ideal class
Let K be a number field. Let π and π be ideals in πͺK (the ring of algebraic integers of K). Define a relation βΌ on the ideals of πͺK in the following way: write πβΌπ if there exist nonzero elements Ξ± and Ξ² of πͺK such that (Ξ±)π=(Ξ²)π.
The relation βΌ is an equivalence relation, and the equivalence classes
under βΌ are known as ideal classes
.
The number of equivalence classes, denoted by h or hK, is called the class number of K.
Note that the set of ideals of any ring R forms an abelian semigroup with the product of ideals as the semigroup operation. By replacing ideals by ideal classes, it is possible to define a group on the ideal classes of πͺK in the following way.
Let π, π be ideals of πͺK. Denote the ideal classes of which π and π are representatives by [π] and [π] respectively. Then define β by
[π]β [π]=[ππ] |
Let π={[π]β£πβ (0),π an ideal of πͺK}.
With the above definition of multiplication, π is an abelian group
, called the ideal class group (or frequently just the class group) of K.
Note that the ideal class group of K is simply the quotient group of the ideal group of K by the subgroup
of principal fractional ideals.
Title | ideal class |
Canonical name | IdealClass |
Date of creation | 2013-03-22 12:36:42 |
Last modified on | 2013-03-22 12:36:42 |
Owner | mathcam (2727) |
Last modified by | mathcam (2727) |
Numerical id | 22 |
Author | mathcam (2727) |
Entry type | Definition |
Classification | msc 11R29 |
Synonym | ideal classes |
Related topic | ExistenceOfHilbertClassField |
Related topic | FractionalIdeal |
Related topic | NumberField |
Related topic | UnramifiedExtensionsAndClassNumberDivisibility |
Related topic | ClassNumberDivisibilityInExtensions |
Related topic | PushDownTheoremOnClassNumbers |
Related topic | MinkowskisConstant |
Related topic | ExtensionsWithoutUnramifiedSubextensionsAndClassNumberDivis |
Defines | class number |
Defines | ideal class group |
Defines | class group |