# Fractional ideal

In mathematics, in particular commutative algebra, the concept of **fractional ideal** is introduced in the context of integral domains and is particularly fruitful in the study of Dedekind domains. In some sense, fractional ideals of an integral domain are like ideals where denominators are allowed. In contexts where fractional ideals and ordinary ring ideals are both under discussion, the latter are sometimes termed * integral ideals* for clarity.

## Definition and basic results

Let *R* be an integral domain, and let *K* be its field of fractions. A **fractional ideal** of *R* is an *R*-submodule *I* of *K* such that there exists a non-zero *r* ∈ *R* such that *rI* ⊆ *R*. The element *r* can be thought of as clearing out the denominators in *I*. The **principal fractional ideals** are those *R*-submodules of *K* generated by a single nonzero element of *K*. A fractional ideal *I* is contained in *R* if, and only if, it is an ('integral') ideal of *R*.

A fractional ideal *I* is called **invertible** if there is another fractional ideal *J* such that *IJ* = *R* (where *IJ* = { a_{1}b_{1} + a_{2}b_{2} + ... + a_{n}b_{n} : a_{i} ∈ *I*, b_{i} ∈ *J*, *n* ∈ **Z**_{>0} } is called the **product** of the two fractional ideals). In this case,
the fractional ideal *J* is uniquely determined and equal to the generalized ideal quotient

The set of invertible fractional ideals form an abelian group with respect to the above product, where the identity is the unit ideal *R* itself. This group is called the **group of fractional ideals** of *R*. The principal fractional ideals form a subgroup. A (nonzero) fractional ideal is invertible if, and only if, it is projective as an *R*-module.

Every finitely generated *R*-submodule of *K* is a fractional ideal and if *R* is noetherian these are all the fractional ideals of *R*.

## Dedekind domains

In Dedekind domains, the situation is much simpler. In particular, every non-zero fractional ideal is invertible. In fact, this property characterizes Dedekind domains: An integral domain is a Dedekind domain if, and only if, every non-zero fractional ideal is invertible.

The set of fractional ideals over a Dedekind domain is denoted . Its quotient group of fractional ideals by the subgroup of principal fractional ideals is an important invariant of a Dedekind domain called the ideal class group.

## Number fields

Recall that the ring of integers of a number field is a Dedekind domain. We call a fractional ideal which is a subset of integral. One of the important structure theorems for fractional ideals of a number field states that every fractional ideal decomposes uniquely up to ordering as

for prime ideals . For example, factors as

Also, because fractional ideals over a number field are all finitely generated we can clear denominators by multiplying by some to get an ideal . Hence

Another useful structure theorem is that integral fractional ideals are generated by up to elements.

There is an exact sequence

associated to every number field, where is the ideal class group of .

## Examples

- is a fractional ideal over
- In we have the factorization . This is because if we multiply it out, we get

Since satisfies , our factorization makes sense.

- In we can multiply the fractional ideals and to get the ideal

## Divisorial ideal

Let denote the intersection of all principal fractional ideals containing a nonzero fractional ideal *I*. Equivalently,

where as above

If then *I* is called **divisorial**.[1] In other words, a divisorial ideal is a nonzero intersection of some nonempty set of fractional principal ideals. If *I* is divisorial and *J* is a nonzero fractional ideal, then (*I* : *J*) is divisorial.

Let *R* be a local Krull domain (e.g., a Noetherian integrally closed local domain). Then *R* is a discrete valuation ring if and only if the maximal ideal of *R* is divisorial.[2]

An integral domain that satisfies the ascending chain conditions on divisorial ideals is called a Mori domain.[3]

## See also

## Notes

## References

- Stein, William,
*A Computational Introduction to Algebraic Number Theory*(PDF) - Chapter 9 of Atiyah, Michael Francis; Macdonald, I.G. (1994),
*Introduction to Commutative Algebra*, Westview Press, ISBN 978-0-201-40751-8 - Chapter VII.1 of Bourbaki, Nicolas (1998),
*Commutative algebra*(2nd ed.), Springer Verlag, ISBN 3-540-64239-0 - Chapter 11 of Matsumura, Hideyuki (1989),
*Commutative ring theory*, Cambridge Studies in Advanced Mathematics,**8**(2nd ed.), Cambridge University Press, ISBN 978-0-521-36764-6, MR 1011461