Editing Construction of the real numbers

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In [[mathematics]], there are several equivalent ways of defining the [[real number]]s. One of them is that they form a [[complete ordered field]] that does not contain any smaller complete ordered field. Such a definition does not prove that such a complete ordered field exists, and the existence proof consists of constructing a [[mathematical structure]] that satisfies the definition.

The article presents several such constructions.{{sfn|Weiss|2015}} They are equivalent in the sense that, given the result of any two such constructions, there is a unique [[isomorphism]] of [[ordered field]] between them. This results from the above definition and is independent of particular constructions. These isomorphisms allow identifying the results of the constructions, and, in practice, to forget which construction has been chosen.

==Axiomatic definitions==
An [[axiomatic method|axiomatic definition]] of the real numbers consists of defining them as the elements of  a complete ordered field.<ref>{{Cite web |title=Real Numbers |url=http://math.colorado.edu/~nita/RealNumbers.pdf |website=[[University of Colorado Boulder]]}}</ref><ref>{{Cite web |last=Saunders |first=Bonnie |date=August 21, 2015 |title=Interactive Notes for Real Analysis |url=http://homepages.math.uic.edu/~saunders/MATH313/INRA/INRA_chapters0and1.pdf |website=[[University of Illinois at Chicago]]}}</ref><ref>{{Cite web |title=Axioms of the Real Number System |url=https://www.math.uci.edu/~mfinkels/140A/Introduction%2520and%2520Logic%2520Notes.pdf |archive-url=https://web.archive.org/web/20101226005339/http://math.uci.edu/~mfinkels/140A/Introduction%20and%20Logic%20Notes.pdf |archive-date=December 26, 2010 |website=[[University of California, Irvine]]}}</ref> This means the following: The real numbers form a [[set (mathematics)|set]], commonly denoted <math>\mathbb{R}</math>, containing two distinguished  elements denoted 0 and 1, and on which are defined  two [[binary operation]]s and one [[binary relation]]; the operations are called ''addition'' and ''multiplication'' of real numbers and denoted respectively with {{math|+}} and {{math|×}}; the binary relation is ''inequality'', denoted <math>\le.</math> Moreover, the following properties called [[axiom]]s must be satisfied.

The existence of such a [[mathematical structure|structure]] is a [[theorem]], which is proved by constructing such a structure. A consequence of the axioms is that this structure is unique [[up to]] an isomorphism, and thus, the real numbers can be used and manipulated, without referring to the method of construction.

=== Axioms ===
# <math>\mathbb{R}</math> is a [[field (mathematics)|field]] under addition and multiplication. In other words,
#* For all ''x'', ''y'', and ''z'' in <math>\mathbb{R}</math>, ''x'' + (''y'' + ''z'') = (''x'' + ''y'') + ''z'' and ''x'' × (''y'' × ''z'') = (''x'' × ''y'') × ''z''. ([[associativity]] of addition and multiplication)
#* For all ''x'' and ''y'' in <math>\mathbb{R}</math>, ''x'' + ''y'' = ''y'' + ''x'' and ''x'' × ''y'' = ''y'' × ''x''. ([[commutative operation|commutativity]] of addition and multiplication)
#* For all ''x'', ''y'', and ''z'' in <math>\mathbb{R}</math>, ''x'' × (''y'' + ''z'') = (''x'' × ''y'') + (''x'' × ''z''). ([[distributivity]] of multiplication over addition)
#* For all ''x'' in <math>\mathbb{R}</math>, ''x'' + 0 = ''x''. (existence of additive [[identity element|identity]])
#* 0 is not equal to 1, and for all ''x'' in <math>\mathbb{R}</math>, ''x'' × 1 = ''x''. (existence of multiplicative identity)
#* For every ''x'' in <math>\mathbb{R}</math>, there exists an element &minus;''x'' in <math>\mathbb{R}</math>, such that ''x'' + (&minus;''x'') = 0. (existence of additive [[inverse element|inverses]])
#* For every ''x'' ≠ 0 in <math>\mathbb{R}</math>, there exists an element ''x''<sup>&minus;1</sup> in <math>\mathbb{R}</math>, such that ''x'' × ''x''<sup>&minus;1</sup> = 1. (existence of multiplicative inverses)
# <math>\mathbb{R}</math> is [[totally ordered set|totally ordered]] for <math>\leq</math>. In other words,
#* For all ''x'' in <math>\mathbb{R}</math>, ''x'' ≤ ''x''. ([[reflexive relation|reflexivity]])
#* For all ''x'' and ''y'' in <math>\mathbb{R}</math>, if ''x'' ≤ ''y'' and ''y'' ≤ ''x'', then ''x'' = ''y''. ([[antisymmetric relation|antisymmetry]])
#* For all ''x'', ''y'', and ''z'' in <math>\mathbb{R}</math>, if ''x'' ≤ ''y'' and ''y'' ≤ ''z'', then ''x'' ≤ ''z''. ([[transitive relation|transitivity]])
#* For all ''x'' and ''y'' in <math>\mathbb{R}</math>, ''x'' ≤ ''y'' or ''y'' ≤ ''x''. ([[total order|totality]])
# Addition and multiplication are compatible with the order. In other words,
#* For all ''x'', ''y'' and ''z'' in <math>\mathbb{R}</math>, if ''x'' ≤ ''y'', then ''x'' + ''z'' ≤ ''y'' + ''z''. (preservation of order under addition)
#* For all ''x'' and ''y'' in <math>\mathbb{R}</math>, if 0 ≤ ''x'' and 0 ≤ ''y'', then 0 ≤ ''x'' × ''y'' (preservation of order under multiplication)
# The order ≤ is ''complete'' in the following sense: every non-empty subset of <math>\mathbb{R}</math> that is [[upper bound|bounded above]] has a [[least upper bound]]. In other words,
#* If ''A'' is a non-empty subset of <math>\mathbb{R}</math>, and if ''A'' has an [[upper bound]] in <math>\R,</math> then ''A'' has a least upper bound ''u'', such that for every upper bound ''v'' of ''A'', ''u'' ≤ ''v''.

==== On the least upper bound property ====
Axiom 4, which requires the order to be [[Dedekind-complete]], implies the [[Archimedean property]].

The axiom is crucial in the characterization of the reals. For example, the totally [[ordered field]] of the rational numbers '''Q''' satisfies the first three axioms, but not the fourth. In other words, models of the rational numbers are also models of the first three axioms.

Note that the axiom is [[Nonfirstorderizability|nonfirstorderizable]], as it expresses a statement about collections of reals and not just individual such numbers. As such, the reals are not given by a [[List of first-order theories|first-order logic theory]].

==== On models ====

A ''model of real numbers'' is a [[mathematical structure]] that satisfies the above axioms.
Several models are given [[#Explicit constructions of models|below]]. Any two models are isomorphic; so, the real numbers are unique [[up to]] isomorphisms.

Saying that any two models are isomorphic means that for any two models <math>(\mathbb{R}, 0_\R, 1_\R, +_\R, \times_\R, \le_\R)</math> and <math>(S, 0_S, 1_S, +_S, \times_S, \le_S),</math> there is a [[bijection]] <math>f\colon\mathbb{R}\to S</math> that preserves both the field operations and the order. Explicitly,
*{{math|''f''}} is both [[injective]] and [[surjective]].
*{{math|1=''f''(0<sub>ℝ</sub>) = 0<sub>''S''</sub>}} and {{math|1=''f''(1<sub>ℝ</sub>) = 1<sub>''S''</sub>}}.
*{{math|1=''f''(''x'' +<sub>ℝ</sub> ''y'') = ''f''(''x'') +<sub>''S''</sub> ''f''(''y'')}} and {{math|1=''f''(''x'' ×<sub>ℝ</sub> ''y'') = ''f''(''x'') ×<sub>''S''</sub> ''f''(''y'')}}, for all {{math|''x''}} and {{math|''y''}} in <math>\mathbb{R}.</math>
* {{math|''x'' ≤<sub>ℝ</sub> ''y''}} [[if and only if]] {{math|''f''(''x'') ≤<sub>''S''</sub> ''f''(''y'')}},  for all {{math|''x''}} and {{math|''y''}} in <math>\mathbb{R}.</math>

===Tarski's axiomatization of the reals===
{{Main|Tarski's axiomatization of the reals}}
An alternative synthetic [[axiomatization]] of the real numbers and their arithmetic was given by [[Alfred Tarski]], consisting of only the 8 [[axiom]]s shown below and a mere four [[primitive notion]]s: a [[Set (mathematics)|set]] called ''the real numbers'', denoted <math>\mathbb{R}</math>, a [[binary relation]] over <math>\mathbb{R}</math> called ''order'', denoted by the [[infix operator]] <, a [[binary operation]] over <math>\mathbb{R}</math> called ''addition'', denoted by the infix operator +, and the constant 1.

''Axioms of order'' (primitives: <math>\mathbb{R}</math>, <):

'''Axiom 1'''. If ''x'' < ''y'', then not ''y'' < ''x''. That is, "<" is an [[asymmetric relation]].

'''Axiom 2'''. If ''x''&nbsp;<&nbsp;''z'', there exists a ''y'' such that ''x''&nbsp;<&nbsp;''y'' and ''y''&nbsp;<&nbsp;''z''. In other words, "<" is [[dense order|dense]] in <math>\mathbb{R}</math>.

'''Axiom 3'''. "<" is [[Dedekind-complete]]. More formally, for all ''X'',&nbsp;''Y''&nbsp;⊆&nbsp;<math>\mathbb{R}</math>, if for all ''x''&nbsp;∈&nbsp;''X'' and ''y''&nbsp;∈&nbsp;''Y'', ''x''&nbsp;<&nbsp;''y'', then there exists a ''z'' such that for all ''x''&nbsp;∈&nbsp;''X'' and ''y''&nbsp;∈&nbsp;''Y'', if ''z''&nbsp;≠&nbsp;''x'' and ''z''&nbsp;≠&nbsp;''y'', then ''x''&nbsp;&lt;&nbsp;''z'' and ''z''&nbsp;&lt;&nbsp;''y''.

To clarify the above statement somewhat, let ''X''&nbsp;⊆&nbsp;<math>\mathbb{R}</math> and ''Y''&nbsp;⊆&nbsp;<math>\mathbb{R}</math>. We now define two common English verbs in a particular way that suits our purpose:

:''X precedes Y'' if and only if for every ''x''&nbsp;&isin;&nbsp;''X'' and every ''y''&nbsp;&isin;&nbsp;''Y'', ''x''&nbsp;<&nbsp;''y''.

:The real number ''z separates'' ''X'' and ''Y'' if and only if for every ''x''&nbsp;&isin;&nbsp;''X'' with ''x''&nbsp;&ne;&nbsp;''z'' and every ''y''&nbsp;&isin;&nbsp;''Y'' with ''y''&nbsp;&ne;&nbsp;''z'', ''x''&nbsp;<&nbsp;''z'' and ''z''&nbsp;<&nbsp;''y''.

Axiom 3 can then be stated as:

:"If a set of reals precedes another set of reals, then there exists at least one real number separating the two sets."

''Axioms of addition'' (primitives: <math>\mathbb{R}</math>, <, +):

'''Axiom 4'''. ''x''&nbsp;+&nbsp;(''y''&nbsp;+&nbsp;''z'')&nbsp;=&nbsp;(''x''&nbsp;+&nbsp;''z'')&nbsp;+&nbsp;''y''.

'''Axiom 5'''. For all ''x'', ''y'', there exists a ''z'' such that ''x''&nbsp;+&nbsp;''z''&nbsp;=&nbsp;''y''.

'''Axiom 6'''. If ''x''&nbsp;+&nbsp;''y''&nbsp;<&nbsp;''z''&nbsp;+&nbsp;''w'', then ''x''&nbsp;<&nbsp;''z'' or ''y''&nbsp;<&nbsp;''w''.

''Axioms for one'' (primitives: <math>\mathbb{R}</math>, <, +, 1):

'''Axiom 7'''. 1&nbsp;∈&nbsp;<math>\mathbb{R}</math>.

'''Axiom 8'''. 1&nbsp;<&nbsp;1&nbsp;+&nbsp;1.

These axioms imply that <math>\mathbb{R}</math> is a [[linearly ordered group|linearly ordered]] [[abelian group]] under addition with distinguished element 1. <math>\mathbb{R}</math> is also [[Dedekind-complete]] and [[divisible group|divisible]].

==Explicit constructions of models==
We shall not prove that any models of the axioms are isomorphic. Such a proof can be found in any number of modern analysis or set theory textbooks. We will sketch the basic definitions and properties of a number of constructions, however, because each of these is important for both mathematical and historical reasons. The first three, due to [[Georg Cantor]]/[[Charles Méray]], [[Richard Dedekind]]/[[Joseph Bertrand]] and [[Karl Weierstrass]] all occurred within a few years of each other. Each has advantages and disadvantages.

===Construction from Cauchy sequences===
A standard procedure to force all [[Cauchy sequence]]s in a [[metric space]] to converge is adding new points to the metric space in a process called [[completeness (topology)|completion]].

<math>\mathbb{R}</math> is defined as the completion of the set <math>\Q</math> of the rational numbers with respect to the metric {{math|{{abs|''x'' − ''y''}}}} Normally, metrics are defined with real numbers as values, but this does not make the construction/definition circular, since all numbers that are implied (even implicitly) are rational numbers.<ref>For completions of <math>\Q</math> with respect to other metrics, see [[p-adic numbers|''p''-adic numbers]]).</ref>

Let ''R'' be the [[Set (mathematics)|set]] of Cauchy sequences of rational numbers. That is, sequences
: {{math|(''x''<sub>''1''</sub>, ''x''<sub>''2''</sub>, ''x''<sub>''3''</sub>,...)}} 
of rational numbers such that for every rational {{math|''ε'' > 0}}, there exists an integer {{math|''N''}} such that for all natural numbers {{math|''m'', ''n'' > ''N''}}, one has {{math| {{!}}''x''<sub>''m''</sub> − ''x''<sub>''n''</sub>{{!}} < ''ε''}}. Here the vertical bars denote the absolute value.

Cauchy sequences {{math|(''x''<sub>''n''</sub>)}} and {{math|(''y''<sub>''n''</sub>)}} can be added and multiplied as follows:
: {{math|1=(''x''<sub>''n''</sub>) + (''y''<sub>''n''</sub>) = (''x''<sub>''n''</sub> + ''y''<sub>''n''</sub>)}}
: {{math|1=(''x''<sub>''n''</sub>) &times; (''y''<sub>''n''</sub>) = (''x''<sub>''n''</sub> &times; ''y''<sub>''n''</sub>)}}.

Two Cauchy sequences {{math|(''x''<sub>''n''</sub>)}} and {{math|(''y''<sub>''n''</sub>)}} are called ''equivalent'' if and only if the difference between them tends to zero; that is, for every rational number {{math|''ε'' > 0}}, there exists an integer {{math|''N''}} such that for all natural numbers {{math|''n'' > ''N''}}, one has {{math| {{!}}''x''<sub>''n''</sub> − ''y''<sub>''n''</sub>{{!}} < ''ε''}}.

This defines an [[equivalence relation]] that is compatible with the operations defined above, and the set '''R''' of all [[equivalence class]]es can be shown to satisfy [[#Axiomatic definitions|all axioms of the real numbers]]. <math>\Q</math> can be considered as a subset of <math>\R</math> by identifying a rational number {{math|''r''}} with the equivalence class of the Cauchy sequence {{math| (''r'', ''r'', ''r'', ...)}}.

Comparison between real numbers is obtained by defining the following comparison between Cauchy sequences: {{math|(''x''<sub>''n''</sub>) ≥  (''y''<sub>''n''</sub>)}} if and only if   
{{math|''x''}} is equivalent to {{math|''y''}} or there exists an integer {{math|''N''}} such that {{math|''x''<sub>''n''</sub> ≥  ''y''<sub>''n''</sub>}} for all    
{{math|''n'' > ''N''}}.

By construction, every real number {{math|''x''}} is represented by a Cauchy sequence of rational numbers. This representation is far from unique; every rational sequence that converges to {{math|''x''}} is a Cauchy sequence representing {{math|''x''}}. This reflects the observation that one can often use different sequences to approximate the same real number.{{sfn|Kemp|2016}}

The only real number axiom that does not follow easily from the definitions is the completeness of {{math|≤}}, i.e. the [[least upper bound property]]. It can be proved as follows: Let {{math|''S''}} be a non-empty subset of <math>\R'</math> and {{math|''U''}} be an upper bound for {{math|''S''}}. Substituting a larger value if necessary, we may assume {{math|''U''}} is rational. Since {{math|''S''}} is non-empty, we can choose a rational number {{math|''L''}} such that {{math|''L'' < ''s''}} for some {{math|''s''}} in {{math|''S''}}. Now define sequences of rationals {{math|(''u''<sub>''n''</sub>)}} and {{math|(''l''<sub>''n''</sub>)}} as follows:

Set {{math|1=''u''<sub>0</sub> = ''U''}} and {{math|1=''l''<sub>0</sub> = ''L''}}. For each {{math|''n''}} consider the number
{{math|1=''m''<sub>''n''</sub> = (''u''<sub>''n''</sub> + ''l''<sub>''n''</sub>)/2}}.
If {{math|''m''<sub>''n''</sub>}} is an upper bound for {{math|''S''}}, set {{math|1=''u''<sub>''n''+1</sub> = ''m''<sub>''n''</sub>}} and {{math|1=''l''<sub>''n''+1</sub> = ''l''<sub>''n''</sub>}}.
Otherwise set
{{math|1=''l''<sub>''n''+1</sub> = ''m''<sub>''n''</sub>}} and {{math|1=''u''<sub>''n''+1</sub> = ''u''<sub>''n''</sub>}}.

This defines two Cauchy sequences of rationals, and so the real numbers {{math|1= ''l'' = (''l''<sub>''n''</sub>)}} and {{math|1= ''u'' = (''u''<sub>''n''</sub>)}}. It is easy to prove, by induction on {{math|''n''}} that {{math|''u''<sub>''n''</sub>}} is an upper bound for {{math|''S''}} for all {{math|''n''}}
and {{math|''l''<sub>''n''</sub>}} is never an upper bound for {{math|''S''}} for any {{math|''n''}}

Thus {{math|''u''}} is an upper bound for {{math|''S''}}. To see that it is a least upper bound, notice that the limit of {{math|(''u''<sub>''n''</sub> − ''l''<sub>''n''</sub>)}} is {{math|0}}, and so {{math|1=''l'' = ''u''}}. Now suppose {{math|1= ''b'' < ''u'' = ''l''}} is a smaller upper bound for {{math|''S''}}. Since {{math|(''l''<sub>''n''</sub>)}} is monotonic increasing it is easy to see that {{math| ''b'' < ''l''<sub>''n''</sub>}} for some {{math|''n''}}. But {{math|''l''<sub>''n''</sub>}} is not an upper bound for {{mvar|S}} and so neither is {{math|''b''}}. Hence {{math|''u''}} is a least upper bound for {{math|''S''}} and {{math|≤}} is complete.

The usual [[decimal notation]] can be translated to Cauchy sequences in a natural way. For example, the notation {{math|1=''π'' = 3.1415...}} means that {{pi}} is the equivalence class of the Cauchy sequence {{math|(3,  3.1,  3.14,  3.141,  3.1415, ...)}}. The equation {{math|1=[[0.999...]] = 1}} states that the sequences {{math|(0,  0.9,  0.99,  0.999,...)}} and {{math|(1, 1, 1, 1,...)}} are equivalent, i.e., their difference converges to {{math|0}}.

An advantage of constructing <math>\R</math> as the completion of <math>\Q</math> is that this construction can be used for every other metric space.

===Construction by Dedekind cuts===
[[File:Dedekind cut at square root of two.svg| thumb| right| 350px| Dedekind used his cut to construct the [[irrational number|irrational]], [[real number]]s.]]

A [[Dedekind cut]] in an ordered field is a [[Partition of a set|partition]] of it, (''A'', ''B''), such that ''A'' is nonempty and closed downwards, ''B'' is nonempty and closed upwards, and ''A'' contains no [[greatest element]]. Real numbers can be constructed as Dedekind cuts of rational numbers.<ref>[https://www.math.ucdavis.edu/~temple/MAT25/HomeworkProblems.pdf Math 25 Exercises] ucdavis.edu</ref><ref>[http://math.furman.edu/~tlewis/math41/Pugh/chap1/sec2.pdf 1.2–Cuts] furman.edu</ref>

For convenience we may take the lower set <math>A\,</math> as the representative of any given Dedekind cut <math>(A, B)\,</math>, since <math>A</math> completely determines <math>B</math>.  By doing this we may think intuitively of a real number as being represented by the set of all smaller rational numbers.  In more detail, a real number <math>r</math> is any subset of the set <math>\textbf{Q}</math> of rational numbers that fulfills the following conditions:{{sfn|Pugh|2002}}
# <math>r</math> is not empty
# <math>r \neq \textbf{Q}</math>
# <math>r</math> is closed downwards.  In other words,  for all <math>x, y \in \textbf{Q}</math> such that <math>x < y</math>, if <math>y \in r</math> then <math>x \in r</math>
# <math>r</math> contains no greatest element.  In other words, there is no <math>x \in r</math> such that for all <math>y \in r</math>, <math>y \leq x</math>

* We form the set <math> \textbf{R} </math> of real numbers as the set of all Dedekind cuts <math>A</math> of <math> \textbf{Q} </math>, and define a [[total ordering]] on the real numbers as follows: <math>x \leq y\Leftrightarrow x \subseteq y</math>
* We [[embedding|embed]] the rational numbers into the reals by identifying the rational number <math>q</math> with the set of all smaller rational numbers <math> \{ x \in \textbf{Q} : x < q \} </math>.{{sfn|Pugh|2002}} Since the rational numbers are [[dense order|dense]], such a set can have no greatest element and thus fulfills the conditions for being a real number laid out above.
* [[Addition]]. <math>A + B := \{a + b: a \in A \land b \in B\}</math>{{sfn|Pugh|2002}}
* [[Subtraction]]. <math>A - B := \{a - b: a \in A \land b \in ( \textbf{Q} \setminus B ) \}</math> where <math> \textbf{Q} \setminus B </math> denotes the [[complement (set theory)|relative complement]] of <math>B</math> in <math>\textbf{Q}</math>, <math> \{ x : x \in \textbf{Q} \land x \notin B \} </math>
* [[Negation of a number|Negation]] is a special case of subtraction: <math>-B := \{a - b: a < 0 \land b \in ( \textbf{Q} \setminus B ) \}</math>
* Defining [[multiplication]] is less straightforward.{{sfn|Pugh|2002}}
** if <math>A, B \geq 0</math> then <math> A \times B := \{ a \times b : a \geq 0 \land a \in A \land b \geq 0 \land b \in B \} \cup \{ x \in \mathrm{Q} : x < 0 \}</math>
** if either <math>A\,</math> or <math>B\,</math> is negative, we use the identities <math> A \times B = -(A \times -B) = -(-A \times B) = (-A \times -B) \,</math> to convert <math>A\,</math> and/or <math>B\,</math> to positive numbers and then apply the definition above.
* We define [[division (mathematics)|division]] in a similar manner:
** if <math> A \geq 0 \mbox{ and } B > 0 </math> then <math> A / B := \{ a / b : a \in A \land b \in ( \textbf{Q} \setminus B ) \}</math>
** if either <math>A\,</math> or <math>B\,</math> is negative, we use the identities <math> A / B = -(A / {-B}) = -(-A / B)= -A / {-B} \, </math> to convert <math>A\, </math> to a non-negative number  and/or <math>B\, </math> to a positive number and then apply the definition above.
* [[Supremum]].  If a nonempty set <math>S</math> of real numbers has any upper bound in <math>\textbf{R}</math>, then it has a least upper bound in <math>\textbf{R}</math> that is equal to <math>\bigcup S</math>.{{sfn|Pugh|2002}}

As an example of a Dedekind cut representing an [[irrational number]], we may take the [[Square root of 2|positive square root of 2]].  This can be defined by the set <math>A = \{ x \in \textbf{Q} : x < 0 \lor x \times x < 2 \}</math>.{{sfn|Hersh|1997}} It can be seen from the definitions above that <math>A</math> is a real number, and that <math>A \times A = 2\,</math>. However, neither claim is immediate. Showing that <math>A\,</math> is real requires showing that <math>A</math> has no greatest element, i.e. that for any positive rational <math>x\,</math> with <math>x \times x < 2\,</math>, there is a rational <math>y\,</math> with <math>x<y\,</math> and <math>y \times y <2\,.</math> The choice <math>y=\frac{2x+2}{x+2}\,</math> works.  Then  <math>A \times A \le 2</math> but to show equality requires showing that if <math>r\,</math> is any rational number with <math>r < 2\,</math>, then there is positive <math>x\,</math> in <math>A</math> with <math>r<x \times x\,</math>.

An advantage of this construction is that each real number corresponds to a unique cut. Furthermore, by relaxing the first two requirements of the definition of a cut, the [[extended real number]] system may be obtained by associating <math>-\infty</math> with the empty set and <math>\infty</math> with all of <math>\textbf{Q}</math>.

===Construction using hyperreal numbers===
As in the [[hyperreal number]]s, one constructs the {{Not typo|hyperrationals}} <math>^*\mathbb{Q}</math> from the rational numbers by means of an [[ultrafilter]].<ref>{{Cite web |last=Krakoff |first=Gianni |date=June 8, 2015 |title=Hyperreals and a Brief Introduction to Non-Standard Analysis |url=https://sites.math.washington.edu/~morrow/336_15/papers/gianni.pdf |website=Department of Mathematics, [[University of Washington]]}}</ref>  Here a hyperrational is by definition a ratio of two [[hyperinteger]]s.  Consider the [[Ring (mathematics)|ring]] <math>B</math> of all limited (i.e. finite) elements in <math>^*\mathbb{Q}</math>.  Then <math>B</math> has a unique [[maximal ideal]] <math>I</math>, the [[infinitesimal]] hyperrational numbers.  The quotient ring <math>B/I</math> gives the [[field (mathematics)|field]] <math>\mathbb{R}</math> of real numbers.<ref>{{cite book
 | last = Goldblatt | first = Robert
 | contribution = Exercise 5.7 (4) | page = 54
 | doi = 10.1007/978-1-4612-0615-6
 | isbn = 0-387-98464-X
 | location = New York
 | mr = 1643950
 | publisher = Springer-Verlag
 | series = Graduate Texts in Mathematics
 | title = Lectures on the Hyperreals: An introduction to nonstandard analysis
 | volume = 188
 | year = 1998}}</ref> This construction uses a non-principal ultrafilter over the set of natural numbers, the existence of which is guaranteed by the [[axiom of choice]].

It turns out that the maximal ideal respects the order on <math>^*\mathbb{Q}</math>.  Hence the resulting field is an ordered field.  Completeness can be proved in a similar way to the construction from the Cauchy sequences.

===Construction from surreal numbers===
Every ordered field can be embedded in the [[surreal number]]s. The real numbers form a maximal subfield that is [[Archimedean group|Archimedean]] (meaning that no real number is infinitely large or infinitely small).  This embedding is not unique, though it can be chosen in a canonical way.

=== Construction from integers (Eudoxus reals)===<!--Linked from 'Eudoxus of Cnidus'-->
A relatively less known construction allows to define real numbers using only the additive group of integers <math>\mathbb{Z}</math> with different versions.{{sfn|Arthan|2004}}{{sfn|A'Campo|2003}}{{sfn|Street|2003}}  {{harvtxt|Arthan|2004}}, who attributes this construction to unpublished work by [[Stephen Schanuel]], refers to this construction as the ''Eudoxus reals'', naming them after ancient Greek astronomer and mathematician [[Eudoxus of Cnidus]].  As noted by {{harvtxt|Shenitzer|1987}} and {{harvtxt|Arthan|2004}}, Eudoxus's treatment of quantity using the behavior of [[Proportionality (mathematics)|proportions]] became the basis for this construction.  This construction has been [[Automated theorem proving|formally verified]] to give a Dedekind-complete ordered field by the IsarMathLib project.{{sfn|IsarMathLib}} 

Let an '''almost homomorphism''' be a map <math>f:\mathbb{Z}\to\mathbb{Z}</math> such that the set <math>\{f(n+m)-f(m)-f(n): n,m\in\mathbb{Z}\}</math> is finite. (Note that <math>f(n) = \lfloor \alpha n\rfloor</math> is an almost homomorphism for every <math> \alpha \in \mathbb{R} </math>.) Almost homomorphisms form an abelian group under pointwise addition. We say that two almost homomorphisms <math>f,g</math> are '''almost equal''' if the set <math>\{f(n)-g(n): n\in \mathbb{Z}\}</math> is finite. This defines an equivalence relation on the set of almost homomorphisms. Real numbers are defined as the equivalence classes of this relation. Alternatively, the almost homomorphisms taking only finitely many values form a subgroup, and the underlying additive group of the real number is the quotient group. To add real numbers defined this way we add the almost homomorphisms that represent them. Multiplication of real numbers corresponds to functional composition of almost homomorphisms. If <math>[f]</math> denotes the real number represented by an almost homomorphism <math>f</math> we say that <math>0\leq [f]</math> if <math>f</math> is bounded or <math>f</math> takes an infinite number of positive values on <math>\mathbb{Z}^+</math>. This defines the [[total order|linear order]] relation on the set of real numbers constructed this way.

=== Other constructions ===
{{harvtxt|Faltin|Metropolis|Ross|Rota|1975}} write: "Few mathematical structures have undergone as many revisions or have been presented in as many guises as the real numbers. Every generation reexamines the reals in the light of its values and mathematical objectives."{{sfn|Faltin|Metropolis|Ross|Rota|1975}}

A number of other constructions have been given, by:
* {{harvtxt|de Bruijn|1976}}, {{harvtxt|de Bruijn|1977}}
* {{harvtxt|Rieger|1982}}
* {{harvtxt|Knopfmacher|Knopfmacher|1987}}, {{harvtxt|Knopfmacher|Knopfmacher|1988}}

For an overview, see {{harvtxt|Weiss|2015}}.

As a reviewer of one noted: "The details are all included, but as usual they are tedious and not too instructive."<ref>{{MR|693180}} (84j:26002) review of {{harvtxt|Rieger1982}}.</ref>

==See also==

* {{annotated link|Constructivism (mathematics)#Example from real analysis}}
* {{annotated link|Decidability of first-order theories of the real numbers}}

==References==
{{reflist}}

==Bibliography==
{{Refbegin|2}}

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{{Refend}}

{{DEFAULTSORT:Construction Of The Real Numbers}}
[[Category:Real numbers]]
[[Category:Constructivism (mathematics)]]