3. Inequalities and ordered fields 7
In Example 2.3, p needs to be a prime number. Property 5) of Proposition 2.1
fails when p is not a prime. We mention without proof that the number of elements
in a finite field must be a power of a prime number. Furthermore, for each prime
p and positive integer n, there exists a finite field with
pn
elements.
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Exercise 1.5. True or false? Every quadratic equation in F3 has a solution.
Fields such as Fp are important in various parts of mathematics and computer
science. For us, they will serve only as examples of fields. The most important
examples of fields for us will be the real numbers and the complex numbers. To
define these fields rigorously will take a bit more effort. We end this section by
giving an example of a field built from the real numbers. We will not use this
example in the logical development.
Example 2.4. Let K denote the collection of rational functions in one variable
x with real coefficients. An element of K can be written
p(x)
q(x)
, where p and q are
polynomials, and we assume that q is not the zero polynomial. (We allow q(x) to
equal 0 for some x, but not for all x.) We add and multiply such rational functions
in the usual way. It is tedious but not difficult to verify the field axioms. Hence
K is a field. Furthermore, K contains R in a natural way; we identify the real
number c with the constant rational function
c
1
. As with the rational numbers,
many different fractions represent the same element of K. To deal rigorously with
such situations, one needs the notion of equivalence relation, discussed in Section 5.
3. Inequalities and ordered fields
Comparing the sizes of a pair of integers or of a pair of rational numbers is both
natural and useful. It does not make sense however to compare the sizes of elements
in an arbitrary ring or field. We therefore introduce a crucial property shared by
the integers Z and the rational numbers Q. For x,y in either of these sets, it makes
sense to say that x y. Furthermore, given the pair x,y, one and only one of three
things must be true: x y, x y, or x = y. We need to formalize this idea in
order to define the real numbers.
Definition 3.1. A field F is called ordered if there is a subset P F, called the
set of positive elements of F, satisfying the following properties:
1) For all x,y in P, we have x + y P and xy P (closure).
2) For each x in F, one and only one of the following three statements is true:
x = 0, x P, -x P (trichotomy).
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Exercise 1.6. Let F be an ordered field. Show that 1 P.
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Exercise 1.7. Show that the trichotomy property can be rewritten as follows.
For each x,y in F, one and only one of the following three statements is true: x = y,
x - y P, y - x P.
The rational number system is an ordered field; a fraction
p
q
is positive if and
only if p and q have the same sign. Note that q is never 0 and that a rational
number is 0 whenever its numerator is 0. It is of course elementary to check in
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