Is an open map also a homeomorphism?

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Discussion Overview

The discussion revolves around the question of whether an open map is necessarily a homeomorphism. Participants explore the implications of the definitions of homeomorphisms and open maps, particularly in the context of bijective and continuous functions.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that while a homeomorphism is an open map, the converse may not hold without additional conditions.
  • One participant suggests that the statement should specify that a bijective continuous map is a homeomorphism if it is open, indicating a potential misunderstanding in the original claim.
  • Another participant provides examples where an open map does not qualify as a homeomorphism, such as the inclusion map from an open subset to a larger space or the projection map from the plane onto a line.
  • There is mention of the open mapping theorem in complex analysis as a relevant consideration, although its implications are not fully explored in the discussion.
  • Some participants express a desire to prove related statements about closed maps and their relationship to homeomorphisms, suggesting further exploration of the topic.

Areas of Agreement / Disagreement

Participants generally disagree on the validity of the original claim that an open map is a homeomorphism, with multiple competing views and examples presented. The discussion remains unresolved regarding the implications of the definitions without additional hypotheses.

Contextual Notes

Limitations include the lack of clarity on the necessary conditions for an open map to be a homeomorphism and the dependence on specific topological properties of the spaces involved.

1MileCrash
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I was told to prove that

f: X -> Y is a homeomorphism iff it is an open map

While I see that if f is a homeomorphism, it is certainly an open map, but is the implication in the other direction even true? Because I see no reason to believe it is.
 
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That is false. I suspect they meant a bijective continuous map ##f## is a homeomorphism iff it is open iff it is closed.
 
1MileCrash said:
I was told to prove that

f: X -> Y is a homeomorphism iff it is an open map

While I see that if f is a homeomorphism, it is certainly an open map, but is the implication in the other direction even true? Because I see no reason to believe it is.

I think it should be "A bijective, continuous map ##f:X\to Y## is a homeomorphism iff it is an open map."

For fun, prove "A bijective, continuous map ##f:X\to Y## is a homeomorphism iff it is a closed map." :-p

Edit: WN got there first. Why you type so fast? :smile:
 
this false without some hypotheses. if X is an open subset of Y, the inclusion map is open but not a homeomorphism.

if f is projection of the plane X onto the y - axis Y, this map is open but not a homeomorphism.

If X and Y are the same set equipped with different topologies so that Y has more open sets than X, then the identity map is open but not a homeomorphism.

etc...
 
1MileCrash said:
I was told to prove that

f: X -> Y is a homeomorphism iff it is an open map

While I see that if f is a homeomorphism, it is certainly an open map, but is the implication in the other direction even true? Because I see no reason to believe it is.

take a look at the open mapping theorem for complex analysis.

Let X be any space and Y a point. Map X to Y.
 
Mandelbroth said:
I think it should be "A bijective, continuous map ##f:X\to Y## is a homeomorphism iff it is an open map."

For fun, prove "A bijective, continuous map ##f:X\to Y## is a homeomorphism iff it is a closed map." :-p

Edit: WN got there first. Why you type so fast? :smile:

Oh, ok. But isn't that kind of trivial? If it is an open map then that is exactly the same (from what I can see) as saying that f-inverse is continuous. So basically that means I'm asked to show that f is a homeomorphism if f is continuous, f-inverse is continuous, and f is a bijection, but that's just what a homeomorphism is.
 

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