# Definition:Partial Derivative/Vector Function

## Definition

Let $\map {\R^n} {x_1, x_2, \ldots, x_n}$ denote the real Cartesian space of $n$ dimensions.

Let $\tuple {\mathbf e_1, \mathbf e_2, \ldots, \mathbf e_n}$ be the standard ordered basis on $\R^n$.

Let $\mathbf f: \R^n \to \R^n$ be a vector-valued function on $\R^n$:

$\forall \mathbf x \in \R^n: \map {\mathbf f} {\mathbf x} := \ds \sum_{k \mathop = 1}^n \map {f_k} {\mathbf x} \mathbf e_k$

where each of $f_k: \R^n \to \R$ are real-valued functions on $\R^n$.

For all $k$, let $f_k$ be differentiable at $a$.

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

$\map {\dfrac {\partial \mathbf f} {\partial x_i} } {\mathbf a} := \ds \sum_{k \mathop = 1}^n \map {g_{k i} '} {a_i} \mathbf e_k$

where:

$g_{k i}$ is the real function defined as $\map {g_k} {x_i} = \map {f_k} {a_1, \ldots, x_i, \dots, a_n}$
$\map {g_{k i}'} {a_i}$ is the derivative of $g_k$ at $a_i$.

### Cartesian $3$-space

Let $\map {\R^3} {x, y, z}$ denote the Cartesian $3$-space.

Let $\tuple {\mathbf i, \mathbf j, \mathbf k}$ be the standard ordered basis on $\R^3$.

Let $\mathbf V$ be a vector field in $\R^3$.

Let $\mathbf v: \R^3 \to \mathbf V$ be a vector-valued function on $\R^3$:

$\forall P = \tuple {x, y, z} \in \R^3: \map {\mathbf v} P := \map {v_1} P \mathbf i + \map {v_2} P \mathbf j + \map {v_3} P \mathbf k$

Let $v_1, v_2, v_3$ be differentiable at $\mathbf a = \tuple {a_x, a_y, a_z}$.

The partial derivatives of $\mathbf v$ with respect to $x$, $y$ and $z$ at $\mathbf a$ are denoted and defined as:

$\map {\dfrac {\partial \mathbf v} {\partial x} } {\mathbf a} := \map {\dfrac {\d v_1} {\d x} } {x, a_y, a_z} \mathbf i + \map {\dfrac {\d v_2} {\d x} } {x, a_y, a_z} \mathbf j + \map {\dfrac {\d v_3} {\d x} } {x, a_y, a_z} \mathbf k$
$\map {\dfrac {\partial \mathbf v} {\partial y} } {\mathbf a} := \map {\dfrac {\d v_1} {\d y} } {a_x, y, a_z} \mathbf i + \map {\dfrac {\d v_2} {\d y} } {a_x, y, a_z} \mathbf j + \map {\dfrac {\d v_3} {\d y} } {a_x, y, a_z} \mathbf k$
$\map {\dfrac {\partial \mathbf v} {\partial z} } {\mathbf a} := \map {\dfrac {\d v_1} {\d z} } {a_x, y, a_z} \mathbf i + \map {\dfrac {\d v_2} {\d z} } {a_x, a_y, z} \mathbf j + \map {\dfrac {\d v_3} {\d z} } {a_x, a_y, z} \mathbf k$