Properties of Homeomorphisms between topological spaces

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Discussion Overview

The discussion revolves around the properties of homeomorphisms between topological spaces, specifically focusing on whether a one-to-one function can exist between the collections of open sets defining the topologies of two homeomorphic spaces. The scope includes theoretical aspects of topology and the implications of homeomorphism.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Goldbeetle initially questions whether a one-to-one function exists between the collections of open sets of two topological spaces.
  • Landau argues that in general, there is no such one-to-one function unless the spaces are homeomorphic, citing examples of different topologies on a set.
  • Goldbeetle acknowledges the oversight and clarifies that the spaces in question are homeomorphic, asking if a one-to-one association among their open sets exists in this case.
  • Participants agree that if the spaces are homeomorphic, then a one-to-one association exists, facilitated by the homeomorphism itself.
  • One participant notes that the function mapping open sets from one topology to another is a bijection and raises a follow-up question regarding the conditions under which an order-preserving bijection implies homeomorphism.
  • Another participant suggests that the conditions for this implication are weak, requiring only that the spaces be sober.
  • Goldbeetle seeks a proof for the bijection, while another participant suggests that the proof is straightforward and encourages Goldbeetle to attempt it.
  • Goldbeetle proposes that the triviality of the proof stems from the bijection between the sets and the continuity of the mappings ensuring the openness of the associated sets.
  • A later reply confirms Goldbeetle's assertion about the triviality of the proof.

Areas of Agreement / Disagreement

Participants generally agree that a one-to-one association exists between the open sets of two homeomorphic spaces. However, the discussion includes varying perspectives on the implications of bijections between topologies and the conditions under which they imply homeomorphism.

Contextual Notes

Some assumptions regarding the properties of the spaces (e.g., being sober) and the nature of the bijections are not fully explored, leaving the discussion open to further clarification.

Who May Find This Useful

Readers interested in topology, particularly those exploring homeomorphisms and the relationships between different topological spaces, may find this discussion relevant.

Goldbeetle
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Dear all,
a homomorphism is a continuous 1-1 function between two topological spaces, that is invertible with continuous inverse. My question is as follows. Let's take the topologies of two topological spaces. Is there a 1-1 function between the two collections of open sets defining the topologies of each of the topological spaces?

Thanks,
Goldbeetle
 
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Goldbeetle said:
Let's take the topologies of two topological spaces. Is there a 1-1 function between the two collections of open sets defining the topologies of each of the topological spaces?
In this generality, the answer is obviously NO. There are usually many, many topologies on a set. An extreme example: take X with the trivial topology and X with the discrete topology. These topologies have cardinality |X| and 2^X, respectively.

Or were you assuming that the two spaces were homeomorphic?
 
Landau,
thanks. You're right. I forgot to add that that the two spaces are homomorphic. Does in this case the 1-1 association among open sets of the two topologies exists?
 
Then, the answer is yes... the association being given by the homeomorphism. And it is because of this property of heomeomorphic space that people say that homeomorphic spaces are "essentially the same" topological spaces.
 
If the spaces are homeomorphic, then the answer is yes. In fact, the function

\mathcal{T}\rightarrow \mathcal{T}^\prime:G\rightarrow f(G)

is a bijection.

An interesting follow-up question is the following: given that there is an (order-preserving) bijection between two topologies, are the spaces homeomorphic?

The answer turns out to be positive under very weak assumptions. We just need the spaces to be sober (a condition much weaker then Hausdorff), and then an order-preserving bijection between the topologies induces a homeomorphism. This question and more is studied in what is called "point-free topology".
 
Thanks. Where can I find a proof of the result of my question?
 
I don't think you'll find a proof anywhere. It's very easy, so try proving yourself that

\mathcal{T}\rightarrow \mathcal{T}^\prime:G\rightarrow f(G)

is a bijection with inverse

\mathcal{T}^\prime\rightarrow \mathcal{T}:G\rightarrow f^{-1}(G).
 
It is indeed trivial: (1) the fact that there's a 1-1 association between the power sets of the sets on which each topology is defined is derivable by the fact that there's a bijection between sets, (2) continuity guarantees that the associated sets are open in the respective topologies.

Right?
Goldbeetle
 
Correct!
 
  • #10
Thanks!
 

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