Definition:Orientation of Contour (Complex Plane)/Positive

Definition
Let $C$ be a contour in the complex plane $\C$ with parameterization $\gamma: \closedint a b \to \C$.

Set $K := \set { t \in \closedint a b : \textrm{ $\gamma$ is not differentiable at $t$ } }$. Let $D \subseteq \C$ be a connected domain.

Let $\Img C \subseteq \partial D$, where $\Img C$ denotes the image of $C$, and $\partial D$ denotes the boundary of $D$.

Then $C$ is positively oriented with respect to $D$, for all $t \in \openint a b \setminus K$, there exists $r \in \R_{>0}$ such that:


 * for all $\epsilon \in \openint 0 r$ : $\map \gamma t + \epsilon i \map {\gamma'} t \in D$, and $\map \gamma t - \epsilon i \map {\gamma'} t \notin D$

Alternatively, we say that $C$ has a positive orientation with respect to $D$.

Informally, we say that $D$ lies to the left of the tangent vector $\map {\gamma'} t$.

Also known as
Some texts say that $C$ is counterclockwise oriented with respect to $D$, or anticlockwise oriented with respect to $D$.

Some texts use the hyphenated form positively-oriented.

Also see

 * Definition:Anticlockwise