# Describing Galois groups of some local fields

We can describe the Galois group of some global fields explicitly, for example, we can describe the Galois group of splitting field of $$x^n-a$$ over the rationals explicitly, especially the cyclotomic fields.

Suppose that $$K/F$$ is an extension of number fields, and $$\mathfrak{P}|\mathfrak{p}$$ are primes in these extensions. I know that decomposition groups of a prime $$\mathfrak{p}$$, are isomorphic to $$Gal(K_{\mathfrak{P}}/F_\mathfrak{p})$$.

Also, I know that for any positive integer $$t$$, there is a unique unramified extension of degree $$t$$ over $$F_\mathfrak{p}$$, which can be constructed by adjoining the $$(q^t-1)^{th}$$ roots of unity, and its Galois group is cyclic.

Even in this case I can not describe the $$Gal(F_\mathfrak{p}(\zeta_{q^t-1})/F_\mathfrak{p})$$ explicitly, I don't even know it well enough. Could you give some insights on these Galois groups?

Also I have no idea about other similar extensions, something like $$Gal(F_\mathfrak{p}(\sqrt[n]a)/F_\mathfrak{p})$$.

• The minimal polynomial of $\zeta_{q^t-1}$ has degree $t$ by Hensel lemma so that $Gal(F_\mathfrak{p}(\zeta_{q^t-1})/F_\mathfrak{p})\cong Gal(k_\mathfrak{p}(\zeta_{q^t-1})/k_\mathfrak{p})=Gal(\Bbb{F}_{q^t}/\Bbb{F}_{q})$ (the residue fields) Jun 12, 2022 at 11:59
• @reuns the same paragraph that I wrote contains almost the same thing as your comment, and if I knew the minimal polynomial then my problem was less hard, and my question is about an explicit description of the Galois group, but the explicit description of the Galois group of that minimal polynomial is not easy for me. Even if there is an explicit description "in terms of Frobenius automorphism" then it would be satisfying for me. Jun 12, 2022 at 12:34
• The Galois group sends $\zeta_{q^t-1}$ to $\zeta_{q^t-1}^{q^j}$ for $j$ in $0\ldots t-1$ (both in the $p$-adic field and the finite field) Jun 12, 2022 at 12:37
• +1 @reuns Your 2nd comment was illuminating enough and solved my problem in the case of $Gal(F_\mathfrak{p}(\zeta_{q^t-1})/F_\mathfrak{p})$. Thanks for your time and your patience. If you write your 2nd comment then it would be an acceptable answer to me. Jun 12, 2022 at 15:12

I will address the unramified extensions and leave it to someone else to discuss some tricks on calculating Galois groups explicitly. But briefly, the Galois theory here is much easier than in the number field case since there is only one prime to deal with and in every extension only two things happen: The residue field grows and/or the valuation becomes finer.

The structure of unramified extensions of $$\mathbb Q_p$$ perfectly mirrors the structure of field extensions of $$\mathbb F_p$$. Some details below:

Let $$p$$ be a prime number and $$q = p^n$$. As you probably know, the units of the field $$\mathbb F_{q^t}$$ form a cyclic group and therefore are exactly the $$q^t-1$$th roots of unity. Moreover, the Galois group Gal$$( \mathbb F_{q^t}/\mathbb F_q)$$ is cyclic and is generated by the $$q$$-Frobenius, i.e. the map that takes $$x \to x^q$$.

Now there is a construction called the ring of Witt vectors that takes a finite field $$\mathbb F_q$$ (more generally a perfect field of characteristic $$p>0$$) and produces a complete local ring of characteristic $$0$$ called $$W(\mathbb F_q)$$ which actually turns out to be a very familiar ring to us! For every positive integer $$n$$, there is a unique unramified extension of $$\mathbb Q_p$$ (call it $$K_n$$) and it's valuation ring is exactly $$W(\mathbb F_q)$$ and there is a canonical isomorphism $$Gal\left( K_n/\mathbb{Q_{p}} \right) \simeq Gal \left(\mathbb F_q/\mathbb F_p\right)$$

So what's the point? The Galois theory of unramified extensions of local fields is as easy and explicit as the Galois theory of finite fields.

I would start playing around with $$\mathbb Q_p$$ and adjoining roots of small degree irreducible polynomials and understand what's going on in each case with the help of Hensel's lemma.

• Thanks for your time, but I knew whatever you wrote except this fact: "The valuation ring of the unique unramified extension of $\mathbb Q_p$ is exactly $W(\mathbb F_q)$", and yet my problem is still there. Jun 12, 2022 at 11:58