# Definition:Dot Product

## Contents

## Definition

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

- $\mathbf a = \displaystyle \sum_{k \mathop = 1}^n a_k \mathbf e_k$
- $\mathbf b = \displaystyle \sum_{k \mathop = 1}^n b_k \mathbf e_k$

where $\tuple {\mathbf e_1, \mathbf e_2, \ldots, \mathbf e_n}$ is the standard ordered basis of $\mathbf V$.

### Definition 1

The **dot product** of $\mathbf a$ and $\mathbf b$ is defined as:

- $\displaystyle \mathbf a \cdot \mathbf b = a_1 b_1 + a_2 b_2 + \cdots + a_n b_n = \sum_{i \mathop = 1}^n a_i b_i$

If the vectors are represented as column matrices:

- $\mathbf a = \begin{bmatrix} a_1 \\ a_2 \\ \vdots \\ a_n \end{bmatrix} , \mathbf b = \begin{bmatrix} b_1 \\ b_2 \\ \vdots \\ b_n \end{bmatrix}$

we can express the dot product as:

- $\mathbf a \cdot \mathbf b = \mathbf a^\intercal \mathbf b$

where:

- $\mathbf a^\intercal = \begin{bmatrix} a_1 & a_2 & \cdots & a_n \end{bmatrix}$ is the transpose of $\mathbf a$
- the operation between the matrices is the matrix product.

### Definition 2

The **dot product** of $\mathbf a$ and $\mathbf b$ is defined as:

- $\mathbf a \cdot \mathbf b = \norm {\mathbf a} \, \norm {\mathbf b} \cos \angle \mathbf a, \mathbf b$

where:

- $\norm {\mathbf a}$ denotes the length of $\mathbf a$
- $\angle \mathbf a, \mathbf b$ is the angle between $\mathbf a$ and $\mathbf b$, taken to be between $0$ and $\pi$.

## Complex Numbers

The definition continues to hold when the vector space under consideration is 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 **dot product** of $z_1$ and $z_2$ is defined as:

- $z_1 \circ z_2 = x_1 x_2 + y_1 y_2$

### Definition 2

The **dot product** of $z_1$ and $z_2$ is defined as:

- $z_1 \circ z_2 = \cmod {z_1} \, \cmod{z_2} \cos \theta$

where:

- $\cmod {z_1}$ denotes the complex modulus of $z_1$
- $\theta$ denotes the angle between $z_1$ and $z_2$.

### Definition 3

The **dot product** of $z_1$ and $z_2$ is defined as:

- $z_1 \circ z_2 := \map \Re {\overline {z_1} z_2}$

where:

- $\map \Re z$ denotes the real 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 **dot product** of $z_1$ and $z_2$ is defined as:

- $z_1 \circ z_2 := \dfrac {\overline {z_1} z_2 + z_1 \overline {z_2} } 2$

where:

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

## Also known as

The **dot product** is also known as:

- The
**scalar product**(but this can be confused with multiplication by a scalar so is less recommended) - The
**standard inner product**.

The symbol used for the dot is variously presented; another version is $\mathbf a \bullet \mathbf b$, which can be preferred if there is ambiguity between the dot product and standard multiplication.

In the complex plane, where it is commonplace to use $\cdot$ to denote complex multiplication, the symbol $\circ$ is often used to denote the **dot product**.

## Also see

- Results about
**dot product**can be found here.

## Sources

- 1992: George F. Simmons:
*Calculus Gems*... (previous) ... (next): Chapter $\text {B}.26$: Extensions of the Complex Number System. Algebras, Quaternions, and Lagrange's Four Squares Theorem