# Definition:Partial Derivative

## Definition

### Real Analysis

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:

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

## Second Derivative

Let $f \left({x, y}\right)$ be a function of the two independent variables $x$ and $y$.

The second partial derivatives of $f$ with respect to $x$ and $y$ are defined and denoted by:

$(1): \quad \dfrac {\partial^2 f}{\partial x^2} = \dfrac {\partial}{\partial x} \left({\dfrac {\partial f}{\partial x}}\right)$
$(2): \quad \dfrac {\partial^2 f}{\partial y^2} = \dfrac {\partial}{\partial y} \left({\dfrac {\partial f}{\partial y}}\right)$
$(3): \quad \dfrac {\partial^2 f}{\partial x \partial y} = \dfrac {\partial}{\partial x} \left({\dfrac {\partial f}{\partial y}}\right)$
$(4): \quad \dfrac {\partial^2 f}{\partial y \partial x} = \dfrac {\partial}{\partial y} \left({\dfrac {\partial f}{\partial x}}\right)$

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

## Also see

• Results about partial differentiation can be found here.