# Finite extensions of local fields

In algebraic number theory, through completion, the study of ramification of a prime ideal can often be reduced to the case of local fields where a more detailed analysis can be carried out with the aid of tools such as ramification groups.

In this article, a local field is non-archimedean and has finite residue field.

## Unramified extension

Let $L/K$ be a finite Galois extension of nonarchimedean local fields with finite residue fields $\ell /k$ and Galois group $G$ . Then the following are equivalent.

• (i) $L/K$ is unramified.
• (ii) ${\mathcal {O}}_{L}/{\mathcal {O}}_{L}{\mathfrak {p}}$ is a field, where ${\mathfrak {p}}$ is the maximal ideal of ${\mathcal {O}}_{K}$ .
• (iii) $[L:K]=[\ell :k]$ • (iv) The inertia subgroup of $G$ is trivial.
• (v) If $\pi$ is a uniformizing element of $K$ , then $\pi$ is also a uniformizing element of $L$ .

When $L/K$ is unramified, by (iv) (or (iii)), G can be identified with $\operatorname {Gal} (\ell /k)$ , which is finite cyclic.

The above implies that there is an equivalence of categories between the finite unramified extensions of a local field K and finite separable extensions of the residue field of K.

## Totally ramified extension

Again, let $L/K$ be a finite Galois extension of nonarchimedean local fields with finite residue fields $l/k$ and Galois group $G$ . The following are equivalent.

• $L/K$ is totally ramified
• $G$ coincides with its inertia subgroup.
• $L=K[\pi ]$ where $\pi$ is a root of an Eisenstein polynomial.
• The norm $N(L/K)$ contains a uniformizer of $K$ .