# Complex Analysis/Complex Functions/Analytic Functions

From our look at complex derivatives, we now examine the analytic functions, the Cauchy-Riemann Equations, and Harmonic Functions.

2.4.1 Holomorphic functions

Note: Holomorphic functions are sometimes referred to as analytic functions. This equivalence will be shown later, though the terms may be used interchangeably until then.

Definition: A complex valued function $f(z)$ is holomorphic on an open set $G$ if it has a derivative at every point in $G$ .

Here, holomorphicity is defined over an open set, however, differentiability could only at one point. If f(z) is holomorphic over the entire complex plane, we say that f is entire. As an example, all polynomial functions of z are entire. (proof)

2.4.2 The Cauchy-Riemann Equations

The definition of holomorphic suggests a relationship between both the real and imaginary parts of the said function. Suppose $f(z)=u(x,y)+v(x,y)i$ is differentiable at $z_{0}=x_{0}+y_{0}i$ . Then the limit

$\lim _{\Delta z\to 0}{\frac {f(z_{0}+\Delta z)-f(z_{0})}{\Delta z}}$ can be determined by letting $\Delta z_{0}(=\Delta x_{0}+\Delta y_{0}i)$ approach zero from any direction in $\mathbb {C}$ .

If it approaches horizontally, we have $f'(z_{0})={\frac {\partial u}{\partial x}}(x_{0},y_{0})+i{\frac {\partial v}{\partial x}}(x_{0},y_{0})$ . Similarly, if it approaches vertically, we have $f'(z_{0})={\frac {\partial v}{\partial y}}(x_{0},y_{0})-i{\frac {\partial u}{\partial y}}(x_{0},y_{0})$ . By equating the real and imaginary parts of these two equations, we arrive at:

${\frac {\partial u}{\partial x}}={\frac {\partial v}{\partial y}},{\frac {\partial v}{\partial x}}=-{\frac {\partial u}{\partial y}}$ These are known as the Cauchy-Riemann Equations, and leads us to an important theorem.

Theorem: Let a function $f(z)=u(x,y)+v(x,y)i$ be defined on an open set $G$ containing a point, $z_{0}$ . If the first partials of $u,v$ exist in $G$ and are continuous at $z_{0}$ and satisfy the Cauchy-Riemann equations, then f is differentiable at $z_{0}$ . Furthermore, if the above conditions are satisfied, $f$ is analytic in $G$ . (proof).

2.4.3 Harmonic Functions

Now we move to Harmonic functions. Recall the Laplace equation, $\nabla ^{2}(\phi ):={\frac {\partial ^{2}(\phi )}{\partial x^{2}}}+{\frac {\partial ^{2}(\phi )}{\partial y^{2}}}=0$ Definition: A real valued function $\phi (x,y)$ is harmonic in a domain $D$ if all of its second partials are continuous in $D$ and if at each point in $D$ , $\phi$ is analytic in a domain $D$ , then both $u(x,y),v(x,y)$ are harmonic in $D$ . (proof)