Schwarz's lemma, complex analysis proof

In summary, The problem involves showing that abs(f(z)) is less than or equal to [(1+abs(z))/(1-abs(z))] for a holomorphic function f on the unit disc with real part greater than or equal to 0 and f(0) = 1. This can be solved by utilizing Schwarz's lemma and creating a function g that maps the half plane with real part greater than 0 into the unit disc, and then applying Schwarz's lemma to the composition of g and f. Some additional algebra is needed to reach the desired conclusion.
  • #1
QuantumLuck
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Homework Statement


Let B1 = {z element C : abs(z) < 1}, f be a holomorphic function on B1 with Re f(z) > greater than or equal to 0 and f(0) =1. then show that:

abs(f(z)) less than or equal to [(1+abs(z))/(1-abs(z))]


Homework Equations


Schwarz's Lemma: Suppose that f is analytic in the unit disc, that abs(f) less than or equal to 1 and that f(0) = 0. Then

i. abs(f(z)) less than or equal to abs(z)
ii. abs(f'(0)) less or equal to 1


The Attempt at a Solution


So I know that the solution to this problem involves utilizing Schwarz's lemma (a hint from my professor), however considering the different value of the point at z = 0 is throwing me for a loop. I am not quite sure how to continue from where I am.
 
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  • #2
You have f from B1 to H, where H is the half plane {z : Re(z)>0}.

Now create a function g which maps H into the unit disc, such that g(1)=0.

Let h = composition of g and f. Apply Schwarz to h. After that, a little extra algebra is needed to get the desired conclusion.
 

What is Schwarz's lemma in complex analysis?

Schwarz's lemma is a fundamental theorem in complex analysis that provides a bound on the absolute value of a holomorphic function at a point in terms of its value at a nearby point. It is named after mathematician Hermann Schwarz.

What is the proof of Schwarz's lemma?

The proof of Schwarz's lemma involves using the Cauchy integral formula and the maximum modulus principle to show that for any holomorphic function f, the ratio of its value at any point z to the maximum value of |f| on a circle centered at z is less than or equal to 1.

What are the applications of Schwarz's lemma?

Schwarz's lemma has a wide range of applications in complex analysis. It is often used to prove other important theorems, such as the Riemann mapping theorem and the open mapping theorem. It also has applications in the study of conformal mappings, which are important in many areas of mathematics and physics.

What is the significance of Schwarz's lemma?

Schwarz's lemma is significant because it provides a powerful tool for understanding the behavior of holomorphic functions and their derivatives. It also helps establish important results in complex analysis, such as the existence of a conformal mapping between any two simply connected domains in the complex plane.

Can Schwarz's lemma be generalized to higher dimensions?

Yes, Schwarz's lemma can be generalized to higher dimensions through the use of Cauchy's integral formula and the maximum modulus principle in several complex variables. This generalization is known as the Cartan-Schwarz lemma and has important applications in the study of complex manifolds.

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