# Number Systems

We begin by assuming there is a set R of real numbers that are characterized by
a set of axioms

listed below. [To see how the integers, rationals and real number system can be
generated from

the axioms of set theory, see e.g. Basic Set Theory by Azriel Levy. Rudin (Chap.
1) demonstrates

one method for constructing the set of real numbers from the set of rational
numbers.]

**The Axioms of R**

These axioms are presented and discussed in Royden (Chap 2) and Rosenlicht (Chap
2). The

discussion of the field and order axioms below largely follows Rosenlicht.

**(A) Field Axioms**

The operations ‘+’ and ‘٠’ define a field on R. That is, for any x, y, z ∈ R:

i. x + y = y + x and x ٠ y = y ٠ x (addition and multiplication are commutative
)

ii. (x + y)+z = x+(y + z) and (x ٠ y) ٠z = x ٠(y ٠ z) (addition and multipliation are associative)

iii. (x + y) ٠ z = x ٠ z + y ٠ z ( distributive law )

iv. x + 0 = x and x ٠ 1 = x, where 1 ≠ 0 (existence of identity elements for
addition and

muliplication)

v. x+(-x) = 0 and (x ٠x^{-1} = 1 if x ≠ 0). (existence of an additive and
multiplicative inverses)

We define x - y ≡ x + ( -y ) and and xy = x ٠ y.

• Many different fields satisfy these axioms. If we define 1 + 1 = 0. Then {0,
1} is a field. What

is -1 in this case?

• The rational and complex number systems are fields. The system of integers is
not a field. Why

not?

The field axioms imply the standard rules of arithmetic that involve just
addition, multiplication

and equality, some of which are listed below. Proofs are provided in the
Appendix.

• 0, 1, -x, and x^{-1} are uniquely defined.

• - (-x) = x and (x^{-1}) ^{-1} = x.

• (a) 0x = 0 and (b) xy = 0 and x ≠ 0 imply y = 0.

• - (x + y) = -x - y and (xy)^{-1} = x^{-1}y^{-1}

• - (xy) = (-x) y

• (-1) x = -x

• (-x)(-y) = xy

**(B) Order Axioms**

To derive the rules of arithmetic that involve inequalities, we require the
additional order axioms.

There is a subset
such that

i. x, y ∈ R_{++} implies x + y ∈ R_{++} and xy ∈ R_{++}.

ii. x ∈ R_{++} implies -x
R_{++}

iii. If x ≠ 0, then either x ∈ R_{++} or -x ∈ R_{++}.

If x ∈ R_{++}, we say that x is a positive number. If
-x ∈ R_{++}, we say that x is a
negative number.

We call R_{+} ≡ R_{++} ∪ {0} the set of nonnegative numbers and R_{ -} ≡ R\R_{++} the set of
nonpositive

numbers. (Observe that B(ii) implies 0
R_{++} since -0 = 0.)

A field that satisfies the order axioms is called an ordered field. We define
the relation (R, ≥) by

x ≥ y if and only if x - y ∈ R_{++} ∪ {0} .

• (R, ≥) is a linear order .

٠ (R, ≥) is transitive since (i) implies that if x -y ∈ R_{+} and y
-z ∈ R_{+} then (x
- y)+(y - z) =

x - z ∈ R_{+}.

٠ (R, ≥) is reflexive since x - x = 0 ∈ implies R_{+}.

٠ (R, ≥) is antisymmetric since (iii) implies x - y ≠ 0 implies x - y ∈ R_{+},
which implies that

y - x
R_{+}.

٠ (R, ≥) is complete since (ii) implies that either x - y ∈ R_{+} ∪ {0} or y
- x ∈
R_{+} ∪ {0}.

Since Q ∩ R_{++} satisfies the order axioms and Q is a subfield of R, it follows
immediately that Q is

an ordered field. However, it is easy to show that the system of complex numbers
defined below is

not an ordered field.

We define x > y if x ≥ y and x ≠ y, and define ‘≤’ and ‘<’ to denote the
respective inverse

relations of ‘≥’ and ‘>’.

The standard rules of arithmetic that involve inequalities can be derived from
the field and order

axioms. The following examples are proved in the Appendix.

• If x > y and w ≥ z, then x + y > y + z

• If x > y > 0 and w ≥ z > 0, then xw > yz.

• Suppose x, y > 0. Then (i) x + y > 0, (ii) (-x) + (-y) < 0, (iii) xy > 0, (iv)
x(-y) < 0, (v)

(-x) (-y) > 0.

• For any x ∈ R, define x^{2} ≡ x
٠ x. Then x^{2} > 0 for any x
≠ 0.

• 1 > 0.

• x > 0 implies x^{-1} > 0.

• x > y > 0 implies 0 < x^{-1} < y^{-1}. In particular, x > 1 implies x^{-1} < 1.

For x, y ∈ R, with x ≥ y, we define max(x, y) = x as the maximum of {x, y} and
min(x, y) = y as

the minimum of {x, y} .

We can extend this notation to arbitrary sets of real numbers. If A
R, then
max A denotes an

element x ∈ A such that y ∈ A implies y ≤ x. Similarly, min A denotes an element
x ∈ A such that

y ∈ A implies y ≥ x.

• For arbitrary sets max A and min A need not exist. Can you provide an example?
However,

when they exist, they are always unique.

We define the absolute value of x as |x| = max{x, -x} . The field and order
axioms imply the

following properties :

• |x| ≥ 0 and |x| = 0 if and only if x ≠ 0.

• |xy| = |x| |y|

• |x|^{2} = x^{2}

• |x + y| ≤ |x| + |y|

For the remainder of the notes, we freely use all of the standard rules of
arithmetic that follow

from the field and order axioms.

**(C) Completeness Axiom**

To uniquely characterize the real number system, distinguishing it from other
ordered fields such

as the rational number system, we require one additional axiom. For any A
R, y
∈ R is an upper

bound for A if x ∈ A implies x ≤ y. And y is a least upper bound for A if y is
an upper bound for

A and y ≤ z for any upper bound z of A. We may then state our final axiom as:

Completeness Axiom: Any nonempty set A
R that has an upper bound has a least
upper

bound, which we denote by sup A.

Similary, we may define a lower bound and greatest lower bound of A, which we
denote by inf A.

Observe that infA = - sup {-x ∈ A} . Therefore the completeness axiom also
implies that if A has

a lower bound, it has a greatest lower bound.

• If sup A ∈ A, then max A = supA. Similarly, if inf A ∈ A, then min A = inf A.

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