The inverse problem for arboreal Galois representations of index two

Abstract

This paper introduces a systematic approach towards the inverse problem for arboreal Galois representations of finite index attached to quadratic polynomials. Let $F$ be a field of characteristic $\neq 2$, $f\in F[x]$ be monic and quadratic and $ρ_f$ be the arboreal Galois representation associated to $f$, taking values in the group $Ω_{\infty}$ of automorphisms of the infinite binary tree. We give a complete description of the maximal closed subgroups of each closed subgroup of index at most two of $Ω_{\infty}$ in terms of linear relations modulo squares among certain universal functions evaluated in elements of the critical orbit of $f$. We use such description in order to derive necessary and sufficient criteria for the image of $ρ_f$ to be a given subgroup of index two of $Ω_\infty$. These depend exclusively on the arithmetic of the critical orbit of $f$. Afterwards, we prove that if $ϕ=x^2+t\in\mathbb Q(t)[x]$, then there exist exactly five distinct subgroups of index two of $Ω_{\infty}$ that can appear as images of $ρ_{ϕ_{t_0}}$ for infinitely many $t_0\in\mathbb Q$, where $ϕ_{t_0}$ is the specialized polynomial. We show that two of them appear infinitely often, and if Vojta's conjecture over $\mathbb Q$ holds true, then so do the remaining ones. Finally, we give an explicit description of the derived series of each subgroup of index two. Using this, we introduce a sequence of combinatorial invariants for subgroups of index two of $Ω_\infty$. With a delicate use of these invariants we are able to establish that such subgroups are pairwise non-isomorphic as topological groups, a result of independent interest. This implies, in particular, that the five aforementioned groups are pairwise distinct topological groups, and therefore yield five genuinely different instances of the infinite inverse Galois problem over $\mathbb Q$.