Mild p-Class Tower Groups of Imaginary Quadratic Fields

Abstract

Let $K$ be an imaginary quadratic number field, let $p$ be an odd prime ($K\neq\mathbb Q(\sqrt{-3})$ if $p=3$), and let $G$ be the Galois group of the maximal everywhere unramified pro-$p$ extension of $K$. To each mod-$p$ character $x$ of $G$ we associate a linear map $D_x$ from $\mathrm{Cl}(K)[p]$ to $\mathrm{Cl}(K)/p$; a formula of Ahlqvist and Carlson expresses it through the class of a norm ideal in the unramified cyclic degree-$p$ extension attached to $x$. These maps determine all triple Massey products on $H^1(G,\mathbb F_p)$, and with them the cubic initial relations of $G$. Suppose the $p$-class rank $d$ of $K$ is at least three. The $(d-1)\times(d-1)$ minors of the family $x\mapsto D_x$ define a subscheme $\Sigma_D$ of $\mathbb P^{d-1}_{\mathbb F_p}$, the norm-degeneracy scheme of $K$. We prove: if the rank condition $\mathrm{rk}\,D_x=d-2$ holds transversally at a point of $\Sigma_D$, over some finite extension of $\mathbb F_p$, then $G$ is mild, and hence of cohomological dimension 2. For $p>3$ transversality means that $\Sigma_D$ is smooth of dimension $d-3$ at the point; at $p=3$ the kernel of the Bockstein map enters as an additional constraint. We treat every imaginary quadratic field of $p$-class rank at least three with $|D_K|<2^{30}$, for every odd prime $p$; only $p=3,5,7$ occur. We prove, unconditionally, that the $p$-class tower group is mild for 864 of the 12749 rank-three fields at $p=3$, for 203 of the 204 fields at $p=5$, and for all three fields at $p=7$; the single rank-four field remains undecided. To the best of our knowledge, these are the first number fields for which the full maximal everywhere unramified pro-$p$ Galois group is proved to be mild; in particular they provide explicit infinite $p$-class towers whose Galois groups have cohomological dimension 2.