Shafarevich–Weil theorem

Short description: Relates fundamental classes of Galois extensions of fields to extensions of Galois groups

In algebraic number theory, the Shafarevich–Weil theorem relates the fundamental class of a Galois extension of local or global fields to an extension of Galois groups. It was introduced by Shafarevich (1946) for local fields and by Weil (1951) for global fields.

Statement

Suppose that F is a global field, K is a normal extension of F, and L is an abelian extension of K. Then the Galois group Gal(L/F) is an extension of the group Gal(K/F) by the abelian group Gal(L/K), and this extension corresponds to an element of the cohomology group H²(Gal(K/F), Gal(L/K)). On the other hand, class field theory gives a fundamental class in H²(Gal(K/F),I_K) and a reciprocity law map from I_K to Gal(L/K). The Shafarevich–Weil theorem states that the class of the extension Gal(L/F) is the image of the fundamental class under the homomorphism of cohomology groups induced by the reciprocity law map (Artin Tate).

Shafarevich stated his theorem for local fields in terms of division algebras rather than the fundamental class (Weil 1967). In this case, with L the maximal abelian extension of K, the extension Gal(L/F) corresponds under the reciprocity map to the normalizer of K in a division algebra of degree [K:F] over F, and Shafarevich's theorem states that the Hasse invariant of this division algebra is 1/[K:F]. The relation to the previous version of the theorem is that division algebras correspond to elements of a second cohomology group (the Brauer group) and under this correspondence the division algebra with Hasse invariant 1/[K:F] corresponds to the fundamental class.

References

• Artin, Emil; Tate, John (2009) [1952], Class field theory, AMS Chelsea Publishing, Providence, RI, ISBN 978-0-8218-4426-7, https://books.google.com/books?isbn=978-0-8218-4426-7 
• Shafarevich, I. R. (1946), "On Galois groups of p-adic fields.", C. R. (Doklady) Acad. Sci. URSS, New Series 53: 15–16  Reprinted in his collected works, pages 4–5
• Weil, André (1951), "Sur la theorie du corps de classes", Journal of the Mathematical Society of Japan 3: 1–35, doi:10.2969/jmsj/00310001, ISSN 0025-5645 , reprinted in volume I of his collected papers, ISBN:0-387-90330-5
• Weil, André (1967), "Appendix III:Shafarevitch's theorem", Basic number theory., Die Grundlehren der mathematischen Wissenschaften, 144, Springer-Verlag New York, Inc., New York, pp. 301–307, ISBN 3-540-58655-5