We shall establish relations between degrees, inertia degrees and ramification indices involved in decomposition and inertia field of a given prime in Galois extension. This is based on Marcus’s *Number Fields* and online notes by R. Ash. It follows a very similar appoach to, but is not based on, the one which can be found in this blog post by Sander Mack-Crane. For this blog post, understanding of Galois theory and basic facts about number fields is necessary.

#### Author: Wojowu (Page 2 of 2)

Based on arguments in Marcus’s *Number Fields* and K. Conrad’s expository paper on ideal factorization. I assume familiarity with the concepts related to ideals and fractional ideals in a commutative ring.

Recall that an integral domain \(R\) with the field of fractions \(K\) is called a *Dedekind domain* if the following conditions hold:

- \(R\) is
*Noetherian*, so every nonempty set of ideals has a maximal element, or equivalently, every ideal is finitely generated, - every prime ideal in \(R\) is a maximal ideal, and
- \(R\) is
*integrally closed*in \(K\), so that every root of a monic polynomial from \(R[x]\) lying in \(K\) lies in \(R\).

Our goal is to prove the following proposition:

**Proposition:** Assume \(R\) is a Dedekind domain. For any nonzero (i.e. containing a nonzero element) fractional ideal \(I\) in \(K\) there exists a fractional ideal \(J\) such that \(IJ=R\).