Definition:Derivative/Complex Function

Definition
Let $f \left({z}\right): \C \to \C$ be a single-valued continuous complex function in a domain $D \subseteq \C$.

Let $z_0 \in D$ be a point in $D$.

Let $f$ be complex-differentiable at the point $z_0$.

That is, suppose the limit $\displaystyle \lim_{h \to 0} \ \frac {f \left({z_0 + h}\right) - f \left({z_0}\right)} h$ exists as a finite number and is independent of how the complex increment $h$ tends to $0$.

Then this limit, which can be denoted $f' \left({z_0}\right)$, is called the derivative of $f$ at the point $z_0$.

Further, let $f$ be complex-differentiable at all points in $D$.

Then $f': D \to \C$ is defined as the complex function whose value at each point $z \in D$ is $f' \left({z}\right)$.