A fundamental question on homeomorphism

In summary, the conversation discusses the possibility of constructing a homeomorphism between R^m and a subset of R^n, given that m>n. It is stated that intuitively it is impossible and Brouwer's Invariance of Domain theorem is used to prove this. The conversation also mentions that the same theorem can be used to answer the question and provides a proof for it.
  • #1
krete
15
0
It is well known that there does NOT exist a homeomorphism between R^m and R^n if m>n. My question is whether it is possible to construct a homeomorphism between R^m (as a whole) and a subset of R^n (note that we also suppose that m>n)?

Intuitively, it is impossible. Is my intuition right? Thank you for your replying in advance!
 
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  • #2
Any subspace of Rn is Rk for k< n< m. And you have already said "there does NOT exist a homeomorphism between R^m and R^k if m>k" (where I have replaced your "n" with "k").
 
  • #3
Hi, HallsofIvy,

How about if the subset of R^n is not the whole R^k (k<n) but some ill-behaved set (e.g., a space filling line)?
 
  • #4
The usual tool for proving the "no homeo thm" is Brouwer's Invariance of Domain theorem:

http://en.wikipedia.org/wiki/Invariance_of_domain

It can in the same way be used to answer your question: Assume a homeo btw S (subset of R^n) and R^m exists. Consider R^n as a subset of R^m (say as R^n x {0,...,0}). Then we have a map

R^m --> S --> R^m

which is the homeomorphism of R^m with S composed with the inclusion of R^n in R^m. This map is not open since the inclusion of R^n in R^m maps any subset of R^n to a non open subset of R^m. This contradicts Brouwer's invariance of domain theorem.
 
  • #5
quasar987 said:
The usual tool for proving the "no homeo thm" is Brouwer's Invariance of Domain theorem:

http://en.wikipedia.org/wiki/Invariance_of_domain

It can in the same way be used to answer your question: Assume a homeo btw S (subset of R^n) and R^m exists. Consider R^n as a subset of R^m (say as R^n x {0,...,0}). Then we have a map

R^m --> S --> R^m

which is the homeomorphism of R^m with S composed with the inclusion of R^n in R^m. This map is not open since the inclusion of R^n in R^m maps any subset of R^n to a non open subset of R^m. This contradicts Brouwer's invariance of domain theorem.

Dear quasar987,

Thank you very much for your helpful answer. It is really a nice proof.
 

1. What is a homeomorphism?

A homeomorphism is a mathematical concept that describes a continuous mapping between two topological spaces that preserves their structure. In simpler terms, it is a function that can transform one space into another without tearing or gluing any points together.

2. How is a homeomorphism different from an isomorphism?

A homeomorphism and an isomorphism are both types of mathematical mappings, but they differ in the types of structures they preserve. A homeomorphism preserves the topological structure of a space, while an isomorphism preserves algebraic or geometric structures.

3. Why is homeomorphism an important concept in mathematics?

Homeomorphisms play a crucial role in topology, which is the study of the properties of geometric shapes that are preserved under continuous deformations. They provide a way to compare and classify different spaces, and are also used in many other areas of mathematics, including differential equations and complex analysis.

4. Can you give an example of a homeomorphism?

One example of a homeomorphism is the mapping between a circle and a square. By continuously stretching and bending the circle, it can be transformed into a square without tearing or gluing any points. This demonstrates how the topological structure of both shapes is preserved by the homeomorphism.

5. Are all homeomorphisms reversible?

No, not all homeomorphisms are reversible. A homeomorphism is only reversible if it has an inverse function that also satisfies the definition of a homeomorphism. In other words, if the inverse function can transform the square back into a circle without tearing or gluing any points, then the homeomorphism is reversible.

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