Cartesian Product

In set theory (and, usually, in other parts of mathematics), a **Cartesian product** is a mathematical operation that returns a set (or **product set** or simply **product**) from multiple sets. That is, for sets *A* and *B*, the Cartesian product is the set of all ordered pairs where and . Products can be specified using set-builder notation, e.g.

^{[1]}

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 .

More generally, a Cartesian product of *n* sets, also known as an ** n-fold Cartesian product**, can be represented by an array of

The Cartesian product is named after René Descartes,^{[2]} whose formulation of analytic geometry gave rise to the concept, which is further generalized in terms of direct product.

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.

returns a set of the form {(A, ?), (A, ?), (A, ?), (A, ?), (K, ?), ..., (3, ?), (2, ?), (2, ?), (2, ?), (2, ?)}.

returns a set of the form {(?, A), (?, K), (?, Q), (?, J), (?, 10), ..., (?, 6), (?, 5), (?, 4), (?, 3), (?, 2)}.

Both sets are distinct, even disjoint.

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; cf. picture. The set of all such pairs (i.e. the Cartesian product with R denoting the real numbers) is thus assigned to the set of all points in the plane.

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, the Kuratowski definition, is . Under this definition, is an element of , and is a subset of that set, where 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.

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

The Cartesian product is not commutative,

because the ordered pairs are reversed unless at least one of the following conditions is satisfied:^{[3]}

*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).

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

The Cartesian product behaves nicely with respect to intersections, cf. left picture.

^{[4]}

In most cases the above statement is not true if we replace intersection with union, cf. middle picture.

In fact, we have that:

For the set difference we also have the following identity:

Here are some rules demonstrating distributivity with other operators (cf. right picture):^{[3]}

^{[4]}

where denotes the absolute complement of *A*.

Other properties related with subsets are:

^{[5]}

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

*A*×*B*= {(a,5), (a,6), (b,5), (b,6)}.

Each element of *A* is paired with each element of *B*. 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*|.

Similarly

- |
*A*×*B*×*C*| = |*A*| · |*B*| · |*C*|

and so on.

The set is infinite if either *A* or *B* is infinite and the other set is not the empty set.^{[6]}

The Cartesian product can be generalized to the ** n-ary Cartesian product** over

of *n*-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 *i* to be the *i*th element of the tuple, then the Cartesian product *X*_{1}×...×*X*_{n} is the set of functions

The **Cartesian square** of a set *X* is the Cartesian product . An example is the 2-dimensional plane where **R** is the set of real numbers: **R**^{2} is the set of all points where *x* and *y* are real numbers (see the Cartesian coordinate system).

The ** n-ary Cartesian power** of a set

An example of this is , with **R** again the set of real numbers, and more generally **R**^{n}.

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*.

It is possible to define the Cartesian product of an arbitrary (possibly infinite) indexed family of sets. If *I* is any index set, and is a family of sets indexed by *I*, then the Cartesian product of the sets in *X* is defined to be

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

For each *j* in *I*, the function

defined by is called the ** jth projection map**.

**Cartesian power** is a Cartesian product where all the factors *X _{i}* are the same set

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 , the natural numbers: this Cartesian product is the set of all infinite sequences with the

can be visualized as a vector with countably infinite real number components. This set is frequently denoted , or .

If several sets are being multiplied together, e.g. *X*_{1}, *X*_{2}, *X*_{3}, ..., then some authors^{[7]} choose to abbreviate the Cartesian product as simply ×*X*_{i}.

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

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

Let be a set and . Then the *cylinder* of with respect to is the Cartesian product of and .

Normally, is considered to be the universe of the context and is left away. For example, if is a subset of the natural numbers , then the cylinder of is .

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.

In graph theory the Cartesian product of two graphs *G* and *H* is the graph denoted by whose vertex set is the (ordinary) Cartesian product and such that two vertices (*u*,*v*) and (*u*?,*v*?) are adjacent in if and only if and *v* is adjacent with *v*? in *H*, *or* 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.

- Binary relation
- Concatenation of sets of strings
- Coproduct
- Empty product
- Euclidean space
- Exponential object
- Finitary relation
- Join (SQL) § Cross join
- Orders on the Cartesian product of totally ordered sets
- Product (category theory)
- Product topology
- Product type
- Ultraproduct

**^**Warner, S. (1990).*Modern Algebra*. Dover Publications. p. 6.**^**"Cartesian".*Merriam-Webster.com*. 2009. Retrieved 2009.- ^
^{a}^{b}Singh, S. (August 27, 2009).*Cartesian product*. Retrieved from the Connexions Web site: http://cnx.org/content/m15207/1.5/ - ^
^{a}^{b}"CartesianProduct".*PlanetMath*. **^**Cartesian Product of Subsets. (February 15, 2011).*ProofWiki*. Retrieved 05:06, August 1, 2011 from https://proofwiki.org/w/index.php?s=Cartesian_Product_of_Subsets&oldid=45868**^**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**^**Osborne, M., and Rubinstein, A., 1994.*A Course in Game Theory*. MIT Press.

- Cartesian Product at ProvenMath
- Hazewinkel, Michiel, ed. (2001) [1994], "Direct product",
*Encyclopedia of Mathematics*, Springer Science+Business Media B.V. / Kluwer Academic Publishers, ISBN 978-1-55608-010-4 - How to find the Cartesian Product, Education Portal Academy

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