# Definition:Vector Cross Product

This page is about cross product in the context of vector algebra. For other uses, see Definition:Cross Product.

## Definition

Let $\mathbf a$ and $\mathbf b$ be vectors in a vector space $\mathbf V$ of $3$ dimensions:

$\mathbf a = a_i \mathbf i + a_j \mathbf j + a_k \mathbf k$
$\mathbf b = b_i \mathbf i + b_j \mathbf j + b_k \mathbf k$

where $\left({\mathbf i, \mathbf j, \mathbf k}\right)$ is the standard ordered basis of $\mathbf V$.

### Definition 1

The vector cross product, denoted $\mathbf a \times \mathbf b$, is defined as:

$\mathbf a \times \mathbf b = \begin{vmatrix} \mathbf i & \mathbf j & \mathbf k\\ a_i & a_j & a_k \\ b_i & b_j & b_k \\ \end{vmatrix}$

where $\begin{vmatrix} \ldots \end{vmatrix}$ is interpreted as a determinant.

More directly:

$\mathbf a \times \mathbf b = \mathbf i \paren {a_j b_k - a_k b_j} - \mathbf j \paren {a_i b_k - a_k b_i} + \mathbf k \paren {a_i b_j - a_j b_i}$

### Definition 2

The vector cross product, denoted $\mathbf a \times \mathbf b$, is defined as:

$\mathbf a \times \mathbf b = \norm {\mathbf a} \, \norm {\mathbf b} \sin \theta \hat {\mathbf n}$

where:

$\norm {\mathbf a}$ denotes the length of $\mathbf a$
$\theta$ denotes the angle from $\mathbf a$ to $\mathbf b$, measured in the positive direction
$\hat {\mathbf n}$ is the unit vector perpendicular to both $\mathbf a$ and $\mathbf b$ in the direction according to the right hand rule.

## Complex Numbers

The definition is slightly different when the vector space under consideration is the complex plane, as the latter is of $2$ dimensions.

This is a modification of the vector cross product in which the resulting product is taken to be the length of the hypothetical vector which would be considered as being perpendicular to the complex plane.

Let $z_1 := x_1 + i y_1$ and $z_2 := x_2 + i y_2$ be complex numbers.

### Definition 1

The cross product of $z_1$ and $z_2$ is defined as:

$z_1 \times z_2 = x_1 y_2 - y_1 x_2$

### Definition 2

The cross product of $z_1$ and $z_2$ is defined as:

$z_1 \times z_2 = \cmod {z_1} \, \cmod {z_2} \sin \theta$

where:

$\cmod {z_1}$ denotes the complex modulus of $z_1$
$\theta$ denotes the angle from $z_1$ to $z_2$, measured in the positive direction.

### Definition 3

The cross product of $z_1$ and $z_2$ is defined as:

$z_1 \times z_2 := \map \Im {\overline {z_1} z_2}$

where:

$\map \Im z$ denotes the imaginary part of a complex number $z$
$\overline {z_1}$ denotes the complex conjugate of $z_1$
$\overline {z_1} z_2$ denotes complex multiplication.

### Definition 4

The cross product of $z_1$ and $z_2$ is defined as:

$z_1 \times z_2 := \dfrac {\overline {z_1} z_2 - z_1 \overline {z_2}} {2 i}$

where:

$\overline {z_1}$ denotes the complex conjugate of $z_1$
$\overline {z_1} z_2$ denotes complex multiplication.

## Also known as

The vector cross product is often called just the cross product when there is no chance of confusion with other types of cross product.

The term vector product can also sometimes be seen, but again this can be ambiguous.

Some sources use $\mathbf A \wedge \mathbf B$, where $\wedge$ is referred to as wedge.

Its $\LaTeX$ symbol is \wedge.

## Also see

• Results about Vector Cross Product can be found here.