# Holomorphic function/Criteria
## Introduction

Holomorphy of a function $f\colon U\to \mathbb {C}$ at a point $z_{0}\in U$ is a neighborhood property of $z_{0}$. There are numerous criteria in complex analysis that can be used to verify holomorphy. Let $U\subseteq \mathbb {C}$ be a domain as a subset of the complex plane and $z_{0}\in U$ a point in this subset.

### Animation - Visualization of the Mapping

The animation shows the function $f(z)={\frac {1}{z}}$. In the animation, $z$ is shown in blue, and the corresponding image point $f(z)$ is shown in red. The point $z$ and $f(z)$ are represented in $\mathbb {C} {\widetilde {=}}\mathbb {R} ^{2}$. The $y$-axis represents the imaginary part of the complex numbers $z$ and $f(z)$. The blue point $z$ moves along the path $\gamma (t):=t\cdot (\cos(t)+i\cdot \sin(t))$

### Complex Differentiability

A function $f\colon U\to \mathbb {C}$ is called complex differentiable at the point $z_{0}$ if the limit $\lim _{h\to 0}{\frac {f(z_{0}+h)-f(z_{0})}{h}}$ exists with $h\in \mathbb {C}$. This is denoted as $f'(z_{0})$.

### Holomorphy

A function $f\colon U\to \mathbb {C}$ is called holomorphic at the point $z_{0}$ if there exists a neighborhood $U_{0}\subseteq U$ of $z_{0}$ such that $f$ is complex differentiable in $U_{0}$. If $f$ is holomorphic on all of $U$, it is simply called holomorphic. If additionally $U=\mathbb {C}$, $f$ is called an entire function.

## Holomorphy Criteria

Let $f:U\to \mathbb {C}$ be a function where $U\subseteq \mathbb {C}$ is a domain, then the following properties of the complex-valued function $f$ are equivalent:

### (HK1) Once Complex Differentiable

The function $f$ is once complex differentiable on $U$.

### (HK2) Arbitrarily Often Complex Differentiable

The function $f$ is arbitrarily often complex differentiable on $U$.

### (HK3) Cauchy-Riemann Differential Equations

The real and imaginary parts satisfy the Cauchy-Riemann equations and are at least once continuously real-differentiable on $U$.

### (HK4) Locally Expansible in Power Series

The function can be locally expanded in a complex power series on $U$.

### (HK5) Path Integrals 0

The function $f$ is continuous, and the path integral of the function over any closed contractible path vanishes (i.e., the winding number of the path integral for all points outside of $U$ is 0).

### (HK6) Cauchy Integral Formula

The function values inside a circular disk can be determined from the function values on the boundary using the Cauchy integral formula.

### (HK7) Cauchy-Riemann Operator

$f$ is real differentiable, and ${\frac {\partial f}{\partial {\bar {z}}}}=0$, where ${\frac {\partial }{\partial {\bar {z}}}}$ is the Cauchy-Riemann operator defined by ${\frac {\partial }{\partial {\bar {z}}}}:={\frac {1}{2}}\left({\frac {\partial }{\partial x}}+i{\frac {\partial }{\partial y}}\right)$.

## Exercises

Let $a,z_{0}\in \mathbb {C}$ be chosen arbitrarily, and assume that $a\neq z_{0}$. Now, develop the function $f(z):={\frac {1}{z-a}}$ for $z\in \mathbb {C} \setminus {a}$ in a power series around $z_{0}\in \mathbb {C}$ and show that the following holds: $f(z)={\frac {1}{z-a}}=\sum _{n=0}^{\infty }-{\frac {1}{(a-z_{0})^{n+1}}}\cdot (z-z_{0})^{n}$
Calculate the radius of convergence of the power series! Explain why the radius of convergence depends on $a,z_{0}\in \mathbb {C}$ in this way and cannot be larger!
It is not true in real analysis that the existence of a once differentiable function implies that the function is infinitely differentiable. Consider the function $g(x):=x\cdot |x|$ defined on all of $\mathbb {R}$.
Explain how the central theorem of Complex Analysis from criterion 1 leads to criterion 2!