Presentations of Galois groups of maximal extensions with restricted ramification

TL;DR

This paper investigates the structure of Galois groups of maximal extensions with restricted ramification using Galois cohomology, generalizes the Euler-Poincaré characteristic, and connects to nonabelian Cohen-Lenstra heuristics.

Contribution

It generalizes the global Euler-Poincaré characteristic and introduces a new group to extend Koch's work to the entire Galois group, linking to Cohen-Lenstra heuristics.

Findings

Proves a generalized global Euler-Poincaré characteristic.

Defines the group B_S(k,A) for Galois cohomology analysis.

Shows the objects in Liu--Wood--Zureick-Brown conjecture are realizable by a specific random group.

Abstract

Motivated by the work of Lubotzky, we use Galois cohomology to study the difference between the number of generators and the minimal number of relations in a presentation of the Galois group $G_S(k)$ of the maximal extension of a global field $k$ that is unramified outside a finite set $S$ of places, as $k$ varies among a certain family of extensions of a fixed global field $Q$. We prove a generalized version of the global Euler-Poincar\'{e} Characteristic, and define a group $B_S(k,A)$, for each finite simple $G_S(k)$-module $A$, to generalize the work of Koch about the pro-$\ell$ completion of $G_S(k)$ to study the whole group $G_S(k)$. In the setting of the nonabelian Cohen-Lenstra heuristics, we prove that the objects studied by the Liu--Wood--Zureick-Brown conjecture are always achievable by the random group that is constructed in the definition the probability measure in the conjecture.