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Fearless Symmetry by Robert Gross, Avner Ash

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180 CHAPTER 16
no matter what element σ is chosen from F (p), χ
r
(σ ) will always be
the same number. The great thing is that in our theorems we will
only need to refer to this character value χ
r
(Frob
p
), and not to the
whole matrix r(Frob
p
), which is not well-defined.
If p is in the set S,thenχ
r
(Frob
p
)isnot well-defined, and we do
not discuss Frob
p
in relationship to r. We cannot. Frustrated, we
often refer to S as the set of “bad” primes for the representation r.
We are not being judgmental. We are just projecting our frustration
onto the primes.
In most of the examples and theorems that are important in
number theory, the set S of bad primes is finite. Usually, knowledge
of S comes if we have knowledge of how a particular representation
r is constructed. You will see examples of this later.
We should be decorating the set of bad primes S with a subscript,
such as S
r
,becauseS depends on what representation r we are
studying. But that just gives us more to worry about, and because
we will only deal with one r at a time, we just leave it out.
In summary, any Galois representation r comes with a set S of
bad primes, and if p is not in S,thenχ
r
(Frob
p
) is a dandy, well-
defined, unambiguous number which we can use in our formulas .
For completeness, we are going to tell you more about how to find
Frob
p
, but you could skim this material during your first reading of
the book.
Algebraic Integers, Discriminants, and Norms
We start by telling you about parts of algebraic number theory that
are not as complicated as they might seem at first:
DEFINITION: An algebraic integer is an element of Q
alg
that
is a root of a Z-polynomial that has a leading coefficient
3
of 1.
The set of all algebraic integers is written
Z.Ifα is an
3
The leading coefficient of a polynomial is the coefficient of the highest power of x in
the polynomial. For example, the leading coefficient of the polynomial 3x
5
2x
2
+ 7x 1
is 3.
FROBENIUS 181
element of Z, then the Z-polynomial of smallest degree with
first coefficient 1 which has α as a root is called the minimal
polynomial for α. Note: Most algebraic integers are not
ordinary integers.
It is easy to find an element of
Z:FindaZ-polynomial whose
first coefficient is 1. For example, any solution of x
5
+ 41x
4
+ 32x +
11 = 0 is an algebraic integer. ( Of course, we write x
5
instead of the
longer 1x
5
, but the coefficient of x
5
is still 1.) It is less obvious—in
fact, rather difficult to show—that sums and products of algebraic
integers are algebraic integers. Quotients of algebraic integers are
not necessarily algebraic integers. The reason for the terminology
“algebraic integer” is that these are the elements of Q
alg
that
mimic ordinary integers: They can be added and multiplied, but not
divided, without leaving the realm of algebraic integers. Of course
you can divide them, but the answer may well be only an algebraic
number, not an algebraic integer any more. Every element of Q
alg
is
a quotient of elements of Z. And one more important fact about Z:
An ordinary integer is always an algebraic integer (e.g., 137 solves
x
1
137 = 0),and,conversely,theonly rational numbers that are
algebraic integers are the ordinary integers. You can symbolize this
last sentence by writing
Z Q = Z.
Next, we take a bit of a detour to discuss the discriminant
of a polynomial. You may remember this term from high-school
algebra: The discriminant of the quadratic polynomial ax
2
+ bx + c
is b
2
4ac. You may even remember that the discriminant of a
quadratic polynomial tells you if the polynomial has 0, 1, or 2 real
roots. This, of course, is another way of saying how many points
there are on the variety S(R) associated with the polynomial. So it
is not surprising that there is a generalization of the discriminant
to higher degrees that will prove useful to us.
DEFINITION: Suppose that f (x) = x
n
+ c
n1
x
n1
+···+c
1
x +
c
0
is a Z-polynomial with roots α
1
, α
2
, ..., α
n
,sothatf (x) =
(x α
1
)(x α
2
) ···(x α
n
). (Notice that because we are
assuming that f (x) starts with the coefficient 1, all of these

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