Conformal map of unit disk to itself

In summary, the problem discussed is about a conformal map of the upper half plane, which is of the form T(z) = \frac{az+b}{cz+d}. It is shown that T(z) leaves the upper half plane invariant when ad-bc>0. The second half of the solution involves using a conformal map of the unit disk to itself, which is of the form W(z) = e^{i\theta}\frac{z-z_{0}}{1-\overline{z_{0}}z}. The author applies this to T(w) and after some algebraic manipulation, obtains the solution for T(w). The author expresses confusion about how this solution was obtained.
  • #1
Dromepalin
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Homework Statement



This problem is an already solved one in Marsden and Hoffman's Basic Complex Analysis, but I can't seem to work out the last step.
Here's the problem:

Suppose a,b,c,d are real and ad-bc>0. Then show that [tex] T(z) = \frac{az+b}{cz+d}[/tex] leaves the upper half plane invariant. Show any conformal map of the upper half plane is of this form.

Homework Equations


The Solution to the second half:
Suppose T maps the upper half plane conformally to itself. Then
[tex] W(z) = \frac{T(\frac{1}{i}*\frac{z+1}{z-1})-i}{T(\frac{1}{i}*\frac{z+1}{z-1})+i}[/tex]
is a conformal map of the unit disk to itself. Now W has the form:
[tex]W(z) = e^{i\theta}\frac{z-z_{0}}{1-\overline{z_{0}}z}[/tex]

and so [tex]e^{i\theta}\frac{\frac{w-i}{w+i}-z_{0}}{1-\overline{z_{0}}\frac{w-i}{w+i}} =\frac{T(w)-i}{T(w)+i}[/tex]

Solving for T(w): <=== This is the part I don't understand!
[tex] T(w) =\frac{w*Re[(1-z_{0})e^{i\theta/2}] - Im[(1+z_{0})e^{i\theta/2}]}{w*Im[(1-z_{0})e^{i\theta/2}] + Re[(1+z_{0})e^{i\theta/2}]}[/tex]

The Attempt at a Solution



How on Earth did the author get that value for T(w). I have tried pages of mindless algebraic manipulation to no avail.
 
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  • #2
I think it has something to do with the fact that T(w) is a fraction, and so the numerator and denominator can't be combined. I am also aware of the fact that Re[z] = \frac{z+\overline{z}}{2} and Im[z] = \frac{z-\overline{z}}{2i}. Any help would be much appreciated!
 

FAQ: Conformal map of unit disk to itself

What is a conformal map?

A conformal map is a type of transformation in mathematics that preserves angles between intersecting curves. In other words, it does not distort the shape of the curves.

What is the unit disk?

The unit disk is a mathematical term that refers to a circle with a radius of 1, centered at the origin (0,0) on a Cartesian coordinate plane.

Why is it important to have a conformal map of the unit disk to itself?

Having a conformal map of the unit disk to itself is important because it allows us to transform the disk in a way that preserves its shape and angles. This can be useful in various mathematical and scientific applications, such as in complex analysis and fluid dynamics.

How is a conformal map of the unit disk to itself represented?

A conformal map of the unit disk to itself is often represented as a function, typically denoted as f(z), where z is a complex variable. This function maps points inside the unit disk to other points inside the disk.

What are some examples of conformal maps of the unit disk to itself?

Some examples of conformal maps of the unit disk to itself include the exponential map, the Möbius transformation, and the Schwarz-Christoffel mapping. These maps have various applications in mathematics, physics, and engineering.

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