Fractional Ideal

Definition

A fractional ideal is a generalization of the concept of an ideal in a ring, specifically in the context of Dedekind domains. It is an additive subgroup of the field of fractions of the integral domain that can be expressed as a fractional multiple of an ideal, allowing for the treatment of elements not necessarily contained in any given ideal. This concept is essential for understanding unique factorization and the structure of the ideal class group.

5 Must Know Facts For Your Next Test

1. Every fractional ideal can be expressed as a quotient of two integral ideals, emphasizing its connection to the structure of the ring.
2. In Dedekind domains, every nonzero fractional ideal can be uniquely factored into prime fractional ideals, reflecting the unique factorization property.
3. The set of fractional ideals forms an abelian group under addition, making it possible to study their structure using group theory.
4. The ideal class group is formed by taking the set of fractional ideals modulo the set of non-zero integral ideals, highlighting how these two concepts interact.
5. The class number, which measures the size of the class group, indicates how far the ring deviates from having unique factorization within its ideals.

Review Questions

• How do fractional ideals relate to integral ideals in terms of their structure and properties?
  • Fractional ideals are essentially extensions of integral ideals, allowing us to include elements that may not belong to any integral ideal directly. They can be constructed as quotients of integral ideals, enabling operations that involve division. This relationship highlights their importance in studying rings like Dedekind domains where unique factorization plays a critical role.
• Discuss how fractional ideals contribute to understanding the class group and its significance in number theory.
  • Fractional ideals are key components in forming the class group, as they allow us to consider equivalence classes of ideals that encapsulate unique factorization properties. By analyzing fractional ideals modulo integral ideals, we can determine how many distinct classes exist, which reflects on the overall structure and behavior of the ring. This insight is significant because it helps identify whether a given ring possesses unique factorization or if it needs further study.
• Evaluate the implications of having a non-trivial class number for a Dedekind domain in relation to fractional ideals.
  • A non-trivial class number indicates that there are fractional ideals that cannot be represented by integral ideals alone, signaling a failure in unique factorization. This situation suggests complexities in how numbers can be expressed within that domain. Understanding this implication helps deepen our insight into the arithmetic properties of algebraic integers and can affect how we approach problems in algebraic number theory.

Related terms

Ideal: An ideal is a special subset of a ring that absorbs multiplication by elements from the ring and is closed under addition, used to generalize certain properties in algebra.

Dedekind Domain: A Dedekind domain is a type of integral domain where every nonzero prime ideal is maximal, allowing for nice properties related to unique factorization in its ideals.

Class Group: The class group is a mathematical structure that measures the failure of unique factorization in a ring; it consists of equivalence classes of fractional ideals.