# Is there a “purely algebraic” proof of the finiteness of the class number?

The background is as follows: I have been whittling away at my commutative algebra notes (or, rather at commutative algebra itself, I suppose) recently for the occasion of a course I will be teaching soon.

I just inserted the statement of the theorem that the ring $\overline{\mathbb{Z}}$ of all algebraic integers is a Bezout domain (i.e., all finitely generated ideals are principal; note that this ring is very far from being Noetherian). I actually don't remember off the top of my head who first proved this result -- and I would be happy to learn, although that's not my main question here. The reference that sticks in my mind is Kaplansky's 1970 text Commutative Rings, where he proves the following nice generalization:

Theorem: Let $R$ be a Dedekind domain with fraction field $K$ and algebraic closure $\overline{K}$. Suppose that for every finite subextension $L$ of $\overline{K}/K$, the ideal class group of the integral closure $R_L$ of $R$ in $L$ is a torsion abelian group. Then the integral closure $S$ of $R$ in $\overline{K}$ is a Bezout domain.

Neat, huh? Then when I got to deducing the result about $\overline{\mathbb{Z}}$, it hit me: nowhere in these notes do I verify that $\mathbb{Z}$ satisfies the hypotheses of Kaplansky's theorem, namely that the ideal class group of the ring of integers of a number field is always finite.

Don't get me wrong: I wasn't expecting anything different -- I actually don't know of any commutative algebra text which proves this result. Indeed, it is generally held that the finiteness of the class number is one of the first results of algebraic number theory which is truly number-theoretic in nature and not part of the general study of commutative rings. But the truth is that I've been bristling at this state of affairs for some time: I would really like there to be a subbranch of mathematics called "abstract algebraic number theory" which proves "general" results like this. (My reasons for this are, so far as I can recall at the moment, purely psychological and aesthetic: I have no specific ulterior motive here, alas.) To see past evidence of me flirting with these issues, see this previous MO question (which does not have an accepted answer) and these other notes of mine (which don't actually get off the ground and establish anything exciting).

So let me try once again:

Is there a purely algebraic proof of the finiteness of the class number?

Unfortunately I don't know exactly what I mean here, because the standard proofs that one finds in algebraic number theory texts are certainly "purely algebraic" in nature or can be made so. (For instance, it is well known that it is convenient but not necessary to use geometry of numbers -- the original proofs of this finiteness result predate Minkowski's work.) Here are some criteria:

1) I want a general -- or "structural" -- condition on a Dedekind domain that implies the finiteness of its class group. (In my previous question, I asked whether finiteness of the residue rings was such a condition. I still don't know the answer to that.)

2) This condition should in particular apply to rings of integers of number fields and also to coordinate rings of regular, integral affine curves over finite fields.

Note that already the standard "purely algebraic proofs" of finiteness of class number in the number field case do not in fact proceed by a general method which also works verbatim in the function field case: additional arguments are usually required. (See for instance Dino Lorenzini's Invitation to Arithmetic Geometry.) As far as number field / function field unity goes, the best approach I know is the adelic one: Fujisaki's Lemma. But this is a topological argument, and the topological and valuation theoretic properties of global fields which go into it are quite particular to global fields: I am (dimly) aware of results of Artin-Whaples which characterize global fields as the ones which have these nice properties: the product formula, and so forth.

It is possible that what I am seeking simply doesn't exist. If you feel like you understand why the finiteness of class number is in some precise way arithmetic rather than algebraic in nature, please do explain it to me!

Added: here are some further musings which might possibly be relevant.

I like to think of three basic theorems of algebraic number theory as being of a kind ("the three finiteness theorems"):
(i) $\mathbb{Z}_K$ is a Dedekind domain which is finitely generated as a $\mathbb{Z}$-module.
(ii) $\operatorname{Pic} \mathbb{Z}_K$ is finite.
(iii) $\mathbb{Z}_K^{\times}$ is finitely generated as a $\mathbb{Z}$-module.

[Yes, there is also a fourth finiteness theorem due to Hermite, on restricted ramification, which is perhaps most important of all...]

The first of these is acceptably "purely algebraic" to me: it is a result about taking the normalization of a Dedekind domain in a finite field extension. The merit of the adelic approach is that it shows that (ii) and (iii) are closely interrelated: the conjunction of the two of them is formally equivalent to the compactness of the norm one idele class group. So perhaps it is a mistake to fixate on conditions only ensuring the finiteness of the class group. For instance, the class of Dedekind domains with finite class group is closed under localization but the class of Dedekind domains which also have finitely generated unit group is not. However the "Hasse domains" -- i.e. $S$-integer rings of global fields -- do have both of these properties.

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As noted in math.stackexchange, I believe it was Dedekind in 1871, that first proved that the ring of algebraic integers is a Bezout domain; in his 1877 exposition he states it by saying that given any algebraic integers $a$ and $b$, there exist algebraic integers $x$ and $y$ such that a greatest common divisor $d$ of $a$ and $b$ can be written as $ax+by = d$. He calls it an important but difficult theorem, and only presents it as an easy corollary after he has developed all the theory, including (if memory serves) finiteness of the class number. –  Arturo Magidin Dec 28 '10 at 17:40
@Arturo: thanks again, that's very helpful. Let me say that I certainly do not know how to prove this result without using the finiteness (or at least the torsionness) of the class numbers of number fields. –  Pete L. Clark Dec 28 '10 at 17:49
This is certainly not an answer, but perhaps of interest as it shows certain limitations: Every abelian group is isomporphic to the class group of a quadratic extensions of a principal ideal domain (Leedham-Green, TAMS 1972). –  quid Jan 19 '11 at 11:21
@unknown: yes, I am aware of this result, having given another proof of it a few years ago. Please see math.uga.edu/~pete/ellipticded.pdf. –  Pete L. Clark Jan 19 '11 at 15:10
@Pete: Thanks for the link to your paper. –  quid Jan 19 '11 at 15:50

A nice account of Dedekind's `greatest common divisor' of two algebraic number is given in Hecke's book, 'Lectures on the Theory of Algebraic Numbers'. You wont of course see an algebraic proof there.

The closest I know of an answer to your question is given by Stickelberger's theorem on ideal class annihilators. In Ireland and Rosen at the end of Chapter 14 you can even find an algebraic proof that the class group of $\mathbb{Q}(\sqrt{-p}), p\equiv 3 \mod 4$ is annihilated by the classical $(N-R)/p$. In fact it is the class number, but I don't know that you can prove this algebraically.

Of course this only applies to abelian extensions of $\mathbb{Q}$, so it doesn't really come close to answering the general question!

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In addition, it only shows that the exponent of the class group is finite. For finiteness, you need to prove algebraically that it is finitely generated. –  Franz Lemmermeyer Jan 19 '11 at 7:48