Conformal Mapping: Transform a Circle to a Rectangle

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Discussion Overview

The discussion revolves around the possibility of transforming a circle into a rectangle using conformal mapping techniques. Participants explore various mathematical approaches and transformations relevant to this problem, including the Riemann mapping theorem and the Schwarz–Christoffel transformation.

Discussion Character

  • Exploratory
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • One participant asks if it is possible to transform a circle into a rectangle and seeks expressions for the transformed coordinates.
  • Another participant asserts that such a transformation is possible under the Riemann mapping theorem, which states that any simply connected nonempty proper subset of the plane can be mapped conformally to another.
  • A request for references to explicit formulas for the transformation is made, indicating a desire for further information.
  • One participant suggests using the transformation from polar coordinates to rectangular coordinates as a potential approach.
  • Another participant mentions the Schwarz–Christoffel transformation, which maps the upper half-plane to a polygon, and notes that this transformation involves an integral related to elliptic functions.
  • A participant outlines a potential method involving a Mobius transformation to map a circle to the upper half-plane, followed by the Schwarz-Christoffel transformation to map to a rectangle, emphasizing the need to determine parameters for the specific problem.
  • One participant reiterates their need for a transformation that maps points from a circle to a rectangle, expressing uncertainty about how to express the transformed coordinates.
  • It is emphasized that the mapping must be conformal, and a suggestion is made to search for relevant terms in complex analysis literature.

Areas of Agreement / Disagreement

Participants generally agree that a conformal transformation from a circle to a rectangle is possible, but there is no consensus on the specific methods or formulas to achieve this transformation. Multiple approaches and models are discussed without resolution.

Contextual Notes

The discussion includes references to complex analysis concepts and transformations that may require further exploration and understanding of specific mathematical techniques. Some assumptions about the nature of the transformations and the parameters involved remain unaddressed.

Carol_m
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Hello!

Please I need some help with this:

Is it possible to transform a circle into a rectangle? If so what would be the expressions of x' and y' in terms of x and y.

Thank you in advance!
 
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The answer is that it is possible (if you mean transforming the interiors). Indeed, any simply connected ("no holes") nonempty proper subset of the plane can be mapped conformally to any other. This is the Riemann mapping theorem.

Though the transformation exists, finding it explicitly is another matter entirely. According to Mathworld it can be explicitly written down using elliptic functions but I don't know how much use that is.
 
Dear henry_m,

Thank you very much for your reply. Could you please give me a reference to this so I can find the explicit formula?

Thanks again!
 
Have you tried using the transformation from polar coordinates to rectangular coordinates?
 
The hardest part of what you are looking for is the Schwarz–Christoffel transformation. It maps the upper half plane to a polygon. The mapping is in the form of an integral; this integral is the source of the elliptic functions henry_m mentioned.

So one approach is to map a circle to the upper half plane using a Mobius transformation, then use a Schwarz-Christoffel to map the upper half plane to rectangle. Both of these have parameters that you will need to determine for your particular problem.

Good luck!

jason
 
Basically my question is:

I have a circle an every point in the circle as (x,y). I need a transformation that changes every point in the circle to a point in a rectangle (x',y') so I could write x' in terms of x and y' in terms of y.

If you look in the attachement you can see what I mean :)

ps. I am not that concern about the base of the rectangle it could be circular so that y=y' but I am stuck with the x'

Thank you!
 

Attachments

  • transformation.jpg
    transformation.jpg
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Carol_m said:
Basically my question is:

I have a circle an every point in the circle as (x,y). I need a transformation that changes every point in the circle to a point in a rectangle (x',y') so I could write x' in terms of x and y' in terms of y.

If you look in the attachement you can see what I mean :)

ps. I am not that concern about the base of the rectangle it could be circular so that y=y' but I am stuck with the x'

Thank you!

If your mapping needs to be conformal, as indicated by your post title, then my previous post should give you the words you need to search for either in Google or a complex analysis book; this should point you in the right direction so that you can figure out your mapping. By the way, the Mobius transformation is the same as the "linear fractional" transformation, which may help in your search.
 
Hi JasonRF,

It certainly needs to be conformal... Thank you for your great answer I will look into this direction, and sorry I posted my reply with the attached figure before reading yours.
 

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