems. Texts in Applied Mathematics 18. Springer. p. 204. ISBN 978-0-387-94377-0.
3. Väisälä, Jussi: Topologia I, Limes RY 1999, p. 63. ISBN 951-745-184-9.

External links

• Hazewinkel, Michiel, ed. (2001), "Homeomorphism", Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
• Homeomorphism at PlanetMath.org.