topics on ideal class groups and discriminants
Ideal Class Groups, Class Numbers and Discriminants (http://planetmath.org/browse/objects/11R29/MSC 11R29)
Let $K$ be a number field^{} (that is, a finite extension^{} of the rational numbers $\mathbb{Q}$) and let ${\mathcal{O}}_{K}$ be the ring of integers^{} in $K$. The ring of integers of $K$ is the analogue of $\mathbb{Z}$ in $\mathbb{Q}$. As we know, $\mathbb{Z}$ enjoys the property that any number can be factored uniquely as a product^{} of powers of primes. In particular, $\mathbb{Z}$ is a UFD and a PID (principal ideal domain^{}). When is ${\mathcal{O}}_{K}$ a UFD or a PID? This is a very hard question to answer. The ideal class group^{} and class number of $K$ are objects that measures how far ${\mathcal{O}}_{K}$ is from actually being a PID. In that sense, the class groups measure the arithmetic^{} complexity of a number field. We include the basic definition of class group here for convenience of the reader:
Definition 1.
The class group, $\mathrm{Cl}\mathit{}\mathrm{(}K\mathrm{)}$, of a number field $K$ is defined to be the quotient group^{} of all fractional ideals^{} of $K$ modulo principal fractional ideals. The size of the class group $\mathrm{}\mathrm{Cl}\mathit{}\mathrm{(}K\mathrm{)}\mathrm{}$ is called the class number of $K$ and it is usually written ${h}_{K}$.
1.1 Basic Definitions

1.
The definition of class group and class number can be found at the entry ideal class. Notice that the ideal classes form an abelian group^{} (the entry also discusses properties of ideal classes).

2.
The Hilbert class field^{} of $K$, usually denoted by $H$, is the maximal unramified abelian extension^{} of $K$. In particular, the Galois group^{} $\mathrm{Gal}(H/K)$ is isomorphic^{} to the class group of $K$ which is the link between ramification, class field theory and class numbers. The entry on the existence of the Hilbert class field (http://planetmath.org/ExistenceOfHilbertClassField) discusses alternative characterizations^{} of $H$.

3.
The concept of ray class group is a generalization^{} of the class group of a field. See also ray class field.
1.2 Computing Class Groups and Class Numbers

1.
The class number formula^{} is one of the most important results in number theory^{}. It relates Dedekind zeta functions and class numbers (and other invariants of the field).

2.
Minkowski’s theorem^{} on lattices provides the wellknown Minkowski’s constant, which in turn can be used to bound class numbers and discriminants^{}.

3.
Using Minkowski’s constant to find a class number (contains examples).
1.3 Divisibility Properties of Class Numbers
The entry on unramified extensions and class number divisibility is a corollary of the existence of the Hilbert class field and clarifies the connection between the prime divisors^{} of ${h}_{K}$ and the unramified abelian extensions of $K$.
The following are theorems that explain the properties of class numbers in extensions^{} of number fields:

1.
Class number divisibility in extensions: $F/K$ Galois, $[F:K]$ not divisible by $p$. Then $p{h}_{K}$ implies $p{h}_{F}$.

2.
Class number divisibility in cyclic extensions: $F/K$ Galois and cyclic with $[F:K]$ not divisible by $p$ and $p$ does not divide the class number of intermediate extensions. Then if $p{h}_{F}$ then ${p}^{f}{h}_{F}$ for some $f$ (see entry for details).

3.
Extensions without unramified subextensions and class number divisibility: $F/K$ such that there are no nontrivial abelian unramified subextensions. Then ${h}_{K}{h}_{F}$.

4.
Class number divisibility in $p$extensions (http://planetmath.org/ClassNumberDivisibilityInPExtensions): $F/K$ is a Galois $p$extension which is ramified at most at one prime. If $p{h}_{F}$ then $p{h}_{K}$.

5.
Pushdown theorem on class numbers: $F/K$ is a $p$extension which is ramified exactly at one prime and this prime is totally ramified. If $p{h}_{F}$ then $p{h}_{K}$.
1.4 Class Numbers of Cyclotomic Fields
Cyclotomic fields^{} have been the object of extensive study. For example, they are crucial in some of the “easy” cases of Fermat’s Last Theorem. For any number $n$, let ${\zeta}_{n}$ be a primitive $n$th root of unity^{}. The field $K=\mathbb{Q}({\zeta}_{n})$ is a cyclotomic field. We denote its class number by ${h}_{n}$.

1.
A prime number^{} $p$ is said to be an irregular prime if ${h}_{p}$ is divisible by $p$ (the entry on regular primes contains Kummer’s criterion for irregularity in terms of Bernoulli numbers^{}). See some examples of regular primes.

2.
Herbrand’s theorem relates Bernoulli numbers and certain subgroups^{} (or $\chi $components) of the ideal class group.

3.
Stickelberger’s theorem on annihilators^{} of the ideal class group of $\mathbb{Q}({\zeta}_{p})$ (it also defines the Stickelberger elements).

4.
Thaine’s theorem is the counterpart of Stickelberger’s theorem for totally real fields.

5.
Vandiver’s conjecture states that a prime number $p$ cannot divide the class number of the maximal real subfield^{} of $\mathbb{Q}({\zeta}_{p})$.

6.
The index of the group of cyclotomic units in the full unit groups is exactly the class number of the maximal real subfield of $\mathbb{Q}({\zeta}_{p})$.
1.5 Discriminants and Related Results

1.
Definition of discriminant (http://planetmath.org/Discriminant) (also discusses the relationship with discriminants in other contexts).

2.
A related concept: the rootdiscriminant.

3.
Hermite’s theorem on extensions which are unramified outside a fixed set of primes.
References

1.
Serge Lang, Algebraic Number Theory^{}. SpringerVerlag, New York.

2.
Daniel A. Marcus, Number Fields, Springer, New York.

3.
K. Ireland, M. Rosen, A Classical Introduction to Modern Number Theory, SpringerVerlag, 1998.

4.
Lawrence C. Washington, Introduction to Cyclotomic Fields, SpringerVerlag, New York.
Note: If you would like to contribute to this entry, please send an email to the author (alozano).
Title  topics on ideal class groups and discriminants 
Canonical name  TopicsOnIdealClassGroupsAndDiscriminants 
Date of creation  20130322 15:07:44 
Last modified on  20130322 15:07:44 
Owner  alozano (2414) 
Last modified by  alozano (2414) 
Numerical id  13 
Author  alozano (2414) 
Entry type  Topic 
Classification  msc 11R29 
Related topic  IdealClass 
Related topic  BibliographyForNumberTheory 
Related topic  ClassNumberDivisibilityInCyclicExtensions 
Related topic  ClassNumberDivisibilityInPExtensions 
Related topic  ClassNumberFormula 
Related topic  UnramifiedExtensionsAndClassNumberDivisibility 
Related topic  PushDownTheoremOnClassNumbers 
Related topic  ClassNumberDivisibilityInExtension 
Defines  ideal class group 