# Abelian number field

From Encyclopedia of Mathematics

(Redirected from Kronecker–Weber theorem)

An Abelian extension of the field of rational numbers $\mathbf{Q}$, i.e. a Galois extension $K$ of $\mathbf{Q}$ such that the Galois group $\mathrm{Gal}(K/\mathbf{Q})$ is Abelian. Examples include: the quadratic number fields $\mathbf{Q}(\sqrt{d})$ and the cyclotomic fields $\mathbf{Q}(\zeta_n)$, $\zeta^n=1$.

The Kronecker–Weber theorem states that every Abelian number field is a subfield of a cyclotomic field. The *conductor* of an abelian number field $K$ is the least $n$ such that $K$ is contained in $\mathbf{Q}(\zeta_n)$, cf. Conductor of an Abelian extension.

See Class field theory.

#### References

- Lawrence C. Washington, " Introduction to Cyclotomic Fields" (2 ed) Graduate Texts in Mathematics
**83**Springer (2012) ISBN 1461219345

**How to Cite This Entry:**

Kronecker–Weber theorem.

*Encyclopedia of Mathematics.*URL: http://www.encyclopediaofmath.org/index.php?title=Kronecker%E2%80%93Weber_theorem&oldid=39899