Investiga tion 37
Finite Fields, the Group of Units in
Z
n
,
and Splitting Fields
Focus Questions
By the end of this investigation, you should be able to give precise and thorough
answers to the questions listed below. You may want to keep these questions in mind
to focus your tho ughts as you complete the investigation.
What is a finite field? How many elements ca n a finite field have?
How are any two finite fields of the same order related?
What is the structure o f the group U
n
of units in Z
n
?
What is the splitting field of a polynomial? Why do we refer to “th e splitting field
instead of “a” splitting field?
How can one construct the splitting field of a polynomial?
Preview Activity 37.1. Let p(x) = x
2
+ x + [1] in Z
2
[x].
(a) Find all of the elements of K = Z
2
[x]/p(x).
(b) Cre a te the addition and multiplication tables for K.
(c) What special kind of rin g is K? Explain.
(d) Wh a t kind of gr oup is U (K), the group of units in K? Explain.
(e) Explain why K contains all of the roots of the polynomial f(x) = x
4
x over Z
2
. In fact,
show that every element of K is a root of f(x) = x
4
x over Z
2
.
Introduction
The structure of the gr oups U
n
, which consist of the units in Z
n
, is not obvious. However, with a bit
of work we can determine the struc ture of th ese groups. To do so, we will first show that the group
U
p
is cyclic when p is prime. We will then use this result to describe every group U
n
, whether n
505

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