Artin Map

Definition

The Artin map is a fundamental concept in class field theory that provides a connection between the ideal class group of a number field and its abelian extensions. It establishes a correspondence between the Galois group of the maximal abelian extension of a number field and the ideals of the ring of integers of that field. This map plays a crucial role in understanding how the arithmetic of number fields relates to their field extensions.

5 Must Know Facts For Your Next Test

1. The Artin map is often denoted as \(\phi: \text{Cl}(K) \to ext{Gal}(L/K)\), where \(\text{Cl}(K)\) is the ideal class group and \(L/K\) is the maximal abelian extension.
2. It encodes how ideals in the ring of integers correspond to elements in the Galois group, allowing for the translation between arithmetic properties and field extensions.
3. The map is injective, meaning distinct classes in the ideal class group correspond to distinct elements in the Galois group, which helps in understanding class numbers.
4. Artin's reciprocity law provides a fundamental relationship between the Artin map and local fields, describing how local conditions impact global behavior.
5. Understanding the Artin map is key to solving problems related to the non-abelian aspects of number theory and can lead to insights about more complex Galois groups.

Review Questions

• How does the Artin map establish a relationship between the ideal class group and the Galois group?
  • The Artin map provides a clear link between the ideal class group of a number field and its Galois group by mapping ideals to automorphisms. Each ideal class corresponds uniquely to an element in the Galois group of its maximal abelian extension. This relationship helps in understanding how the properties of ideals influence the structure of field extensions, allowing mathematicians to analyze abelian extensions through their arithmetic properties.
• Discuss how Artin's reciprocity law enhances our understanding of the Artin map's implications for local fields.
  • Artin's reciprocity law connects local conditions at various places of a number field to global properties through the Artin map. It essentially shows that if we understand how ideals behave locally, we can predict their behavior globally in terms of class numbers and Galois groups. This deepens our insight into not just how abelian extensions function but also how they interact with different valuations and primes within number fields.
• Evaluate the significance of the Artin map in solving problems related to non-abelian extensions in number theory.
  • The Artin map's significance extends beyond just abelian extensions; it lays groundwork for approaching non-abelian Galois theory. By establishing a bridge between ideals and their corresponding Galois actions, it allows mathematicians to frame problems regarding non-abelian groups through an understanding of their abelian counterparts. This insight is crucial for tackling more complex questions regarding solvability by radicals and exploring deeper connections between algebraic structures within number theory.

Related terms

Class Field Theory:

A branch of algebraic number theory that studies abelian extensions of number fields and their connection to the ideal class group.

Galois Group:

A group that describes symmetries in the roots of polynomial equations, specifically representing automorphisms of field extensions.

Maximal Abelian Extension: The largest extension of a number field that is abelian over that field, containing all abelian extensions.