Ordered pair
In mathematics, an ordered pair (a, b) is a pair of mathematical objects. The order in which the objects appear in the pair is significant: the ordered pair (a, b) is different from the ordered pair (b, a) unless a = b. (In contrast, the unordered pair {a, b} equals the unordered pair {b, a}.)
Ordered pairs are also called 2tuples, or sequences of length 2; ordered pairs of scalars are also called 2dimensional vectors. The entries of an ordered pair can be other ordered pairs, enabling the recursive definition of ordered ntuples (ordered lists of n objects). For example, the ordered triple (a,b,c) can be defined as (a, (b,c)), i.e., as one pair nested in another.
In the ordered pair (a, b), the object a is called the first entry, and the object b the second entry of the pair. Alternatively, the objects are called the first and second coordinates, or the left and right projections of the ordered pair.
Cartesian products and binary relations (and hence functions) are defined in terms of ordered pairs.
Generalities
Let and be ordered pairs. Then the characteristic (or defining) property of the ordered pair is:
The set of all ordered pairs whose first entry is in some set A and whose second entry is in some set B is called the Cartesian product of A and B, and written A × B. A binary relation between sets A and B is a subset of A × B.
If one wishes to employ the notation for a different purpose (such as denoting open intervals on the real number line) the ordered pair may be denoted by the variant notation
The left and right projection of a pair p is usually denoted by π_{1}(p) and π_{2}(p), or by π_{l}(p) and π_{r}(p), respectively. In contexts where arbitrary ntuples are considered, π^{n}_{i}(t) is a common notation for the ith component of an n tuple t.
Defining the ordered pair using set theory
The above characteristic property of ordered pairs is all that is required to understand the role of ordered pairs in mathematics. Hence the ordered pair can be taken as a primitive notion, whose associated axiom is the characteristic property. This was the approach taken by the N. Bourbaki group in its Theory of Sets, published in 1954, long after Kuratowski discovered his reduction (below). The Kuratowski definition was added in the second edition of Theory of Sets, published in 1970.
If one agrees that set theory is an appealing foundation of mathematics, then all mathematical objects must be defined as sets of some sort. Hence if the ordered pair is not taken as primitive, it must be defined as a set.^{} Several settheoretic definitions of the ordered pair are given below.
Wiener's definition
Norbert Wiener proposed the first set theoretical definition of the ordered pair in 1914:^{}
He observed that this definition made it possible to define the types of Principia Mathematica as sets. Principia Mathematica had taken types, and hence relations of all arities, as primitive.
Wiener used {{b}} instead of {b} to make the definition compatible with type theory where all elements in a class must be of the same "type". With nesting b within an additional set its type is made equal to 's.
Hausdorff's definition
About the same time as Wiener (1914), Felix Hausdorff proposed his definition:
"where 1 and 2 are two distinct objects different from a and b."^{}
Kuratowski definition
In 1921 Kazimierz Kuratowski offered the nowaccepted definition^{}^{} of the ordered pair (a, b):
Note that this definition is used even when the first and the second coordinates are identical:
Given some ordered pair p, the property "x is the first coordinate of p" can be formulated as:
The property "x is the second coordinate of p" can be formulated as:
In the case that the left and right coordinates are identical, the right conjunct is trivially true, since Y_{1} ≠ Y_{2} is never the case.
This is how we can extract the first coordinate of a pair (using the notation for arbitrary intersection and arbitrary union):
This is how the second coordinate can be extracted:
Variants
The above Kuratowski definition of the ordered pair is "adequate" in that it satisfies the characteristic property that an ordered pair must satisfy, namely that . In particular, it adequately expresses 'order', in that is false unless . There are other definitions, of similar or lesser complexity, that are equally adequate:
 ^{}
The reverse definition is merely a trivial variant of the Kuratowski definition, and as such is of no independent interest. The definition short is socalled because it requires two rather than three pairs of braces. Proving that short satisfies the characteristic property requires the Zermelo–Fraenkel set theory axiom of regularity.^{} Moreover, if one uses von Neumann's settheoretic construction of the natural numbers, then 2 is defined as the set {0, 1} = {0, {0}}, which is indistinguishable from the pair (0, 0)_{short}. Yet another disadvantage of the short pair is the fact, that even if a and b are of the same type, the elements of the short pair are not. (However, if a = b then the short version keeps having cardinality 2, which is something one might expect of any "pair", including any "ordered pair". Also note that the short version is used in Tarski–Grothendieck set theory, upon which the Mizar system is founded.)
Proving that definitions satisfy the characteristic property
Prove: (a, b) = (c, d) if and only if a = c and b = d.
Kuratowski:
If. If a = c and b = d, then {{a}, {a, b}} = {{c}, {c, d}}. Thus (a, b)_{K} = (c, d)_{K}.
Only if. Two cases: a = b, and a ≠ b.
If a = b:
 (a, b)_{K} = {{a}, {a, b}} = {{a}, {a, a}} = {{a}}.
 (c, d)_{K} = {{c}, {c, d}} = {{a}}.
 Thus {c} = {c, d} = {a}, which implies a = c and a = d. By hypothesis, a = b. Hence b = d.
If a ≠ b, then (a, b)_{K} = (c, d)_{K} implies {{a}, {a, b}} = {{c}, {c, d}}.
 Suppose {c, d} = {a}. Then c = d = a, and so {{c}, {c, d}} = {{a}, {a, a}} = {{a}, {a}} = {{a}}. But then {{a}, {a, b}} would also equal {{a}}, so that b = a which contradicts a ≠ b.
 Suppose {c} = {a, b}. Then a = b = c, which also contradicts a ≠ b.
 Therefore {c} = {a}, so that c = a and {c, d} = {a, b}.
 If d = a were true, then {c, d} = {a, a} = {a} ≠ {a, b}, a contradiction. Thus d = b is the case, so that a = c and b = d.
Reverse:
(a, b)_{reverse} = {{b}, {a, b}} = {{b}, {b, a}} = (b, a)_{K}.
If. If (a, b)_{reverse} = (c, d)_{reverse}, (b, a)_{K} = (d, c)_{K}. Therefore b = d and a = c.
Only if. If a = c and b = d, then {{b}, {a, b}} = {{d}, {c, d}}. Thus (a, b)_{reverse} = (c, d)_{reverse}.
Short:^{}
If: If a = c and b = d, then {a, {a, b}} = {c, {c, d}}. Thus (a, b)_{short} = (c, d)_{short}.
Only if: Suppose {a, {a, b}} = {c, {c, d}}. Then a is in the left hand side, and thus in the right hand side. Because equal sets have equal elements, one of a = c or a = {c, d} must be the case.
 If a = {c, d}, then by similar reasoning as above, {a, b} is in the right hand side, so {a, b} = c or {a, b} = {c, d}.
 If {a, b} = c then c is in {c, d} = a and a is in c, and this combination contradicts the axiom of regularity, as {a, c} has no minimal element under the relation "element of."
 If {a, b} = {c, d}, then a is an element of a, from a = {c, d} = {a, b}, again contradicting regularity.
 Hence a = c must hold.
Again, we see that {a, b} = c or {a, b} = {c, d}.
 The option {a, b} = c and a = c implies that c is an element of c, contradicting regularity.
 So we have a = c and {a, b} = {c, d}, and so: {b} = {a, b} \ {a} = {c, d} \ {c} = {d}, so b = d.
Quine–Rosser definition
Rosser (1953)^{} employed a definition of the ordered pair due to Quine which requires a prior definition of the natural numbers. Let be the set of natural numbers and be the set of the elements of not in . Define
Applying this function simply increments every natural number in x. In particular, does not contain the number 0, so that for any sets x and y,
Define the ordered pair (A, B) as
Extracting all the elements of the pair that do not contain 0 and undoing yields A. Likewise, B can be recovered from the elements of the pair that do contain 0.
In type theory and in outgrowths thereof such as the axiomatic set theory NF, the Quine–Rosser pair has the same type as its projections and hence is termed a "typelevel" ordered pair. Hence this definition has the advantage of enabling a function, defined as a set of ordered pairs, to have a type only 1 higher than the type of its arguments. This definition works only if the set of natural numbers is infinite. This is the case in NF, but not in type theory or in NFU. J. Barkley Rosser showed that the existence of such a typelevel ordered pair (or even a "typeraising by 1" ordered pair) implies the axiom of infinity. For an extensive discussion of the ordered pair in the context of Quinian set theories, see Holmes (1998).^{}
Morse definition
Morse–Kelley set theory makes free use of proper classes.^{}Morse defined the ordered pair so that its projections could be proper classes as well as sets. (The Kuratowski definition does not allow this.) He first defined ordered pairs whose projections are sets in Kuratowski's manner. He then redefined the pair
where the component Cartesian products are Kuratowski pairs of sets and where
This renders possible pairs whose projections are proper classes. The QuineRosser definition above also admits proper classes as projections. Similarly the triple is defined as a 3tuple as follows:
The use of the singleton set which has an inserted empty set allows tuples to have the uniqueness property that if a is an ntuple and b is an mtuple and a = b then n = m. Ordered triples which are defined as ordered pairs do not have this property with respect to ordered pairs.
Category theory
A categorytheoretic product A × B in a category of sets represents the set of ordered pairs, with the first element coming from A and the second coming from B. In this context the characteristic property above is a consequence of the universal property of the product and the fact that elements of a set X can be identified with morphisms from 1 (a one element set) to X. While different objects may have the universal property, they are all naturally isomorphic.
References
