Complex Mapping - Is transforming boundaries enough?

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    Complex Mapping
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Discussion Overview

The discussion revolves around the transformation of regions and boundaries in complex mapping, specifically from the ##z## plane to the ##w## plane. Participants explore the implications of these transformations on the inside-outside relationship of regions and boundaries, focusing on the conditions under which such mappings preserve these relationships.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions why knowing the equations for a boundary ##C## allows for the conclusion that the region ##R## will remain inside the transformed boundaries in the ##w## plane.
  • Another participant suggests that the nature of the transformation, particularly if it is linear or continuous, is crucial for preserving the inside-outside relationship.
  • A participant reflects on the role of continuity in understanding the mapping, noting that neighborhoods around points in ##R## will also map to neighborhoods in the transformed region ##R'##.
  • A counterexample is presented where a specific transformation maps a closed boundary to itself while the corresponding region is mapped outside of it, challenging the initial assumption.
  • There is a discussion about the conditions under which analytic functions maintain the inside-outside relationship, with some uncertainty expressed regarding the applicability of this principle.

Areas of Agreement / Disagreement

Participants express differing views on the conditions necessary for the preservation of the inside-outside relationship during transformations. There is no consensus on whether analytic functions universally guarantee this preservation, as some participants highlight specific counterexamples and conditions that complicate the discussion.

Contextual Notes

The discussion reveals limitations in the assumptions made about the transformations and the dependence on the specific characteristics of the boundary ##C##. The implications of continuity and analyticity are not fully resolved, indicating a need for further exploration of these concepts.

davidbenari
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Say I will make the transformation from the ##z## plane to the ##w## plane. Moreover, I'll transform a region ##R## with boundary ##C## in the ##z## plane to something in the ##w## plane.

Why is it that if I know the equations for ##C## then I can transform these and immediately know that ##R## will be inside the transformed boundaries? Why isn't it the case that some point in ##R## maps into some point not inside ##C'## (which is my transformed boundary)?

Thanks.
 
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You have to say something about the transformation. Normally, you are using linear, or at least continuous mappings which by definition will preserve this inside-outside relationship.
 
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According to what you say I think continuous mappings would be what I'm dealing with. Strangely I didn't think of continuity, which makes this more intuitive, but not obvious to me.

I see that every point in the neighborhood of ##z_o## in ##R## will also be mapped in ##R'## as a simple closed region. I guess I can visualize these small neighborhoods expanding until they reach the transformed contour...

Is this the argument?

I was told this was because analytic transformations are "open maps" is this what they mean by it?
 
davidbenari said:
Say I will make the transformation from the ##z## plane to the ##w## plane. Moreover, I'll transform a region ##R## with boundary ##C## in the ##z## plane to something in the ##w## plane.

Why is it that if I know the equations for ##C## then I can transform these and immediately know that ##R## will be inside the transformed boundaries? Why isn't it the case that some point in ##R## maps into some point not inside ##C'## (which is my transformed boundary)?

Thanks.
Counterexample: Let [itex]C= (z:\lvert z \lvert = 1)[/itex], then [itex]R= (z:\lvert z \lvert < 1)[/itex]. Now transform this using [itex]w=\frac{1}{z}[/itex]. This maps C onto C, but R is mapped to the outside of C.
 
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Svein, but if the function ##w## is analytic in every point in ##R## then what I say will hold right?
 
davidbenari said:
Svein, but if the function ww is analytic in every point in RR then what I say will hold right?
Sorry, I do not know. I checked out Ahlfors, and he carefully does not make that statement. It depends very much on C and the concept of inside.
 
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