# Cartesian product

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

In mathematics, specifically set theory, 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

## Contents

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

## Examples

### A deck of cards

An illustrative example is the standard 52-card deck. The standard playing card ranks {A, K, Q, J, 10, 9, 8, 7, 6, 5, 4, 3, 2} form a 13-element set. The card suits {♠, , , ♣} form a four-element set. The Cartesian product of these sets returns a 52-element set consisting of 52 ordered pairs, which correspond to all 52 possible playing cards.

Ranks × Suits returns a set of the form {(A, ♠), (A, ), (A, ), (A, ♣), (K, ♠), …, (3, ♣), (2, ♠), (2, ), (2, ), (2, ♣)}.

Suits × Ranks returns a set of the form {(♠, A), (♠, K), (♠, Q), (♠, J), (♠, 10), …, (♣, 6), (♣, 5), (♣, 4), (♣, 3), (♣, 2)}.

These two sets are distinct, even disjoint, but there is a natural bijection between them, under which (3, ♣) corresponds to (♣, 3) and so on.

### A two-dimensional coordinate system

The main historical example is the Cartesian plane in analytic geometry. In order to represent geometrical shapes in a numerical way, and extract numerical information from shapes' numerical representations, René Descartes assigned to each point in the plane a pair of real numbers, called its coordinates. Usually, such a pair's first and second components are called its x and y coordinates, respectively (see picture). The set of all such pairs (i.e., the Cartesian product ℝ×ℝ, with ℝ denoting the real numbers) is thus assigned to the set of all points in the plane.[ citation needed ]

## 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: 

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).

### Intersections, unions, and subsets

Example sets

A={y  :1y4}, B={x : 2x5},
and C = {x : 4x7}, demonstrating
A × (BC) = (A×B) ∩ (A×C),
A × (BC) = (A×B) ∪ (A×C), and

A × (B\C) = (A×B) \ (A×C)
Example sets

A={x : 2x5}, B={x : 3x7},
C={y :1y3}, D={y : 2y4}, demonstrating

(AB) × (CD) = (A×C) ∩ (B×D).
(AB) × (CD) ≠ (A×C) ∪ (B×D) can be seen from the same example.

The Cartesian product satisfies the following property with respect to intersections (see middle picture).

$(A\cap B)\times (C\cap D)=(A\times C)\cap (B\times D)$ In most cases, the above statement is not true if we replace intersection with union (see rightmost picture).

$(A\cup B)\times (C\cup D)\neq (A\times C)\cup (B\times D)$ In fact, we have that:

$(A\times C)\cup (B\times D)=[(A\setminus B)\times C]\cup [(A\cap B)\times (C\cup D)]\cup [(B\setminus A)\times D]$ For the set difference, we also have the following identity:

$(A\times C)\setminus (B\times D)=[A\times (C\setminus D)]\cup [(A\setminus B)\times C]$ Here are some rules demonstrating distributivity with other operators (see leftmost picture): 

{\begin{aligned}A\times (B\cap C)&=(A\times B)\cap (A\times C),\\A\times (B\cup C)&=(A\times B)\cup (A\times C),\\A\times (B\setminus C)&=(A\times B)\setminus (A\times C),\end{aligned}} $(A\times B)^{\complement }=\left(A^{\complement }\times B^{\complement }\right)\cup \left(A^{\complement }\times B\right)\cup \left(A\times B^{\complement }\right)\!,$ where $A^{\complement }$ denotes the absolute complement of A.

Other properties related with subsets are:

${\text{if }}A\subseteq B{\text{, then }}A\times C\subseteq B\times C;$ ${\text{if both }}A,B\neq \emptyset {\text{, then }}A\times B\subseteq C\times D\!\iff \!A\subseteq C{\text{ and }}B\subseteq D.$ ### 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, ..., Xn 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 × ⋯ × Xn−1) × Xn. 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×⋯×Xn 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 (see the Cartesian coordinate system).

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

$\prod _{i\in I}X_{i}=\left\{\left.f:I\to \bigcup _{i\in I}X_{i}\ \right|\ (\forall i\in I)(f(i)\in X_{i})\right\},$ 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 Xi. Even if each of the Xi 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 Xi 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 XI. 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 Xi. 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 ×Xi.

### Cartesian product of functions

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

$(f\times g)(x,y)=(f(x),g(y)).$ 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 $B\times \mathbb {N}$ .

## Definitions outside set theory

### Category theory

Although the Cartesian product is traditionally applied to sets, category theory provides a more general interpretation of the product of mathematical structures. This is distinct from, although related to, the notion of a Cartesian square in category theory, which is a generalization of the fiber product.

Exponentiation is the right adjoint of the Cartesian product; thus any category with a Cartesian product (and a final object) is a Cartesian closed category.

### Graph theory

In graph theory, the Cartesian product of two graphs G and H is the graph denoted by G × H, whose vertex set is the (ordinary) Cartesian product V(G) × V(H) and such that two vertices (u,v) and (u′,v′) are adjacent in G × H, if and only if u = u and v is adjacent with v′ in H, orv = v and u is adjacent with u′ in G. The Cartesian product of graphs is not a product in the sense of category theory. Instead, the categorical product is known as the tensor product of graphs.

## Related Research Articles

In mathematics, a set is countable if either it is finite or it can be made in one to one correspondence with the set of natural numbers. Equivalently, a set is countable if there exists an injective function from it into the natural numbers; this means that each element in the set may be associated to a unique natural number, or that the elements of the set can be counted one at a time, although the counting may never finish due to an infinite number of elements.

In mathematics, one can often define a direct product of objects already known, giving a new one. This generalizes the Cartesian product of the underlying sets, together with a suitably defined structure on the product set. More abstractly, one talks about the product in category theory, which formalizes these notions.

In mathematics, a directed set is a nonempty set together with a reflexive and transitive binary relation , with the additional property that every pair of elements has an upper bound. In other words, for any and in there must exist in with and A directed set's preorder is called a direction. In mathematics, an ordered pair is a pair of objects. The order in which the objects appear in the pair is significant: the ordered pair is different from the ordered pair unless a = b. In mathematics, especially order theory, a partially ordered set formalizes and generalizes the intuitive concept of an ordering, sequencing, or arrangement of the elements of a set. A poset consists of a set together with a binary relation indicating that, for certain pairs of elements in the set, one of the elements precedes the other in the ordering. The relation itself is called a "partial order."

In mathematics, a product is the result of multiplication, or an expression that identifies objects to be multiplied, called factors. For example, 30 is the product of 6 and 5, and is the product of and .

In mathematics, a base for the topology τ of a topological space (X, τ) is a family of open subsets of X such that every open set of the topology is equal to the union of some sub-family of . For example, the set of all open intervals in the real number line is a basis for the Euclidean topology on because every open interval is an open set, and also every open subset of can be written as a union of some family of open intervals.

A mathematical symbol is a figure or a combination of figures that is used to represent a mathematical object, an action on mathematical objects, a relation between mathematical objects, or for structuring the other symbols that occur in a formula. As formulas are entirely constituted with symbols of various types, many symbols are needed for expressing all mathematics.

In the mathematical discipline of set theory, forcing is a technique for proving consistency and independence results. It was first used by Paul Cohen in 1963, to prove the independence of the axiom of choice and the continuum hypothesis from Zermelo–Fraenkel set theory.

In database theory, relational algebra is a theory that uses algebraic structures with a well-founded semantics for modeling data, and defining queries on it. The theory was introduced by Edgar F. Codd. In mathematics, a function from a set X to a set Y assigns to each element of X exactly one element of Y. The set X is called the domain of the function and the set Y is called the codomain of the function. In mathematics, the symmetric difference of two sets, also known as the disjunctive union, is the set of elements which are in either of the sets, but not in their intersection. For example, the symmetric difference of the sets and is .

In the foundations of mathematics, von Neumann–Bernays–Gödel set theory (NBG) is an axiomatic set theory that is a conservative extension of Zermelo–Fraenkel–choice set theory (ZFC). NBG introduces the notion of class, which is a collection of sets defined by a formula whose quantifiers range only over sets. NBG can define classes that are larger than sets, such as the class of all sets and the class of all ordinals. Morse–Kelley set theory (MK) allows classes to be defined by formulas whose quantifiers range over classes. NBG is finitely axiomatizable, while ZFC and MK are not. In mathematics, the restriction of a function is a new function, denoted or obtained by choosing a smaller domain for the original function The function is then said to extend

A Dynkin system, named after Eugene Dynkin is a collection of subsets of another universal set satisfying a set of axioms weaker than those of 𝜎-algebra. Dynkin systems are sometimes referred to as 𝜆-systems or d-system. These set families have applications in measure theory and probability.

In mathematics, a π-system on a set is a collection of certain subsets of such that

In mathematics, a filter on a set is a family of subsets such that:

1. and
2. if and ,then
3. If ,and ,then

In mathematical analysis and its applications, a function of several real variables or real multivariate function is a function with more than one argument, with all arguments being real variables. This concept extends the idea of a function of a real variable to several variables. The "input" variables take real values, while the "output", also called the "value of the function", may be real or complex. However, the study of the complex-valued functions may be easily reduced to the study of the real-valued functions, by considering the real and imaginary parts of the complex function; therefore, unless explicitly specified, only real-valued functions will be considered in this article. In the mathematical field of set theory, an ultrafilter is a maximal proper filter: it is a filter on a given non-empty set which is a certain type of non-empty family of subsets of that is not equal to the power set of and that is also "maximal" in that there does not exist any other proper filter on that contains it as a proper subset. Said differently, a proper filter is called an ultrafilter if there exists exactly one proper filter that contains it as a subset, that proper filter (necessarily) being itself.

1. Weisstein, Eric W. "Cartesian Product". mathworld.wolfram.com. Retrieved September 5, 2020.
2. Warner, S. (1990). Modern Algebra. Dover Publications. p. 6.
3. Nykamp, Duane. "Cartesian product definition". Math Insight. Retrieved September 5, 2020.
4. "Cartesian Product". web.mnstate.edu. Archived from the original on July 18, 2020. Retrieved September 5, 2020.
5. "Cartesian". Merriam-Webster.com. 2009. Retrieved December 1, 2009.
6. Singh, S. (August 27, 2009). Cartesian product. Retrieved from the Connexions Web site: http://cnx.org/content/m15207/1.5/
7. Cartesian Product of Subsets. (February 15, 2011). ProofWiki. Retrieved 05:06, August 1, 2011 from https://proofwiki.org/w/index.php?title=Cartesian_Product_of_Subsets&oldid=45868
8. Peter S. (1998). A Crash Course in the Mathematics of Infinite Sets. St. John's Review, 44(2), 35–59. Retrieved August 1, 2011, from http://www.mathpath.org/concepts/infinity.htm
9. Osborne, M., and Rubinstein, A., 1994. A Course in Game Theory. MIT Press.