Artin Symbol

Definition

The Artin symbol is an important notation in algebraic number theory that describes the action of the Galois group of a number field on the ideal class group of another number field. It plays a crucial role in understanding how primes in one field relate to primes in another through their factorization, particularly when considering extensions and residue fields. This symbol connects the notions of Galois theory and class field theory, making it essential for examining the arithmetic properties of number fields.

5 Must Know Facts For Your Next Test

1. The Artin symbol is denoted as $(a/K)$, where 'a' is a prime ideal and 'K' is a field extension over which the ideal is considered.
2. This symbol helps in expressing how a prime ideal splits or remains inert in extensions, which is critical for understanding the distribution of prime ideals.
3. The Artin symbol can be interpreted as an element of the Galois group acting on the class group, offering insights into the interplay between different number fields.
4. In class field theory, Artin symbols provide a way to encode information about the local behavior of primes across different completions of number fields.
5. The Artin reciprocity law states that the Artin symbol is compatible with both the local and global structures of number fields, reflecting their deep arithmetic relationships.

Review Questions

• How does the Artin symbol relate to the behavior of prime ideals when considering extensions of number fields?
  • The Artin symbol provides a way to describe how prime ideals behave in different extensions. Specifically, it indicates whether a prime ideal remains inert, splits into several prime ideals, or ramifies when moving from one field to another. This understanding is fundamental for studying the arithmetic properties of number fields and plays an essential role in class field theory, which analyzes these behaviors in depth.
• Discuss the relationship between Frobenius automorphisms and the Artin symbol in the context of Galois groups.
  • Frobenius automorphisms are specific instances within the Galois group that correspond to prime ideals. The Artin symbol essentially encapsulates this relationship by showing how Frobenius automorphisms act on these ideals as elements of the Galois group. This connection allows mathematicians to utilize the structure provided by Frobenius automorphisms to understand how primes behave across extensions, reinforcing the link between Galois theory and class field theory.
• Evaluate how the Artin reciprocity law links local and global properties of number fields through Artin symbols.
  • The Artin reciprocity law establishes a profound connection between local and global properties by illustrating how Artin symbols reflect these interactions. It asserts that if you know how primes split or behave locally (in local completions), you can derive information about their behavior globally across extensions. This interplay is vital for gaining insights into more complex structures in algebraic number theory and illustrates the unifying nature of Artin symbols in understanding ideal class groups in various fields.

Related terms

Frobenius Automorphism:

An automorphism associated with a prime ideal in a number field, indicating how that prime ideal behaves under the action of the Galois group.

Galois Group:

The group of field automorphisms that leave a given field fixed, used to analyze the symmetries of polynomial roots and their relationships.

Class Field Theory:

A branch of algebraic number theory that studies abelian extensions of number fields and provides a connection between the Galois group of a field and its ideal class group.