Estimation Lemma for Contour Integrals

Theorem
Let $C$ be a contour.

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

Then:


 * $\ds \size {\int_C \map f z \rd z} \le \max_{z \mathop \in \Img C} \size {\map f z} \map L C$

where $\map L C$ denotes the length of $C$.

Proof
By definition of contour, $C$ is a concatenation of a finite sequence $C_1, \ldots, C_n$ of directed smooth curves.

Let $C_k$ be parameterized by the smooth path $\gamma_k: \closedint {a_k} {b_k} \to \C$ for all $k \in \set {1, \ldots, n}$.

Then:

Also known as
Some sources give this as Triangle Inequality for Contour Integrals.

Both names are used on in order to enhance accessibility.