Hilbert class field
In algebraic number theory, the Hilbert class field E of a number field K is the maximal abelian unramified extension of K. Its degree over K equals the class number of K and the Galois group of E over K is canonically isomorphic to the ideal class group of K using Frobenius elements for prime ideals in K.
In this context, the Hilbert class field of K is not just unramified at the finite places (the classical ideal theoretic interpretation) but also at the infinite places of K. That is, every real embedding of K extends to a real embedding of E (rather than to a complex embedding of E).
If the ring of integers of K is a unique factorization domain, in particular, if then K is its own Hilbert class field.
By contrast, let . By analyzing ramification degrees over , one can show that is an everywhere unramified extension of K, and it is certainly abelian. Hence the Hilbert class field of K is a nontrivial extension and the ring of integers of K cannot be a unique factorization domain. (In fact, using the Minkowski bound, one can show that K has class number exactly 2.) Hence, the Hilbert class field is .
To see why ramification at the archimedean primes must be taken into account, consider the real quadratic field K obtained by adjoining the square root of 3 to Q. This field has class number 1, but the extension K(i)/K is unramified at all prime ideals in K, so K admits finite abelian extensions of degree greater than 1 in which all primes of K are unramified. This doesn't contradict the Hilbert class field of K being K itself: every proper finite abelian extension of K must ramify at some place, and in the extension K(i)/K there is ramification at the archimedean places: the real embeddings of K extend to complex (rather than real) embeddings of K(i).
The existence of a Hilbert class field for a given number field K was conjectured by David Hilbert and proved by Philipp Furtwängler. The existence of the Hilbert class field is a valuable tool in studying the structure of the ideal class group of a given field.
Additional properties 
The Hilbert class field E also satisfies the following:
- E is a finite Galois extension of K and [E : K]=hK, where hK is the class number of K.
- The ideal class group of K is isomorphic to the Galois group of E over K.
- Every ideal of OK is a principal ideal of the ring extension OE (principal ideal theorem).
- Every prime ideal P of OK decomposes into the product of hK/f prime ideals in OE, where f is the order of [P] in the ideal class group of OK.
In fact, E is the unique field satisfying the first, second, and fourth properties.
Explicit constructions 
In class field theory, one studies the ray class field with respect to a given modulus, which is a formal product of prime ideals (including, possibly, archimedean ones). The ray class field is the maximal abelian extension unramified outside the primes dividing the modulus and satisfying a particular ramification condition at the primes dividing the modulus. The Hilbert class field is then the ray class field with respect to the trivial modulus 1.
The narrow class field is the Hilbert class field with respect to the modulus consisting of all infinite primes. For example, the argument above shows that is the narrow class field of .
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- Furtwängler, Philipp (1906), "Allgemeiner Existenzbeweis für den Klassenkörper eines beliebigen algebraischen Zahlkörpers", Mathematische Annalen 63 (1): 1–37, doi:10.1007/BF01448421, JFM 37.0243.02, MR 1511392, retrieved 2009-08-21
- J. S. Milne, Class Field Theory (Course notes available at http://www.jmilne.org/math/). See the Introduction chapter of the notes, especially p. 4.
- Silverman, Joseph H. (1994), Advanced topics in the arithmetic of elliptic curves, Graduate Texts in Mathematics 151, New York: Springer-Verlag, ISBN 978-0-387-94325-1
- Gras, Georges (second corrected printing 2005), Class field theory: From theory to practice, New York: Springer