The Cartiesian Product

Cartesian product $\scriptstyle A\times B$ of the sets $\scriptstyle A=\{x,y,z\}$ and $\scriptstyle B=\{1,2,3\}$

The Cartesian product of two sets A and B, denoted A×B, is the set of all ordered pairs (a, b) where a is in A and b is in B. In terms of set-builder notation, that is

$A\times B=\{(a,b)\mid a\in A\ {\mbox{ and }}\ b\in B\}.$

A table can be created by taking the Cartesian product of a set of rows and a set of columns. If the Cartesian product rows × columns is taken, the cells of the table contain ordered pairs of the form (row value, column value).

One can similarly define the Cartesian product of n sets, also known as an n-fold Cartesian product, which can be represented by an n-dimensional array, where each element is an n-tuple. An ordered pair is a 2-tuple or couple. More generally still, one can define the Cartesian product of an indexed family of sets.

The Cartesian product is named after René Descartes, whose formulation of analytic geometry gave rise to the concept, which is further generalized in terms of direct product.

Most common implementation (set theory)

A formal definition of the Cartesian product from set-theoretical principles follows from a definition of ordered pair. The most common definition of ordered pairs, Kuratowski's definition, is $(x,y)=\{\{x\},\{x,y\}\}$. Under this definition, $(x,y)$ is an element of ${\mathcal {P}}({\mathcal {P}}(X\cup Y))$, and $X\times Y$ is a subset of that set, where ${\mathcal {P}}$ represents the power set operator. Therefore, the existence of the Cartesian product of any two sets in ZFC follows from the axioms of pairing, union, power set, and specification. Since functions are usually defined as a special case of relations, and relations are usually defined as subsets of the Cartesian product, the definition of the two-set Cartesian product is necessarily prior to most other definitions.

Non-commutativity and non-associativity

Let A, B, C, and D be sets.

The Cartesian product A × B is not commutative,

$A\times B\neq B\times A,$

because the ordered pairs are reversed unless at least one of the following conditions is satisfied:

• A is equal to B, or
• A or B is the empty set.

For example:

A = {1,2}; B = {3,4}

A × B = {1,2} × {3,4} = {(1,3), (1,4), (2,3), (2,4)}

B × A = {3,4} × {1,2} = {(3,1), (3,2), (4,1), (4,2)}

A = B = {1,2}

A × B = B × A = {1,2} × {1,2} = {(1,1), (1,2), (2,1), (2,2)}

A = {1,2}; B = ∅

A × B = {1,2} × ∅ = ∅

B × A = ∅ × {1,2} = ∅

Strictly speaking, the Cartesian product is not associative (unless one of the involved sets is empty).

$(A\times B)\times C\neq A\times (B\times C)$

If for example A = {1}, then (A × A) × A = {((1, 1), 1)} ≠ {(1, (1, 1))} = A × (A × A).

Cardinality

The cardinality of a set is the number of elements of the set. For example, defining two sets: A = {a, b} and B = {5, 6}. Both set A and set B consist of two elements each. Their Cartesian product, written as A × B, results in a new set which has the following elements:

A × B = {(a,5), (a,6), (b,5), (b,6)}.

where each element of A is paired with each element of B, and where each pair makes up one element of the output set. The number of values in each element of the resulting set is equal to the number of sets whose Cartesian product is being taken; 2 in this case. The cardinality of the output set is equal to the product of the cardinalities of all the input sets. That is,

|A × B| = |A| · |B|.

In this case, |A × B| = 4

Similarly

|A × B × C| = |A| · |B| · |C|

and so on.

The set A × B is infinite if either A or B is infinite, and the other set is not the empty set.

Cartesian products of several sets

n-ary Cartesian product

The Cartesian product can be generalized to the n-ary Cartesian product over n sets X₁, ..., X_n as the set

$X_{1}\times \cdots \times X_{n}=\{(x_{1},\ldots ,x_{n})\mid x_{i}\in X_{i}\ {\text{for every}}\ i\in \{1,\ldots ,n\}\}$

of n-tuples. If tuples are defined as nested ordered pairs, it can be identified with (X₁ × ⋯ × X_n−1) × X_n. If a tuple is defined as a function on {1, 2, …, n} that takes its value at i to be the ith element of the tuple, then the Cartesian product X₁×⋯×X_n is the set of functions

$\{x:\{1,\ldots ,n\}\to X_{1}\cup \cdots \cup X_{n}\ |\ x(i)\in X_{i}\ {\text{for every}}\ i\in \{1,\ldots ,n\}\}.$

n-ary Cartesian power

The Cartesian square of a set X is the Cartesian product X² = X × X. An example is the 2-dimensional plane R² = R × R where R is the set of real numbers: R² is the set of all points (x,y) where x and y are real numbers.

The n-ary Cartesian power of a set X, denoted $X^{n}$, can be defined as

$X^{n}=\underbrace {X\times X\times \cdots \times X} _{n}=\{(x_{1},\ldots ,x_{n})\ |\ x_{i}\in X\ {\text{for every}}\ i\in \{1,\ldots ,n\}\}.$

An example of this is R³ = R × R × R, with R again the set of real numbers, and more generally Rⁿ.

The n-ary Cartesian power of a set X is isomorphic to the space of functions from an n-element set to X. As a special case, the 0-ary Cartesian power of X may be taken to be a singleton set, corresponding to the empty function with codomain X.

Infinite Cartesian products

It is possible to define the Cartesian product of an arbitrary (possibly infinite) indexed family of sets. If I is any index set, and $\{X_{i}\}_{i\in I}$ is a family of sets indexed by I, then the Cartesian product of the sets in $\{X_{i}\}_{i\in I}$ is defined to be

$\prod _{i\in I}X_{i}=\left\{\left.f:I\to \bigcup _{i\in I}X_{i}\ \right|\ (\forall i)(f(i)\in X_{i})\right\},$

that is, the set of all functions defined on the index set such that the value of the function at a particular index i is an element of X_i. Even if each of the X_i is nonempty, the Cartesian product may be empty if the axiom of choice, which is equivalent to the statement that every such product is nonempty, is not assumed.

For each j in I, the function

$\pi _{j}:\prod _{i\in I}X_{i}\to X_{j},$

defined by $\pi _{j}(f)=f(j)$ is called the jth projection map.

Cartesian power is a Cartesian product where all the factors X_i are the same set X. In this case,

$\prod _{i\in I}X_{i}=\prod _{i\in I}X$

is the set of all functions from I to X, and is frequently denoted X^I. This case is important in the study of cardinal exponentiation. An important special case is when the index set is $\mathbb {N}$, the natural numbers: this Cartesian product is the set of all infinite sequences with the ith term in its corresponding set X_i. For example, each element of

$\prod _{n=1}^{\infty }\mathbb {R} =\mathbb {R} \times \mathbb {R} \times \cdots$

can be visualized as a vector with countably infinite real number components. This set is frequently denoted $\mathbb {R} ^{\omega }$, or $\mathbb {R} ^{\mathbb {N} }$.

Other forms

Abbreviated form

If several sets are being multiplied together (e.g., X₁, X₂, X₃, …), then some authors choose to abbreviate the Cartesian product as simply ×X_i.

Cartesian product of functions

If f is a function from A to B and g is a function from X to Y, then their Cartesian product f × g is a function from A × X to B × Y with

$(f\times g)(a,x)=(f(a),g(x)).$

This can be extended to tuples and infinite collections of functions. This is different from the standard Cartesian product of functions considered as sets.

Cylinder

Let $A$ be a set and $B\subseteq A$. Then the cylinder of $B$ with respect to $A$ is the Cartesian product $B\times A$ of $B$ and $A$.

Normally, $A$ is considered to be the universe of the context and is left away. For example, if $B$ is a subset of the natural numbers $\mathbb {N}$, then the cylinder of $B$ is $B\times \mathbb {N}$.

Licensing

Content obtained and/or adapted from:

• Cartesian product, Wikipedia under a CC BY-SA license