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ideal class (Definition)

Let $ K$ be a number field. Let $ \mathfrak{a}$ and $ \mathfrak{b}$ be ideals in $ \mathcal{O}_{K}$ (the ring of algebraic integers of $ K$). Define a relation $ \sim$ on the ideals of $ \mathcal{O}_{K}$ in the following way: write $ \mathfrak{a} \sim \mathfrak{b}$ if there exist nonzero elements $ \alpha$ and $ \beta$ of $ \mathcal{O}_K$ such that $ (\alpha)\mathfrak{a}=(\beta)\mathfrak{b}$.

The relation $ \sim$ is an equivalence relation, and the equivalence classes under $ \sim$ are known as ideal classes.

The number of equivalence classes, denoted by $ h$ or $ h_K$, 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 $ \mathcal{O}_{K}$ in the following way.

Let $ \mathfrak{a}$, $ \mathfrak{b}$ be ideals of $ \mathcal{O}_{K}$. Denote the ideal classes of which $ \mathfrak{a}$ and $ \mathfrak{b}$ are representatives by $ [\mathfrak{a}]$ and $ [\mathfrak{b}]$ respectively. Then define $ \cdot$ by

$\displaystyle [\mathfrak{a}] \cdot [\mathfrak{b}]=[\mathfrak{a} \mathfrak{b}] $

Let $ {\cal C} = \{ [\mathfrak{a}] \mid \mathfrak{a} \neq (0), \mathfrak{a}$    an ideal of $ \mathcal{O}_{K} \}$. With the above definition of multiplication, $ \cal C$ 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.



"ideal class" is owned by mathcam. [ full author list (2) | owner history (1) ]
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See Also: existence of Hilbert class field, fractional ideal, number field, unramified extensions and class number divisibility, class number divisibility in extensions, push-down theorem on class numbers, Minkowski's constant, extensions without unramified subextensions and class number divisibility, class number divisibility in $p$-extensions, class number divisibility in cyclic extensions, topics on ideal class groups and discriminants

Other names:  ideal classes
Also defines:  class number, ideal class group, class group

Attachments:
ideal classes form an abelian group (Theorem) by mathcam
ideal class group is finite (Theorem) by rm50
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Cross-references: fractional ideals, subgroup, ideal group, quotient group, abelian group, multiplication, group, operation, semigroup, product of ideals, Abelian semigroup, equivalence classes, equivalence relation, relation, algebraic integers, ring, ideals, number field
There are 28 references to this entry.

This is version 19 of ideal class, born on 2002-04-23, modified 2008-01-23.
Object id is 2869, canonical name is IdealClass.
Accessed 7666 times total.

Classification:
AMS MSC11R29 (Number theory :: Algebraic number theory: global fields :: Class numbers, class groups, discriminants)

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