Homeomorphism

Not to be confused with homomorphism.

For homeomorphisms in graph theory, see homeomorphism (graph theory).

"Topological equivalence" redirects here. For topological equivalence in dynamical systems, see Topological conjugacy.

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 or same and μορφή (morphē) = shape, form, introduced to mathematics by Henri Poincaré in 1895.^[1][2]

Very 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. However, this description can be misleading. Some continuous deformations are not homeomorphisms, such as the deformation of a line into a point. Some homeomorphisms are not continuous deformations, such as the homeomorphism between a trefoil knot and a circle.

An often-repeated mathematical joke is that topologists can't tell the difference between a coffee cup and a donut,^[3] 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.

Contents

• 
  1 Definition
  


• 
  2 Examples
  
  • 
    2.1 Non-examples
    
  
  


• 
  3 Notes
  


• 
  4 Properties
  


• 
  5 Informal discussion
  


• 
  6 See also
  


• 
  7 References
  


• 
  8 External links
  

Definition

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

• 
  fdisplaystyle f is a bijection (one-to-one and onto),


• 
  fdisplaystyle f is continuous,


• the inverse function f−1displaystyle f^-1 is continuous (fdisplaystyle f is an open mapping).

A function with these three properties is sometimes called bicontinuous. If such a function exists, we say (X,TX)displaystyle (X,mathcal T_X) and (Y,TY)displaystyle (Y,mathcal T_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 torus, but not isotopic in R³. Continuous mappings are not always realizable as deformations.

• The open interval (a,b)textstyle (a,b) is homeomorphic to the real numbers Rtextstyle mathbf R for any a<btextstyle a<b. (In this case, a bicontinuous forward mapping is given by f(x)=1a−x+1b−xtextstyle f(x)=frac 1a-x+frac 1b-x while other such mappings are given by scaled and translated versions of the tan or arg tanh functions).


• The unit 2-disc D2textstyle D^2 and the unit square in R² are homeomorphic; since the unit disc can be deformed into the unit square. An example of a bicontinuous mapping from the square to the disc is, in polar coordinates, (ρ,θ)↦(ρmax(|cos⁡θ|,|sin⁡θ|),θ)displaystyle (rho ,theta )mapsto left(rho max(.


• The graph of a differentiable function is homeomorphic to the domain of the function.


• A differentiable parametrization of a curve is an homeomorphism between the domain of the parametrization and the curve.


• A chart of a manifold is an homeomorphism between an open subset of the manifold and an open subset of a Euclidean space.


• 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 Gdisplaystyle G is a topological group, its inversion map x↦x−1displaystyle xmapsto x^-1 is a homeomorphism. Also, for any x∈Gdisplaystyle xin G, the left translation y↦xydisplaystyle ymapsto xy, the right translation y↦yxdisplaystyle ymapsto yx, and the inner automorphism y↦xyx−1displaystyle ymapsto 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−1textstyle f^-1 be continuous, is essential. Consider for instance the function f:[0,2π)→S1textstyle f:[0,2pi )to S^1 (the unit circle in R2textstyle mathbb R ^2) defined byf(ϕ)=(cos⁡ϕ,sin⁡ϕ)textstyle f(phi )=(cos phi ,sin phi ). This function is bijective and continuous, but not a homeomorphism (S1textstyle S^1 is compact but [0,2π)textstyle [0,2pi ) is not). The function f−1textstyle f^-1 is not continuous at the point (1,0)textstyle (1,0), because although f−1textstyle f^-1 maps (1,0)textstyle (1,0) to 0textstyle 0, any neighbourhood of this point also includes points that the function maps close to 2πtextstyle 2pi , but the points it maps to numbers in between lie outside the neighbourhood.^[4]

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→Xtextstyle Xto X forms a group, called the homeomorphism group of X, often denoted Homeo(X)textstyle textHomeo(X). This group can be given a topology, such as the compact-open topology, which under certain assumptions makes it a topological group.^[5]

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)textstyle textHomeo(X,Y), is a torsor for the homeomorphism groups Homeo(X)textstyle textHomeo(X) and Homeo(Y)textstyle textHomeo(Y), and, given a specific homeomorphism between Xdisplaystyle X and Ydisplaystyle 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 and 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 S1displaystyle S^1 can be extended to a self-homeomorphism of the whole disk D2displaystyle 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. In this case, for example, the line segment possesses infinitely many points, and therefore cannot be put into a bijection with a set containing only a finite number of points, including a single point.

This characterization of a homeomorphism often leads to a 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
  


• Homeomorphism group


• Dehn twist


• 
  Homeomorphism (graph theory) (closely related to graph subdivision)


• Homotopy#Isotopy


• Mapping class group


• Poincaré conjecture


• Universal homeomorphism

References

1. 
   ^ "Analysis Situs selon Poincaré (1895)". serge.mehl.free.fr. Archived from the original on 11 June 2016. Retrieved 29 April 2018. 
   


2. 
   ^ Gamelin, T. W.; Greene, R. E. (1999). Introduction to Topology. Courier. p. 67. 
   


3. 
   ^ 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. 
   


4. 
   ^ Väisälä, Jussi: Topologia I, Limes RY 1999, p. 63. ISBN 951-745-184-9.
   


5. 
   ^ Dijkstra, Jan J. (1 December 2005). "On Homeomorphism Groups and the Compact-Open Topology" (PDF). The American Mathematical Monthly. 112 (10): 910. doi:10.2307/30037630. Archived (PDF) from the original on 16 September 2016. 
   

External links

• 
  Hazewinkel, Michiel, ed. (2001) [1994], "Homeomorphism", Encyclopedia of Mathematics, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 
  


• 
  "Homeomorphism". PlanetMath. 
  

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