Counting number fields whose Galois group is a wreath product of symmetric groups

Abstract

Let $K$ be a number field and $k\ge 2$ be an integer. Let $(n_1,n_2,\dots ,n_k)$ be a vector with entries $n_i\in {\mathbb{Z}}_{\ge 2}$. Given a number field extension $L/K$, we denote by $\tilde{L}$ the Galois closure of $L$ over $K$. We prove asymptotic lower bounds for the number of number field extensions $L/K$ with $[L:K]={\prod }_{i=1}^k{n}_i$, such that $Gal(\tilde{L}/K)$ is isomorphic to the iterated wreath product of symmetric groups $S_{n_1}\wr S_{n_2}\wr \cdots \wr S_{n_k}$. Here, the number fields $L$ are ordered according to discriminant $|{\Delta }_L|:=|Nor{m}_{K/\mathbb{Q}}({\Delta }_{L/K})|$. The results in this paper are motivated by Malle's conjecture. When $n_1=n_2=\cdots =n_k$, these wreath products arise naturally in the study of arboreal Galois representations associated to rational functions over $K$. We prove our results by developing Galois theoretic techniques that have their origins in the study of dynamical systems.

arXiv commentVersion 3: 25 pages, minor corrections, accepted for publication in Ann. Math. Quebec