Cauchy-Goursat Theorem/Corollary 1
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Corollary
Let $f: U \to \C$ be a holomorphic function, where $U \subseteq \C$ is an open set.
Let $C$ be a simple closed contour in $U$.
Let $\Int C \subseteq U$, where $\Int C$ denotes the interior of $C$.
Then:
- $\ds \oint_C \map f z \rd z = 0$
Proof
By Interior of Simply Closed Contour Extends to Simply Connected Domain, there exists a simply connected domain $V$ such that $\Int C \subseteq V \subseteq U$, and $C$ is a contour in $V$.
The result now follows from the main Cauchy-Goursat Theorem.
$\blacksquare$
Sources
- 2001: Christian Berg: Kompleks funktionsteori: $\S 3.1$