# Definition:Contour Integral/Complex

## Definition

Let $C$ be a contour defined by a finite sequence $C_1, \ldots, C_n$ of directed smooth curves.

Let $C_i$ be parameterized by the smooth path $\gamma_i: \closedint {a_i} {b_i} \to \C$ for all $i \in \set {1, \ldots, n}$.

Let $f: \Img C \to \C$ be a continuous complex function, where $\Img C$ denotes the image of $C$.

The **contour integral of $f$ along $C$** is defined by:

- $\displaystyle \int_C \map f z \rd z = \sum_{i \mathop = 1}^n \int_{a_i}^{b_i} \map f {\map {\gamma_i} t} \map {\gamma_i'} t \rd t$

From Contour Integral is Well-Defined, it follows that the complex Riemann integral on the right side is defined and is independent of the parameterizations of $C_1, \ldots, C_n$.

### Contour Integral along Closed Contour

Let $C$ be a closed contour in $\C$.

Then the symbol $\displaystyle \oint$ is used for the **contour integral** on $C$.

The definition remains the same:

- $\displaystyle \oint_C f \left({z}\right) \rd z := \sum_{i \mathop = 1}^n \int_{a_i}^{b_i} f \left({\gamma_i \left({t}\right) }\right) \gamma_i' \left({t}\right) \rd t$

## Also known as

A **contour integral** is called a **line integral** or a **curve integral** in many texts.

## Sources

- 2001: Christian Berg:
*Kompleks funktionsteori*$\S 2.2$