# Product (mathematics)

Calculation results
Subtraction (−)
minuend − subtrahend = difference
Multiplication (×)
multiplicand × multiplier = product
Division (÷)
dividend ÷ divisor = quotient
Exponentiation
baseexponent = power
nth root (√)
Logarithm
logbase(power) = exponent

In mathematics, a product is the result of multiplying, or an expression that identifies factors to be multiplied. Thus, for instance, 6 is the product of 2 and 3 (the result of multiplication), and $x\cdot (2+x)$ is the product of $x$ and $(2+x)$ (indicating that the two factors should be multiplied together).

The order in which real or complex numbers are multiplied has no bearing on the product; this is known as the commutative law of multiplication. When matrices or members of various other associative algebras are multiplied, the product usually depends on the order of the factors. Matrix multiplication, for example, and multiplication in other algebras is in general non-commutative.

## Product of two numbers

### Product of two natural numbers

3 by 4 is 12

Placing several stones into a rectangular pattern with $r$ rows and $s$ columns gives

$r \cdot s = \sum_{i=1}^s r = \sum_{j=1}^r s$

stones.

### Product of two integers

Integers allow positive and negative numbers. The two numbers are multiplied just like natural numbers, except we need an additional rule for the signs:

$\begin{array}{|c|c c|}\hline \cdot & - & + \\ \hline - & + & - \\ + & - & + \\ \hline \end{array}$

In words, we have:

• Minus times Minus gives Plus
• Minus times Plus gives Minus
• Plus times Minus gives Minus
• Plus times Plus gives Plus

### Product of two fractions

Two fractions can be multiplied by multiplying their numerators and denominators:

$\frac{z}{n} \cdot \frac{z'}{n'} = \frac{z\cdot z'}{n\cdot n'}$

### Product of two real numbers

The rigorous definition of the product of two real numbers is too complicated for this article. But the idea is that one takes a decimal approximation to each real and multiplies the approximations together, and then take better and better approximations.

### Product of two complex numbers

Two complex numbers can be multiplied by the distributive law and the fact that $\mathrm i^2=-1$, as follows:

\begin{align} (a + b\,\mathrm i)\cdot (c+d\,\mathrm i) & = a\cdot c + a \cdot d\,\mathrm i + b\cdot c \,\mathrm i + b\cdot d \cdot \mathrm i^2\\ & = (a \cdot c - b\cdot d) + (a\cdot d + b\cdot c) \,\mathrm i \end{align}

#### Geometric meaning of complex multiplication

A complex number in polar coordinates.

Complex numbers can be written in polar coordinates:

$a + b\,\mathrm i = r \cdot ( \cos(\varphi) + \mathrm i \sin(\varphi) ) = r \cdot \mathrm e ^{\mathrm i \varphi}$

Furthermore,

$c + d\,\mathrm i = s \cdot ( \cos(\psi) + \mathrm i \sin(\psi) ) = s \cdot \mathrm e ^{\mathrm i \psi}$, from which we obtain:
$(a \cdot c - b\cdot d) + (a\cdot d + b\cdot c) \,\mathrm i = r\cdot s \cdot ( \cos(\varphi+\psi) + \mathrm i \sin(\varphi+\psi) ) = r\cdot s \cdot \mathrm e ^{\mathrm i (\varphi+\psi)}$

The geometric meaning is that we multiply the magnitudes and add the angles.

### Product of two quaternions

The product of two quaternions can be found in the article on quaternions. However, it is interesting to note that in this case, $a\cdot b$ and $b\cdot a$ are different.

## Product of sequences

The product operator for the product of a sequence is denoted by the capital Greek letter Pi (in analogy to the use of the capital Sigma as summation symbol). The product of a sequence consisting of only one number is just that number itself. The product of no factors at all is known as the empty product, and is equal to 1.

## Further examples for commutative rings

### Residue classes of integers

Residue classes in the rings $\Z/N\Z$ can be added:

$(a+N\Z) + (b+N\Z) = a+b + N\Z$

and multiplied:

$(a+N\Z) \cdot (b+N\Z) = a\cdot b + N\Z$

### Rings of functions

Functions to the real numbers can be added or multiplied by adding or multiplying their outputs:

$(f+g)(m) : = f(m) + g(m)$
$(f\cdot g) (m) := f(m) \cdot g(m)$

#### Convolution

The convolution of the square wave with itself gives the triangular function

Two functions from the reals to itself can be multiplied in another way, called the convolution.

If :$\int\limits_{-\infty}^\infty |f(t)|\,\mathrm{d}\,t \;<\;\infty\quad\mbox{and } \int\limits_{-\infty}^\infty |g(t)|\,\mathrm{d}\,t \;<\; \infty$

then the integral

$(f*g) (t) \;:= \int\limits_{-\infty}^\infty f(\tau)\cdot g(t-\tau)\,\mathrm{d}\tau$

is well defined and is called the convolution.

Under the Fourier transform, convolution becomes multiplication.

### Polynomial rings

The product of two polynomials is given by the following:

$\left(\sum_{i=0}^n a_i X^i\right) \cdot \left(\sum_{j=0}^m b_j X^j\right) = \sum_{k=0}^{n+m} c_k X^k$

with

$c_k = \sum_{i+j=k} a_i \cdot b_j$

## Products in linear algebra

### Scalar multiplication

By the very definition of a vector space, one can form the product of any scalar with any vector, giving a map $\R \times V \rightarrow V$.

### Scalar product

A scalar product is a bilinear map:

$\cdot : V \times V \rightarrow \R$

with the following conditions, that $v\cdot v > 0$ for all $0 \not= v \in V$.

From the scalar product, one can define a norm by letting $\|v\| := \sqrt{v\cdot v}$.

The scalar product also allows one to define an angle between two vectors:

$\cos \angle (v,w) = \frac{v\cdot w}{\|v\| \cdot \|w\|}$

In $n$-dimensional Euclidean space, the standard scalar product (called the dot product) is given by:

$\left(\sum_{i=1}^n \alpha_i e_i \right) \cdot \left(\sum_{i=1}^n \beta_i e_i \right) = \sum_{i=1}^n \alpha_i\,\beta_i$

### Cross product in 3-dimensional space

The cross product of two vectors in 3-dimensions is a vector perpendicular to the two factors, with length equal to the area of the parallelogram spanned by the two factors.

The cross product can also be expressed as the formala determinant:

$\mathbf{u\times v}=\begin{vmatrix} \mathbf{i}&\mathbf{j}&\mathbf{k}\\ u_1&u_2&u_3\\ v_1&v_2&v_3\\ \end{vmatrix}$

### Product of two matrices

Given two matrices

$A = (a_{i,j})_{i=1\ldots s;j=1\ldots r} \in \R^{s\times r}$ and $B = (b_{j,k})_{j=1\ldots r;k=1\ldots t}\in \R^{r\times t}$

their product is given by

$B \cdot A = \left( \sum_{j=1}^r a_{i,j} \cdot b_{j,k} \right)_{i=1\ldots s;k=1\ldots t} \;\in\R^{s\times t}$

## Set theoretical product

In set theory, a Cartesian product is a mathematical operation which returns a set (or product set) from multiple sets. That is, for sets A and B, the Cartesian product A × B is the set of all ordered pairs (a, b) where a ∈ A and b ∈ B.

## Empty product

The empty product has the value of 1 (the identity element of multiplication) just like the empty sum has the value of 0 (the identity element of addition).

## Other products

Many different kinds of products are studied in mathematics: