Certain Unramified Metabelian Extensions Using Lemmermeyer Factorizations

Abstract

We study solutions to the Brauer embedding problem with restricted ramification. Suppose $G$ and $A$ are a abelian groups, $E$ is a central extension of $G$ by $A$, and $f:\text{Gal}(\overline{\mathbf{Q}}/\mathbf{Q})\rightarrow G$ a continuous homomorphism. We determine conditions on the discriminant of $f$ that are equivalent to the existence of an unramified lift $\widetilde{f}:\text{Gal}(\overline{\mathbf{Q}}/\mathbf{Q})\rightarrow E$ of $f$. As a consequence of this result, we use conditions on the discriminant of $K$ for $K/\mathbf{Q}$ abelian to classify and count unramified nonabelian extensions $L/K$ normal over $\mathbf{Q}$ where the (nontrivial) commutator subgroup of $\text{Gal}(L/\mathbf{Q})$ is contained in its center. This generalizes a result due to Lemmermeyer, which states that a quadratic field $\mathbf{Q}(\sqrt{d})$ has an unramified extension normal over $\mathbf{Q}$ with Galois group $H_8$ the quaternion group if and only if the discriminant factors $d=d_1 d_2 d_3$ as a product of three coprime discriminants, at most one of which is negative, satisfying the following condition on Legendre symbols: \[ \left(\frac{d_i d_j}{p_k}\right)=1 \] for $\{i,j,k\}=\{1,2,3\}$ and $p_i$ any prime dividing $d_i$.