The $2$-adic Absolute Galois Group

Give a presentation for the absolute Galois group of the field of $2$-adic numbers as a profinite group.

Notability: Solid result
Solved: No
Contributor: David Roe
Field: Number Theory

About the problem

The absolute Galois group of a field $K$ is the projective limit of all of the finite Galois groups $\operatorname{Gal}(E/K)$. It packages together the information about all finite extensions, and studying the Galois group of the rational field $\mathbb{Q}$ is a central problem in algebraic number theory. One method for approaching this group is to study the analogous Galois groups of p-adic fields $\mathbb{Q}_p$. In this case, there is an explicit presentation of $\operatorname{Gal}(\overline{\mathbb{Q}}_p/\mathbb{Q}_p)$ for $p>2$. For $p=2$, we have a description for the absolute Galois group of some extensions of $\mathbb{Q}_2$, but not for $\mathbb{Q}_2$ itself. Finding such a description would fill in a gap in our explicit understanding of Galois groups, and would have consequences in terms of counting p-adic fields with a given Galois group.

Warm-up: We ask for a presentation of the absolute Galois group of $\mathbb{Q}_3$, which is known.

Full write-up

Attempts by AI

We have evaluated the following models on this problem. “Warm-up” refers to an easier variant of the problem with a known solution.

Model
Problem
Solved
Logs
GPT-5.2 Pro
Full Problem
No
Gemini 3 Deep Think
Full Problem
No

AI Prompts

Warm-up

Find a presentation for the absolute Galois group of the field of 3-adic numbers. It may be helpful to know that the maximal pro-3 quotient of the absolute Galois group is a free group on 2 generators, and that for every finite tamely ramified extension $K/\mathbb{Q}_3$ the maximal pro-3 quotient of $\Gal(\overline{K}/K)$ is a Demuskin group. You should give your output as two or three sections, separated by blank lines: variables, definitions (optional) and relations. The variables section should consist of a comma separated list of variable names on one line corresponding to the generators of the presentation. The first and second generators (say $\sigma$ and $\tau$ respectively) are distinguished, and must satisfy the implicit relation $\tau^\sigma = \tau^3$ (which should not be included in the \texttt{relations} section). All later generators should be contained within the $3$-core of the group (the intersection of all $3$-Sylow subgroups). In the definitions section, you may define additional variables in terms of the generators and variables defined earlier within the definitions section. Each line should take the form "var = expr", where expr is an expression built using group operations. In the relations section, you should give the relations as expressions in terms of variables defined in the previous two sections. If there are multiple relations, give one on each line. Expressions in profinite groups may involve exponents lying in $\hat{\Z}$. To specify such a constant, you should provide its reduction modulo $5354228880 = 2^4 \cdot 3^2 \cdot 5 \cdot 7 \cdot 11 \cdot 13 \cdot 17 \cdot 19 \cdot 23$. Example format for the solution (this is the presentation for the maximal pro-2 quotient of the 2-adic absolute Galois group z,tau,x,y x^2*y^3*y^z

Full problem

Find a presentation for the absolute Galois group of the field of 2-adic numbers. It may be helpful to know that the maximal pro-2 quotient of the absolute Galois group can be presented with three generators x,y,z and one relation, x^2*y^3*y^z=1. The theory of Demuskin groups may also be useful. You should give your output as two or three sections, separated by blank lines: variables, definitions (optional) and relations. The variables section should consist of a comma separated list of variable names on one line corresponding to the generators of the presentation. The first and second generators (say $\sigma$ and $\tau$ respectively) are distinguished, and must satisfy the implicit relation $\tau^\sigma = \tau^2$ (which should not be included in the \texttt{relations} section). All later generators should be contained within the $2$-core of the group (the intersection of all $2$-Sylow subgroups). In the definitions section, you may define additional variables in terms of the generators and variables defined earlier within the definitions section. Each line should take the form "var = expr", where expr is an expression built using group operations. In the relations section, you should give the relations as expressions in terms of variables defined in the previous two sections. If there are multiple relations, give one on each line. Expressions in profinite groups may involve exponents lying in $\hat{\Z}$. To specify such a constant, you should provide its reduction modulo $85667662080 = 2^8 \cdot 3^2 \cdot 5 \cdot 7 \cdot 11 \cdot 13 \cdot 17 \cdot 19 \cdot 23$. Example format for the solution (this is the presentation for the presentation when p=3) sigma,tau,x0,x1 projp = 12880 proj2 = 1701 x0part = sigma^-1*x0^-1*sigma * (x0*tau*x0^-1*tau)^projp sigma2 = sigma^proj2 tau2 = tau^proj2 stau2 = sigma2*tau2 tau4 = tau2^2 stau4 = sigma2*tau4 tau8 = tau4^2 tau16 = tau8^2 inner = (x1 * tau16)^(2*projp) outer = (inner * stau2^2 * inner^-1 * stau2^2)^projp y1 = x1^tau8 * inner^stau2 * outer^stau4 * outer^tau4 x1part = x1^4 * y1 * x1^-1 * y1^-1 x0part * x1part

Mathematician survey

The author assessed the problem as follows.

# of mathematicians highly familiar with the problem: a majority of those working on a specialized topic (≈10)
# of mathematicians who have made a serious attempt to solve the problem: 2–4
Rough guess of how long it would take an expert human to solve the problem: 1–3 months
Notability of a solution: a solid result in a subfield
A solution would be published: in a standard specialty journal
Likelihood of a solution generating more interesting math: somewhat likely: an especially innovative solution could, but is hardly guaranteed
Probability that the problem is solvable as stated: 95-99%