
History and Origin of the Ideal Class Group
The first ideal class groups encountered in mathematics were part of the theory of quadratic forms: in the case of binary integral quadratic forms, as put into something like a final form by Gauss, a composition law was defined on certain equivalence classes of forms. This gave a finite abelian group, as was recognised at the time.
Later Kummer[?] was working towards a theory of cyclotomic fields. It had been realised (probably by several people) that failure to complete proofs in the general case of Fermat's Last Theorem by factorisation using the roots of unity was for a very good reason: a failure of the fundamental theorem of arithmetic to hold, in the rings generated by those roots of unity, was a major obstacle. Out of Kummer's work for the first time came a study of the obstruction to the factorisation. We now recognise this as part of the ideal class group: in fact Kummer had isolated the ptorsion in that group for the field of proots of unity, for any prime number p, as the reason for the failure of the standard method of attack on the Fermat problem.
Somewhat later again Dedekind formulated the concept of ideal, Kummer having worked in a different way. At this point the existing examples could be unified. It was shown that while rings of algebraic integers do not always have unique factorization into primes (because they need not be principal ideal domains), they do have the property that every proper ideal admits a unique factorization as a product of prime ideals (that is, every ring of algebraic integers is a Dedekind domain)). The ideal class group gives some answer to the question: which ideals are principal ideals? The answer comes in the form all of them, if and only if the ideal class group (which is a finite group) has just one element.
If R is an integral domain, define a relation ~ on nonzero ideals of R by I ~ J whenever there exist nonzero elements a and b of R such that (a)I = (b)J. (Here the notation (a) means the principal ideal of R consisting of all the multiples of a.) It is easily shown that this is an equivalence relation. The equivalence classes are called the ideal classes of R. Ideal classes can be multiplied: if [I] denotes the equivalence class of the ideal I, then the multiplication [I][J] = [IJ] is welldefined and abelian. The principal ideals form the ideal class [R] which serves as an identity element for this multiplication.
If R is a ring of algebraic integers, or more generally a Dedekind domain, the multiplication defined above turns the set of ideal classes into an abelian group, the ideal class group of R.
The ideal class group is trivial (i.e. contains only its identity element) if and only if all ideals of R are principal. In this sense, the ideal class group measures how far R is from being a principal ideal domain, and hence from satisfying unique prime factorization (Dedekind domains are unique factorization domains if and only if they are principal ideal domains).
The number of ideal classes (the class number of R) may be infinite in general. But if R is in fact a ring of algebraic integers, then this number is always finite. This is one of the main results of classical algebraic number theory.
It turns out that an alternative characterization of Dedekind domain is that every fractional ideal[?] of R be invertible[?] under ideal multiplication. The definition of fractional ideal relies on the definitions of module and field of quotients. Since R is a Dedekind domain it is an integral domain, and so it possesses a field of quotients K. A fractional ideal of R, then, is a nonzero finitely generated Rsubmodule of K. A fractional ideal I is contained in R if and only if it is an honest ideal of R. (In this case it is normally said that I is an integral ideal of R.) Fractional ideals can be multiplied in a natural manner, and tt can be shown that in a Dedekind domain, the fractional ideals form an abelian multiplicative group with R as identity element. The principal fractional ideals are those Rsubmodules of K which are generated by a single nonzero element of K. It is easy to see that they form a subgroup of the group of all fractional ideals. The quotient group of fractional ideals divided by principal fractional ideals is the ideal class group of R; it is isomorphic to the one defined above. Part of the reason for introducing fractional ideals is to realize the ideal class group as an actual quotient group, rather than with the ad hoc multiplication of equivalence classes described above.
Computation of the class group is hard, in general; it can be done by hand for algebraic number fields of small discriminant[?], using a theorem of Minkowski[?]. This result gives a bound on the norms of the ideals[?] in a particular class. In general the bound is not sharp enough to make the calculation practical for fields with large discriminant, but computers are well suited to the task.
It was remarked above that the ideal class group provides part of the answer to the question of how much ideals behave like ring elements in a Dedekind domain. The other part of the answer is provided by the multiplicative group of units of the Dedekind domain (and this is the rest of the reason for introducing the concept of fractional ideal, as well).
Define a map from K\{0} to the set of all nonzero fractional ideals of R by sending every element to the principal (fractional) ideal it generates. This is a group homomorphism; its kernel is the group of units of R, and its cokernel is the ideal class group of R. The failure of these groups to be trivial is a precise measure of the failure of the map to be an isomorphism: that is the failure of ideals to act like ring elements, that is to say, like numbers.
Examples of ideal class groups
The rings Z, Z[i], and Z[w], (where i is a square root of 1 and w is a cube root of 1) are all principal ideal domains, and so have class number 1: that is, they have trivial ideal class groups. If k is a field, then the polynomial ring k[X_{1}, X_{2}, X_{3}, ...] is an integral domain. It has a countably infinite set of ideal classes.
If d is a squarefree integer (in other words, a product of distinct primes) other than 1 or 1, then Q(√ d) is a finite extension of Q. In particular it is a 2dimensional vector space over Q, called a quadratic extension. If d is less than 0, then the class number of the ring R of algebraic integers of Q(√ d) is equal to 1 for precisely the following values of d: d = 1, 2, 3, 7, 11, 19, 43, 67, and 163. This result was first conjectured by Gauss and proven by Heegner[?], although Heegner's proof was not believed until Stark gave a later proof in 1967, which Stark showed was actually equivalent to Heegner's. This is a special case of the famous class number conjecture[?].
If, on the other hand, d > 0, then it is unknown whether there are infinitely many fields Q(√ d) with class number 1. Computational results indicate that there are a great many such fields to say the least.
Connections to class field theory
Class field theory is a more advanced branch of number theory which seeks to classify all the abelian extensions of a given algebraic number field, meaning Galois extensions with abelian Galois group. A particularly beautiful example is found in the Hilbert class field of a number field, which can be defined as the maximal unramified[?] abelian extension of such a field. The Hilbert class field L of a number field Kis unique and has the following properties:
Neither property is particularly easy to prove.
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