Matthew Emerton posts really good answers

I asked a question on what Tate’s thesis was really about on math.SE, and Matthew Emerton has posted a very thoughtful and detailed answer. You should go read it. Actually, I recommend looking at all his answers on the website, which are easily some of the best answers there.

(Apparently I am not the first person to notice this.)

 

The Artin reciprocity law

It is now time to prove the reciprocity law, the primary result in class field theory.  I know I haven’t posted on this topic in a little while, so new readers (if they don’t already know this material) may want to review the strategy of the proof and the meaning of the Artin lemma (which is useful in reducing this to the cyclotomic case).

1. The cyclic reciprocity law

Well, I’ve already stated it before multiple times, but here it is:

Theorem 1 (Reciprocity law, cyclic case) Let be a cyclic extension of number fields of degree . Then the reciprocity law holds for : there is an admissible cycle such that the kernel of the map is , and the Artin map consequently induces an isomorphism

 

The proof of this theorem is a little sly and devious.

Recall that, for any admissible cycle , we have

by the conjunction of the first and second inequalities, and the Artin map is surjective. If we prove that the kernel of the Artin map is contained in , then we’ll be done by the obvious count.

This is what we shall do.

 

The proof of Artin’s lemma

This is the lemma we shall use in the proof of the reciprocity law, to reduce the cyclic case to the cyclotomic case:

Lemma 4 (Artin) Let be a cyclic extension of degree and a prime of unramified in . Then we can find a field , a subextension of , with such that in the lattice of fields

we have:

1. splits completely in

2. , so that is cyclotomic

3. is unramified in

Moreover, we can choose such that it is divisible only by arbitrarily large primes.

The proof of this will use the previous number-theory lemmas and the basic tools of Galois theory.

So, first of all, we know that is a subextension of some . We don’t know what is, but pretend we do, and will start carrying out the proof. As we do so, we will learn more and more about what has to be like, and eventually choose it.

 

Towards a lemma of Artin: some elementary number theory

So, the key to proving the Artin reciprocity law in general is to reduce it to the cyclotomic case (which has already been handled, cf. this). To carry out this reduction, start with a number field and a cyclic extension . We will prove that there is an extension of and of such that we have a lattice of fields

(where lattice means , ) such that is cyclotomic and a given prime of splits completely in . This means that, in a sense, the Artin law for becomes reduced to that of , at least for the prime , in that . From this, we shall be able to deduce the Artin law for cyclic extensions, whence the general case will be a “mere” corollary.

1. A funny lemma

The thing is, though, finding the appropriate root of unity to use is not at all trivial. In fact, it requires some tricky number-theoretic reasoning, which we shall carry out in this post. The first step is a lemma in elementary number theory. Basically, we will need two large cyclic subgroups of multiplicative groups of residue classes modulo an integer, one of which is generated by a fixed integer known in advance. We describe now how to do this.

Proposition 1 Let . Then there exists , prime to , such that has order divisible by . In addition, there exists of order divisible by such that the cyclic subgroups generated by have trivial intersection.

Finally if , we can assume that is divisible only by primes .

 

Towards the reciprocity law: motivation and the cyclotomic case

It is now time to begin the final descent towards the Artin reciprocity law, which states that for an abelian extension , there is an isomorphism

We will actually prove the Artin reciprocity law in the idealic form, because we have only defined the Artin map on idelas. In particular, we will show that if is a suitable cycle in , then the Artin map induces an isomorphism

The proof is a bit strange; as some have said, the theorems of class field theory are true because they could not be otherwise. In fact, the approach I will take (which follows Lang’s Algebraic Number Theory, in turn following Emil Artin himself).

So, first of all, we know that there is a map via the Artin symbol, and we know that it vanishes on . It is also necessarily surjective (a consequence of the first inequality). We don’t know that it factors through , however.

Once we prove that (for a suitable ) is in the kernel, then we see that the Artin map actually factors through this norm class group. By the second inequality, the norm class group has order at most that of , which implies that the map must be an isomorphism, since it is surjective.

In particular, we will prove that there is a conductor for the Artin symbol. If is sufficieintly close to 1 at a large set of primes, then the ideal has trivial Artin symbol. This is what we need to prove.

Our strategy will be as follows. We will first analyze the situation for cyclotomic fields, which is much simpler. Then we will use some number theory to reduce the general abelian case to the cyclotomic case (in a kind of similar manner as we reduced the second inequality to the Kummer case). Putting all this together will lead to the reciprocity law.

 

The algebraic proof of the second inequality II

We continue (and finish) the proof started in the previous post of the second inequality.

3. Construction of

So, here’s the situation. We have a cyclic extension of degree , a prime number, and contains the -th roots of unity. In particular, we can write (by Kummer theory) for a subgroup of such that , in particular can be taken to be generated by one element .

We are going to prove that the norm index of the ideles is at most . Then, by the reductions made earlier, we will have proved the second inequality.

3.1. Setting the stage

Now take a huge but finite set of primes such that:

1.

2. is an -unit

3. contains all the primes dividing

4. contains the ramified primes We will now find a bigger extension of whose degree is a prime power. We consider the tower for the -units. We have the extension whose degree we can easily compute; it is

 

The algebraic proof of the second inequality I

So, it turns out there’s another way to prove the second inequality, due to Chevalley in 1940. It’s purely arithmetical, where “arithmetic” is allowed to include cohomology and ideles. But the point is that no analysis is used, which was apparently seen as good for presumably the same reasons that the standard proof of the prime number theorem is occasionally shunned. I’m not going into the proof so much for the sake of number-theory triumphalism but rather because I can do it more completely, and because the ideas will resurface when we prove the existence theorem. Anyhow, the proof is somewhat involved, and I am going to split it into steps. The goal, remember, is to prove that if is a finite abelian extension of degree , then

Here is an outline of the proof:

1. Technical abstract nonsense: Reduce to the case of cyclic of a prime degree and containing the -th roots of unity

2. Explicitly construct a group and prove that

3. Compute the index . The whole proof is too long for one blog post, so I will do step 1 (as well as some preliminary index computations—yes, these are quite fun—today).

 

The analytic proof of the second inequality

Today, we will prove the second inequality: the norm index of the ideles is at most the degree of the field extension. We will prove this using ideles (cf. the discussion of how ideles and ideals connect to each other), and some analysis.

1. A Big Theorem

We shall use one key fact from the theory of L-series. Namely, it is that:

Theorem 1 If is a number field, we have

as . Here ranges over the primes of . The notation means that the two differ by a bounded quantity as .

This gives a qualititative expression for what the distribution of primes must kinda look like—with the aid of some Tauberian theorems, one can deduce that the number of primes of norm at most is asymptotically for , i.e. an analog of the standard prime number theorem. In number fields. We actually need a slight refinement thereof.

Theorem 2 More generally, if is a character of the group , we have

if , and otherwise it tends either to a finite limit or .

Instead of just stating this as a random, isolated fact, I’d like to give some sort of context.  Recall that the Riemann-zeta function was defined as . There is a generalization of this to number fields, called the Dedekind zeta function. The Dedekind-zeta function is not defined by summing over for in the ring of integers (minus 0). Why not? Because the ring of integers is not a unique factorization domain in general, and therefore we don’t get a nice product formula.

 

Why the idele class group is really an ideal group

(Argh. So, the spacing isn’t working as well as I would like on the post and it reads non-ideally (sorry). So I’ve also included a PDF of the post if it makes things better.  -AM)

So, we have defined this thing called the Artin map on the ideals prime to some set of primes. But we really care about the ideles. There has to be some way to relate ideals and ideles. In this post, we give a translation guide between the idealic and ideleic framework. In the good ol’ days, one apparently developed class field theory using only ideal theory, but now the language of the ideles is convenient too (and as we saw, the ideles lend themselves very nicely to computing Herbrand quotients).  But they are not as good for the Artin map, unless one already has local class field theory. We don’t—we could if we developed a lot of cohomological machinery and some delightful pieces of abstract nonsense—but that’s not what we’re going to do (at least not until I manage to muster some understanding of said machinery).

1. Some subgroups of the ideles

Fix a number field . Let’s first look at the open subgroups of . For this, we determine a basis of open subgroups in when is a place. When is real, will do. When is complex, (the full thing) is the smallest it gets. When is -adic, we can use the subgroups . Motivated by this, we define the notion of a cycle : by this we mean a formal product of an ideal and real places induced by real embeddings . Say that an idele is congruent to 1 modulo if for all primes and for . We have subgroups consisting of ideles congruent to 1 modulo . Note that in view of the approximation theorem. We define the subgroup consisting of ideles that are congruent to 1 modulo and units everywhere. Fix a finite Galois extension . If is large enough (e.g. contains the ramified primes and to a high enough power), then consists of norms—this is because any unit is a local norm, and any idele in is very close to 1 (or positive) at the ramified primes. These in fact form a basis of open subgroups of .

 

The Artin map on ideals

We shall now consider a number field and an abelian extension . Let be a finite set of primes (nonarchimedean valuations) of containing the ramified primes, and consider the group of fractional ideals prime to the elements of . This is a free abelian group on the primes not in . We shall define a map, called the Artin map from .

1. How does this work?

Specifically, let be a prime in . There is a prime of lying above it. If are the rings of integers in , respectively, then we have a field extension . As is well-known, there is a surjective homomoprhism of the decomposition group of onto whose kernel, called the inertia group, is of degree .

But, we know that the extension is cyclic, because these are finite fields. The Galois group is generated by a canonically determined Frobenius element which sends . We can lift this to an element of , still called the Frobenius element.