Uniqueness of analytic functions

Oct 13, 2025 by wbrum.org 2 views

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Introduction

The exponential function is one of the most fundamental and versatile functions in mathematics, and its behavior on different domains is a crucial aspect of complex analysis. In this article, we will explore the behavior of the function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ on the unit ball $B(0,1)$ , which is a closed and bounded domain in the complex plane. This problem is significant because it involves the exponential function, singularity, and the maximum principle, which are essential concepts in complex analysis.

Background

The exponential function is defined as $\exp(z) = e^z = \sum_{n=0}^{\infty} \frac{z^n}{n!}$ , where $z$ is a complex number. This function is analytic everywhere in the complex plane, meaning that it is differentiable at every point. The exponential function has a number of important properties, including the fact that it is periodic with period $2\pi i$ and that it satisfies the equation $\exp(z+w) = \exp(z)\exp(w)$ .

The Unit Ball $B(0,1)$

The unit ball $B(0,1)$ is a closed and bounded domain in the complex plane, defined as the set of all complex numbers $z$ such that $|z| \leq 1$ . This domain is a closed disk centered at the origin with radius 1. The boundary of the unit ball is the unit circle $S^1$ , which is a closed curve that separates the interior of the ball from the exterior.

The Function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$

The function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ is a composition of the exponential function and the rational function $\frac{z+1}{z-1}$ . The rational function $\frac{z+1}{z-1}$ has a pole at $z=1$ and a zero at $z=-1$ . The exponential function is analytic everywhere, so the composition of the two functions is analytic wherever the rational function is analytic.

Singularity of the Function

The function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ has a singularity at $z=1$ , which is a point where the function is not analytic. This singularity is a pole of order 1, meaning that the function has a Laurent series expansion around this point with a non-zero coefficient of the $1/(z-1)$ term.

Maximum Principle

The maximum principle is a fundamental theorem in complex analysis that states that a non-constant analytic function on a bounded domain cannot attain its maximum value on the interior of the domain. In other words, if $f$ is an analytic function on a bounded domain $D$ , then $|f(z)| < \max_{w \in \partial D} |f(w)|$ for all $z \in D$ .

Behavior of the Function on the Unit Ball

To determine the behavior of the function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ on the unit ball $B(0,1)$ , we need to consider the singularity at $z=1$ and the maximum principle. Since the function has a pole at $z=1$ , it is not analytic at this point, and the maximum principle does not apply. However, we can still use the maximum principle to determine the behavior of the function on the interior of the ball.

Proof of the Maximum Principle

To prove the maximum principle, we can use the following argument. Suppose that $f$ is an analytic function on a bounded domain $D$ , and suppose that $|f(z)| \leq M$ for all $z \in \partial D$ . Then, by the mean value theorem, we have

f(z) = \frac{1}{2\pi i} \int_{\partial D} \frac{f(w)}{w-z} dw

for all $z \in D$ . Taking the absolute value of both sides, we get

|f(z)| \leq \frac{1}{2\pi} \int_{\partial D} \frac{|f(w)|}{|w-z|} |dw| \leq \frac{M}{\text{dist}(z,\partial D)}

for all $z \in D$ . Since $D$ is bounded, we have $\text{dist}(z,\partial D) > 0$ for all $z \in D$ , and therefore $|f(z)| < M$ for all $z \in D$ . This proves the maximum principle.

Application to the Function

We can now apply the maximum principle to the function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ on the unit ball $B(0,1)$ . Since the function has a pole at $z=1$ , it is not analytic at this point, and the maximum principle does not apply. However, we can still use the maximum principle to determine the behavior of the function on the interior of the ball.

Conclusion

In conclusion, the behavior of the function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ on the unit ball $B(0,1)$ is determined by the singularity at $z=1$ and the maximum principle. Since the function has a pole at $z=1$ , it is not analytic at this point, and the maximum principle does not apply. However, we can still use the maximum principle to determine the behavior of the function on the interior of the ball.

References

Ahlfors, L. V. (1979). Complex Analysis. McGraw-Hill.
Rudin, W. (1987). Real and Complex Analysis. McGraw-Hill.
Conway, J. B. (1995). Functions of One Complex Variable. Springer-Verlag.

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Q: What is the unit ball $B(0,1)$ ?

A: The unit ball $B(0,1)$ is a closed and bounded domain in the complex plane, defined as the set of all complex numbers $z$ such that $|z| \leq 1$ . This domain is a closed disk centered at the origin with radius 1.

Q: What is the function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ ?

A: The function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ is a composition of the exponential function and the rational function $\frac{z+1}{z-1}$ . The rational function $\frac{z+1}{z-1}$ has a pole at $z=1$ and a zero at $z=-1$ . The exponential function is analytic everywhere, so the composition of the two functions is analytic wherever the rational function is analytic.

Q: What is the singularity of the function?

A: The function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ has a singularity at $z=1$ , which is a point where the function is not analytic. This singularity is a pole of order 1, meaning that the function has a Laurent series expansion around this point with a non-zero coefficient of the $1/(z-1)$ term.

Q: What is the maximum principle?

A: The maximum principle is a fundamental theorem in complex analysis that states that a non-constant analytic function on a bounded domain cannot attain its maximum value on the interior of the domain. In other words, if $f$ is an analytic function on a bounded domain $D$ , then $|f(z)| < \max_{w \in \partial D} |f(w)|$ for all $z \in D$ .

Q: How does the maximum principle apply to the function?

A: Since the function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ has a pole at $z=1$ , it is not analytic at this point, and the maximum principle does not apply. However, we can still use the maximum principle to determine the behavior of the function on the interior of the ball.

Q: What is the behavior of the function on the unit ball?

A: The behavior of the function $\exp\Bigl(\frac{z+1}{z-1}\Bigr)$ on the unit ball $B(0,1)$ is determined by the singularity at $z=1$ and the maximum principle. Since the function has a pole at $z=1$ , it is not analytic at this point, and the maximum principle does not apply. However, we can still use the maximum principle to determine the behavior of the function on the interior of the ball.

Q: What are some common applications of the maximum principle?

A: The maximum principle has many applications in complex analysis, including:

Uniqueness of analytic functions: The maximum principle can be used to prove that an analytic function is unique if it is analytic on a bounded domain and attains its maximum value on the boundary.
Extremal problems: The maximum principle can be used to solve extremal problems, such as finding the maximum or minimum value of an analytic function on a bounded domain.
Boundary value problems: The maximum principle can be used to solve boundary value problems, such as finding the solution to a partial differential equation on a bounded domain.

Q: What are some common mistakes to avoid when applying the maximum principle?

A: Some common mistakes to avoid when applying the maximum principle include:

Failing to check for singularities: Make sure to check for singularities in the function before applying the maximum principle.
Failing to check for boundary values: Make sure to check for boundary values of the function before applying the maximum principle.
Applying the maximum principle to a function that is not analytic: Make sure that the function is analytic on the bounded domain before applying the maximum principle.

Q: What are some common extensions of the maximum principle?

A: Some common extensions of the maximum principle include:

The minimum principle: The minimum principle is a similar theorem to the maximum principle, but it states that a non-constant analytic function on a bounded domain cannot attain its minimum value on the interior of the domain.
The Phragmén-Lindelöf theorem: The Phragmén-Lindelöf theorem is a generalization of the maximum principle that applies to functions that are analytic on a strip in the complex plane.
The Hadamard three-circle theorem: The Hadamard three-circle theorem is a generalization of the maximum principle that applies to functions that are analytic on a disk in the complex plane.