F must be a Diffeomorphism: Why & What if?

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Discussion Overview

The discussion centers on the necessity of F being a diffeomorphism in the context of F-related vector fields. Participants explore the implications of F being smooth and bijective but not a diffeomorphism, examining the consequences for vector fields on manifolds.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Some participants assert that F must be a diffeomorphism to ensure a unique F-related vector field exists on M for every vector field on N.
  • Others argue that the definition of F-related vector fields does not require F to be bijective, suggesting that non-bijective mappings could lead to complications in defining related vector fields.
  • One participant points out that if F is merely smooth and bijective, the notion of F-relatedness can still be defined, although it may not have the same properties as when F is a diffeomorphism.
  • Another participant emphasizes that the authors of the referenced book likely define F as a diffeomorphism to ensure the existence of a unique F-related vector field on M for every vector field on N.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of F being a diffeomorphism, with some supporting its necessity for unique vector field correspondence and others questioning this requirement. The discussion remains unresolved regarding the implications of F being smooth and bijective without being a diffeomorphism.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the definitions of F-related vector fields and the implications of smoothness and bijectiveness without diffeomorphism. The discussion also reflects varying interpretations of the definitions presented in the referenced book.

fk378
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In the definition of F-related vector fields, F must be a diffeomorphism. Why must it be a diffeomorphism? What if F is smooth and bijective, but not a diffeo?
 
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If you post the definition I might be able to help. I left my manifolds book at school.
 
The definition does not even require F-->N to be bijective. If F is not bijective, then a vector field on M might to push-foward to a vector field on N, and a vector field on N might not have an F-related vector field on M. But if F is a diffeo, then we are in the nice situation where to every vector field on N, then exists a unique F-related vector field on M given, of course, by the pushfoward by F^-1.
 
Tinyboss said:
If you post the definition I might be able to help. I left my manifolds book at school.

Suppose F: M-->N is a diffeomorphism. For every Y in TM (tangent bundle to M), there is a unique smooth vector field on N that is F-related to Y.
 
quasar987 said:
The definition does not even require F-->N to be bijective. If F is not bijective, then a vector field on M might to push-foward to a vector field on N, and a vector field on N might not have an F-related vector field on M. But if F is a diffeo, then we are in the nice situation where to every vector field on N, then exists a unique F-related vector field on M given, of course, by the pushfoward by F^-1.

Yes, I understand the smooth and bijective part, but what about the non-diffeo part?
 
I said that the notion of "F-relatedness" can be defined if F is merely smooth. In particular, it makes sense to speak about F-relatedness if F is smooth and bijective.

Then I hinted to the fact that your book defines the notion when F is diffeo probably because in that case, we are in the nice situation where to every vector field on N there exists a unique F-related vector field on M... which is probably the property that the authors needed.
 

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