# Definition:Directed Smooth Curve

## Definition

Let $\R^n$ be a real cartesian space of $n$ dimensions.

Let $\rho: \left[{a \,.\,.\, b}\right] \to \R^n$ be a smooth path in $\R^n$.

The **directed smooth curve** with **parameterization** $\rho$ is defined as an equivalence class of smooth paths as follows:

A smooth path $\sigma: \left[{a \,.\,.\, b}\right] \to \R^n$ belongs to the equivalence class of $\rho$ if and only if:

- there exists a bijective differentiable strictly increasing real function:
- $\phi: \left[{c \,.\,.\, d}\right] \to \left[{a \,.\,.\, b}\right]$

- such that $\sigma = \rho \circ \phi$.

It follows from Directed Smooth Curve Relation is Equivalence and Fundamental Theorem on Equivalence Relations that this does in fact define an equivalence class.

If a **directed smooth curve** is only defined by a smooth path $\rho$, then it is often denoted with the same symbol $\rho$.

### Parameterization

Let $\R^n$ be a real cartesian space of $n$ dimensions.

Let $C$ be a directed smooth curve in $\R^n$.

Let $\rho: \left[{a \,.\,.\, b}\right] \to \C$ be a smooth path in $\R^n$.

Then $\rho$ is a **parameterization** of $C$ if and only if $\rho$ is an element of the equivalence class that constitutes $C$.

### Endpoints

Let $C$ be parameterized by a smooth path $\rho: \left[{a \,.\,.\, b}\right] \to \C$.

Then:

- $\rho \left({a}\right)$ is the
**start point**of $C$

- $\rho \left({b}\right)$ is the
**end point**of $C$.

Collectively, $\rho \left({a}\right)$ and $\rho \left({b}\right)$ are known as the **endpoints** of $\rho$.

## Complex Plane

The definition carries over to the complex plane, in which context it is usually applied:

Let $\gamma : \left[{ a \,.\,.\, b }\right] \to \C$ be a smooth path in $\C$.

The **directed smooth curve** with **parameterization** $\gamma$ is defined as an equivalence class of smooth paths as follows:

A smooth path $\sigma: \left[{ a \,.\,.\, b }\right] \to \C$ belongs to the equivalence class of $\gamma$ if and only if:

- there exists a bijective differentiable strictly increasing real function:
- $\phi: \left[{c \,.\,.\, d}\right] \to \left[{a \,.\,.\, b}\right]$

- such that $\sigma = \gamma \circ \phi$.

## Also known as

A **directed smooth curve** is called an **oriented smooth curve**, a **smooth curve with orientation** or simply a **curve** in many texts.