Construction of real numbers

From Academic Kids

In mathematics, there are a number of ways of defining the real number system as an ordered field. The synthetic approach gives a list of axioms for the real numbers as a complete ordered field. Under the usual axioms of set theory, one can show that these axioms are categorical, in the sense that there is a model for the axioms, and any two such models are isomorphic. Any one of these models must be explicitly constructed, and all of these models are built using the basic properties of the rational number system as an ordered field.

Contents

Synthetic approach

The synthetic approach axiomatically defines the real number system as a complete ordered field. Precisely, this means the following. A model for the real number system consists of a set R, two distinct elements 0 and 1 of R, two binary operations + and * on R (called addition and multiplication, resp.), a total order ≤ on R, satisfying the following properties.

1. (R, +, *) forms a field. In other words,

  • For all x, y, and z in R, x + (y + z) = (x + y) + z and x * (y * z) = (x * y) * z. (associativity of addition and multiplication)
  • For all x and y in R, x + y = y + x and x * y = y * x. (commutativity of addition and multiplication)
  • For all x, y, and z in R, x * (y + z) = (x * y) + (x * z). (distributivity of multiplication over addition)
  • For all x in R, x + 0 = x. (existence of additive identity)
  • 0 is not equal to 1, and for all x in R, x * 1 = x. (existence of multiplicative identity)
  • For every x in R, there exists an element −x in R, such that x + (−x) = 0. (existence of additive inverses)
  • For every x ≠ 0 in R, there exists an element x−1 in R, such that x * x−1 = 1. (existence of multiplicative inverses)

2. (R, ≤) forms a totally ordered set. In other words,

  • For all x in R, xx. (reflexivity)
  • For all x and y in R, if xy and yx, then x = y. (antisymmetry)
  • For all x, y, and z in R, if xy and yz, then xz. (transitivity)
  • For all x and y in R, xy or yx. (totalness)

3. The field operations + and * on R are compatible with the order ≤. In other words,

  • For all x, y and z in R, if xy, then x + zy + z. (preservation of order under addition)
  • For all x and y in R, if 0 ≤ x and 0 ≤ y, then 0 ≤ x * y (preservation of order under multiplication)

4. The order ≤ is complete in the following sense: every non-empty subset of R bounded above has a least upper bound. In other words,

  • If A is a non-empty subset of R, and if A has an upper bound, then A has an upper bound u, such that for every upper bound v of A, uv.

When we say that any two models of the above axioms are isomorphic, we mean that for any two models (R, 0R, 1R, +R, *R, ≤R) and (S, 0S, 1S, +S, *S, ≤S), there is a bijection f : RS preserving both the field operations and the order. Explicitly,

  • f is both 1-1 and onto.
  • f(0R) = 0S and f(1R) = 1S.
  • For all x and y in R, f(x +R y) = f(x) +S f(y) and f(x *R y) = f(x) *S f(y).
  • For all x and y in R, xR y if and only if f(x) ≤S f(y).

The final axiom above is most crucial. Without this axiom, we simply have the axioms which define an ordered field, and there are many non-isomorphic models which satisfy these axioms. However, when the completeness axiom is added, it can be shown that any two models must be isomorphic, and so in this sense, there is only one complete ordered field.

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.

Construction from Cauchy sequences

If we have a space where Cauchy sequences are meaningful (such as a metric space, i.e., a space where distance is defined, or more generally a uniform space), a standard procedure to force all Cauchy sequences to converge is adding new points to the space (a process called completion). By starting with rational numbers and the metric d(x,y) = |x - y|, we can construct the real numbers, as will be detailed below. (If we started with a different metric on the rationals, we'd end up with the p-adic numbers instead.)

Let R be the set of Cauchy sequences of rational numbers. Cauchy sequences (xn) and (yn) can be added, multiplied and compared as follows:

(xn) + (yn) = (xn + yn)
(xn) × (yn) = (xn × yn)
(xn) ≥ (yn) if and only if for every ε > 0, there exists an integer N such that xnyn - ε for all n > N.

Two Cauchy sequences are called equivalent if the sequence (xn - yn) has limit 0. This does indeed define an equivalence relation, it is compatible with the operations defined above, and the set R of all equivalence classes can be shown to satisfy all the usual axioms of the real numbers. We can embed the rational numbers into the reals by identifying the rational number r with the sequence (r,r,r,...).

A practical and concrete representative for an equivalence class representing a real number is provided by the representation to base b -- in practice, b is usually 2 (binary), 8 (octal), 10 (decimal) or 16 (hexadecimal). For example, the number π = 3.14159... corresponds to the Cauchy sequence (3,3.1,3.14,3.141,3.1415,...). Notice that the sequence (0,0.9,0.99,0.999,0.9999,...) is equivalent to the sequence (1,1.0,1.00,1.000,1.0000,...); this shows that 0.9999... = 1.

Construction by Dedekind cuts

A Dedekind cut in an ordered field is a partition of it, (A, B), such that A is nonempty and closed downwards, B is nonempty and closed upwards. Real numbers can be constructed as Dedekind cuts of rational numbers. In detail, one can make the following definitions. (These are of value in extending some definitions to combinatorial game theory.)

Certain arithmetic operations and set-theoretic notions which apply to the real numbers can be defined correspondingly for Dedekind cuts as follows:

1. Comparison. Two Dedekind cuts, (Ax, Bx) and (Ay, By) are equal:

<math> (A_x, B_x) = (A_y, B_y) \Leftrightarrow A_x \subseteq A_y \and B_x \subseteq B_y<math>

and (Ax, Bx) is less than, or equal to, (Ay, By):

<math> (A_x, B_x) \leq (A_y, B_y) \Leftrightarrow A_x \subseteq A_y \and B_y \subseteq B_x. <math>

2. Addition. The sum of two Dedekind cuts:

<math>(A_x, B_x) + (A_y, B_y) = (A_\mathrm{sum}, B_\mathrm{sum}) := <math>
<math>(\{x + y: x \in A_x \and y \in A_y\}, \{x + y: x \in B_x \and y \in B_y\}).<math>

3. Subtraction is defined analogously to addition.

4. Multiplication. The product of two Dedekind cuts, in case

<math> {\mathbf \forall b_x \in B_x : 0 \leq b_x \and \forall b_y \in B_y : 0 \leq b_y } <math>
<math> {\mathbf (A_x, B_x) \times (A_y, B_y) = (A_\mathrm{prod}, B_\mathrm{prod}) := } <math>
<math> { (\{ a_\mathrm{prod} \in \textbf{Q} : a_\mathrm{prod} \, {\not \in} \, \{ b_\mathrm{prod} \in \textbf{Q} : b_\mathrm{prod} = b_x \times b_y \and b_x \in B_x \and b_y \in B_y \} \},} <math>
<math> { \{ b_\mathrm{prod} \in \textbf{Q} : b_\mathrm{prod} = b_x \times b_y \and b_x \in B_x \and b_y \in B_y \})}. <math>

5. Division. The quotient of two Dedekind cuts, in case <math> {\mathbf \forall b_x \in B_x : 0 \leq b_x \and \exists q \in A_y : 0 < q } <math>

<math> {\mathbf (A_x, B_x) / (A_y, B_y) = (A_\mathrm{quot}, B_\mathrm{quot}) := } <math>
<math> { (\{ a_\mathrm{quot} \in \textbf{Q} : a_\mathrm{quot} \, {\not \in} \, \{ b_\mathrm{quot} \in \textbf{Q} : b_\mathrm{quot} = b_x / q \and b_x \in B_x \and q \in A_y \and 0 < q \} \},} <math>
<math> { \{ b_\mathrm{quot} \in \textbf{Q} : b_\mathrm{quot} = b_x / q \and b_x \in B_x \and q \in A_y \and 0 < q \} )}. <math>

6. Completeness. The supremum of a set of Dedekind cuts which is bounded above:

<math> {\mathbf \sup( \{ (A_n, B_n) \} ) = (A_\mathrm{sup}, B_\mathrm{sup}) := } <math>
<math> { (\{ a_\mathrm{sup} \in \textbf{Q} : a_\mathrm{sub} \, \in \, \bigcup_n A_n \}, } <math>
<math> { \{ b_{\sup} \in \textbf{Q} : b_{\sup} \, {\not \in} \, \{ a_{\sup} \in \textbf{Q} : a_\mathrm{sub} \, \in \, \bigcup_n A_n \} \} )} <math>

and the infimum of a set of Dedekind cuts which is bounded below:

<math> {\mathbf \inf( \{ (A_n, B_n) \} ) = (A_{\inf}, B_{\inf}) := } <math>
<math> { (\{ a_{\inf} \in \textbf{Q} : a_{\inf} \, {\not \in} \, \{ b_{\inf} \in \textbf{Q} : b_{\inf} \, \in \, \bigcup_n B_n \} \},} <math>
<math> { \{ b_{\inf} \in \textbf{Q} : b_{\inf} \, \in \, \bigcup_n B_n \} ). } <math>

Based on the above definitions it is perhaps worth noting that the sum of certain pairs of Dedekind cuts is not necessarily itself a Dedekind cut. Considering for instance the sum <math> (A_{\mathcal S}, B_{\mathcal S}) <math> of

<math> {\mathbf (A_l, B_l) = ( \{ a_l \in \textbf{Q} : a_l < 0 \}, \{ b_l \in \textbf{Q} : b_l \ge 0 \} ) } <math>

and <math> {\mathbf (A_h, B_h) = ( \{ a_h \in \textbf{Q} : a_h \le 0 \}, \{ b_h \in \textbf{Q} : b_h > 0 \} ) } <math>,

i. e. <math> {\mathbf (A_l, B_l) + (A_h, B_h) = (A_{\mathcal S}, B_{\mathcal S}) = ( \{ a_{\mathcal S} \in \textbf{Q} : a_{\mathcal S} < 0 \}, \{ b_{\mathcal S} \in \textbf{Q} : b_{\mathcal S} > 0 \} ) } <math>,

is not a Dedekind cut at all; it is not a partition of the set <math> \textbf{Q} <math> of rational numbers because the rational number <math> \textbf{0} <math> is neither an element of set <math> A_{\mathcal S} <math>, nor an element of set <math> B_{\mathcal S} <math>. In this regard (at least) the real numbers are apparently not represented as Dedekind cuts of the rational numbers.

Construction by decimal expansions

We can take the infinite decimal expansion to be the definition of a real number, considering expansions like 0.9999... and 1.0000... to be equivalent, and define the arithmetical operations formally.

Construction from ultrafilters

As in the hyperreal numbers, we construct *Q from the rational numbers using an ultrafilter. We take then the ring of all elements in *Q whose absolute value is less than some nonzero natural number (it doesn't matter which). This ring has a unique maximal ideal, the infinitesimal numbers. Factoring a ring by a maximal ideal gives a field, in this case the field of reals. It turns out that the maximal ideal respects the order on *Q, so the field we get is an ordered field. Completeness can be proven in a similar way to the construction from the Cauchy sequences.

Construction from surreal numbers

Every ordered field can be embedded in the surreal numbers. The real numbers form the largest subfield that is Archimedean (meaning that no real number is infinitely large).

fr:Construction des nombres réels

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