Definition:Partial Derivative/Real Analysis

Definition

At a Point

Let $U\subset\R^n$ be an open set.

Let $f : U \to \R$ be a real-valued function.

Let $a = (a_1,\ldots,a_n)^\intercal \in U$.

Let $f$ be differentiable at $a$.

Let $i\in\{1,\ldots, n\}$.

Definition 1

The partial derivative of $f$ with respect to $x_i$ at $a$ is denoted and defined as:

$\dfrac {\partial f}{\partial x_i}(a) := g_i'(a_i)$

where:

$g_i$ is the real function defined as $g \left({x_i}\right) = f \left({a_1, \ldots, x_i, \dots, a_n}\right)$
$g'(a_i)$ is the derivative of $g$ at $a_i$.

Definition 2

The $i$th partial derivative of $f$ at $a$ is the limit:

$\dfrac{\partial f}{\partial x_i}(a) = \displaystyle \lim_{x_i \to a_i} \frac {f\left( a_1,\ldots, x_i, \ldots,a_n\right) - f\left(a\right)}{x_i - a}$

When spoken, $\dfrac {\partial y}{\partial x}$, "the partial derivative of $y$ with respect to $x$" is often shortened to "partial $y$ partial $x$", or "del $y$ del $x$".

On an Open Set

Let $U\subset\R^n$ be an open set.

Let $f : U \to \R$ be a real-valued function.

Let $f$ be differentiable in $U$.

The $i$th partial derivative (function) of $f$ with respect to $x_i$ is the real-valued function which sends each $x\in U$ to the $i$th partial derivative at $x$.

Notation

There are various notations for the $i$th partial derivative of a function:

• $\dfrac {\partial f} {\partial x_i}$
• $\dfrac {\partial} {\partial x_i} f$
• $f_{x_i} \left({\mathbf x}\right)$
• $f_{x_i} \left({x_1, x_2, \cdots, x_n}\right)$
• $f_{x_i}$
• $\partial_{x_i}f$
• $\partial_i f$
• $D_i f$
• $\dfrac {\partial z} {\partial x_i}$
• $z_{x_i}$

where $z = f \left({x_1, x_2, \cdots, x_n}\right)$.