Cartesian product explained

In mathematics, specifically set theory, the Cartesian product of two sets and, denoted, is the set of all ordered pairs where is in and is in . In terms of set-builder notation, that is$$A\backslash times\; B\; =\; \backslash .$$^{[1] [2]}

A table can be created by taking the Cartesian product of a set of rows and a set of columns. If the Cartesian product is taken, the cells of the table contain ordered pairs of the form .^[3]

One can similarly define the Cartesian product of sets, also known as an -fold Cartesian product, which can be represented by an -dimensional array, where each element is an -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,^[4] whose formulation of analytic geometry gave rise to the concept, which is further generalized in terms of direct product.

Set-theoretic definition

A rigorous definition of the Cartesian product requires a domain to be specified in the set-builder notation. In this case the domain would have to contain the Cartesian product itself. For defining the Cartesian product of the sets

A and B , with the typical Kuratowski's definition of a pair (a,b) as \{\{a\},\{a,b\}\} , an appropriate domain is the set l{P}( l{P}(A\cup B)) where l{P} denotes the power set. Then the Cartesian product of the sets A and B would be defined as^[5] $$A\backslash times\; B=\backslash .$$

Examples

A deck of cards

An illustrative example is the standard 52-card deck. The standard playing card ranks 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.

returns a set of the form .

returns a set of the form .

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 and coordinates, respectively (see picture). The set of all such pairs (i.e., the Cartesian product

\R x \R , with \R denoting the real numbers) is thus assigned to the set of all points in the plane.^[6]

Most common implementation (set theory)

See main article: article and Implementation of mathematics in 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 l{P}( l{P}(X \cup Y)) , and X x Y is a subset of that set, where l{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,,, and be sets.

The Cartesian product is not commutative,$$A\; \backslash times\; B\; \backslash neq\; B\; \backslash times\; A,$$^[3] because the ordered pairs are reversed unless at least one of the following conditions is satisfied:

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

For example:

;

Strictly speaking, the Cartesian product is not associative (unless one of the involved sets is empty).$$(A\backslash times\; B)\backslash times\; C\; \backslash neq\; A\; \backslash times\; (B\; \backslash times\; C)$$If for example, then .

Intersections, unions, and subsets

See also: List of set identities and relations.

The Cartesian product satisfies the following property with respect to intersections (see middle picture).$$(A\; \backslash cap\; B)\; \backslash times\; (C\; \backslash cap\; D)\; =\; (A\; \backslash times\; C)\; \backslash cap\; (B\; \backslash times\; D)$$

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

In fact, we have that:$$(A\; \backslash times\; C)\; \backslash cup\; (B\; \backslash times\; D)\; =\; [(A\; \backslash setminus\; B)\; \backslash times\; C]\; \backslash cup\; [(A\; \backslash cap\; B)\; \backslash times\; (C\; \backslash cup\; D)]\; \backslash cup\; [(B\; \backslash setminus\; A)\; \backslash times\; D]$$

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

Here are some rules demonstrating distributivity with other operators (see leftmost picture):^[7] $$(A\; \backslash times\; B)^\backslash complement\; =\; \backslash left(A^\backslash complement\; \backslash times\; B^\backslash complement\backslash right)\; \backslash cup\; \backslash left(A^\backslash complement\; \backslash times\; B\backslash right)\; \backslash cup\; \backslash left(A\; \backslash times\; B^\backslash complement\backslash right)\backslash !,$$where

A ^{\complement} denotes the absolute complement of .

Other properties related with subsets are:

$$\backslash text\; A\; \backslash subseteq\; B\; \backslash text\; A\; \backslash times\; C\; \backslash subseteq\; B\; \backslash times\; C;$$$$\backslash text\; A,B\; \backslash neq\; \backslash emptyset\; \backslash text\; A\; \backslash times\; B\; \backslash subseteq\; C\; \backslash times\; D\; \backslash !\backslash iff\backslash !\; A\; \backslash subseteq\; C\; \backslash text\; B\; \backslash subseteq\; D.$$^[8]

Cardinality

The cardinality of a set is the number of elements of the set. For example, defining two sets: and . Both set and set consist of two elements each. Their Cartesian product, written as, results in a new set which has the following elements:

.

where each element of is paired with each element of, 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,

.^[3] In this case,

Similarly,

and so on.

The set is infinite if either or is infinite, and the other set is not the empty set.^[9]

Cartesian products of several sets

n-ary Cartesian product

The Cartesian product can be generalized to the -ary Cartesian product over sets as the set$$X\_1\backslash times\backslash cdots\backslash times\; X\_n\; =\; \backslash$$

of -tuples. If tuples are defined as nested ordered pairs, it can be identified with . If a tuple is defined as a function on that takes its value at to be the -th element of the tuple, then the Cartesian product is the set of functions$$\backslash .$$

n-ary Cartesian power

The Cartesian square of a set is the Cartesian product .An example is the 2-dimensional plane where is the set of real numbers: is the set of all points where and are real numbers (see the Cartesian coordinate system).

The -ary Cartesian power of a set, denoted

X ^{n} , can be defined as$$X^n\; =\; \backslash underbrace\_=\; \backslash .$$

An example of this is, with again the set of real numbers, and more generally .

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

Infinite Cartesian products

See main article: Direct product.

It is possible to define the Cartesian product of an arbitrary (possibly infinite) indexed family of sets. If is any index set, and

\{X _{i\}} _{i\in} is a family of sets indexed by, then the Cartesian product of the sets in \{X _{i\}} _{i\in} is defined to be$$\backslash prod\_\; X\_i\; =\; \backslash left\backslash ,$$that is, the set of all functions defined on the index set such that the value of the function at a particular index 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. \prod _{i\in} X _{i} may also be denoted X {} _{i\in} X _{i} .^[10]

For each in, the function$$\backslash pi\_:\; \backslash prod\_\; X\_i\; \backslash to\; X\_,$$defined by

\pi _{j} (f) = f(j) is called the -th projection map.

Cartesian power is a Cartesian product where all the factors X_i are the same set . In this case,$$\backslash prod\_\; X\_i\; =\; \backslash prod\_\; X$$is the set of all functions from to, 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

N , the natural numbers: this Cartesian product is the set of all infinite sequences with the -th term in its corresponding set X_i. For example, each element of$$\backslash prod\_^\backslash infty\; \backslash mathbb\; R\; =\; \backslash mathbb\; R\; \backslash times\; \backslash mathbb\; R\; \backslash times\; \backslash cdots$$can be visualized as a vector with countably infinite real number components. This set is frequently denoted R ^{\omega} , or R ^{N} .

Other forms

Abbreviated form

If several sets are being multiplied together (e.g.,), then some authors^[11] choose to abbreviate the Cartesian product as simply .

Cartesian product of functions

If is a function from to and is a function from to, then their Cartesian product is a function from to with$$(f\backslash times\; g)(x,\; y)\; =\; (f(x),\; g(y)).$$

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 x 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 N , then the cylinder of B is B x 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 and is the graph denoted by, whose vertex set is the (ordinary) Cartesian product and such that two vertices and are adjacent in, if and only if and is adjacent with ′ in, or and is adjacent with ′ in . 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.

See also

• Axiom of power set (to prove the existence of the Cartesian product)
• Direct product
• Empty product
• Finitary relation
• Join (SQL) § Cross join
• Orders on the Cartesian product of totally ordered sets
• Outer product
• Product (category theory)
• Product topology
• Product type

External links

• Cartesian Product at ProvenMath
  • How to find the Cartesian Product, Education Portal Academy

Notes and References

1. Book: Warner, S. . Modern Algebra . 6 . . 1990.
2. Web site: Nykamp . Duane . Cartesian product definition . September 5, 2020 . Math Insight.
3. Web site: Cartesian Product . 2020-09-05 . web.mnstate.edu . July 18, 2020 . https://web.archive.org/web/20200718035119/http://web.mnstate.edu/peil/MDEV102/U1/S7/Cartesian4.htm . dead.
4. Web site: Cartesian . 2009 . Merriam-Webster.com . December 1, 2009 .
5. Web site: Corry . S. . A Sketch of the Rudiments of Set Theory . May 5, 2023 .
6. Book: Goldberg, Samuel. Probability: An Introduction. Dover Books on Mathematics. Courier Corporation. 1986. 9780486652528. 41.
7. Singh, S. (August 27, 2009). Cartesian product. Retrieved from the Connexions Web site: http://cnx.org/content/m15207/1.5/
8. 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
9. 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
10. F. R. Drake, Set Theory: An Introduction to Large Cardinals, p. 24. Studies in Logic and the Foundations of Mathematics, vol. 76 (1978). ISBN 0-7204-2200-0.
11. Osborne, M., and Rubinstein, A., 1994. A Course in Game Theory. MIT Press.