$\ell$-class groups of fields in Kummer towers

Abstract

Let $\ell$ and $p$ be prime numbers and $K_{n,m}=\mathbb Q(p^{\frac{1}{\ell^n}},\zeta_{2\ell^{m}})$. We study the $\ell$-class group of $K_{n,m}$ in this paper. When $\ell=2$, we determine the structure of the $2$-class group of $K_{n,m}$ for all $(n,m)\in \mathbb Z_{\geq 0}^2$ in the case $p\equiv 3, 5\bmod{8}$, and for $(n,m)=(n,0)$, $(n,1)$, or $(1,m)$ in the case $p\equiv 7\bmod{16}$, generalizing the results of Parry about the $2$-divisibility of the class number of $K_{2,0}$. We also obtain results about the $\ell$-class group of $K_{n,m}$ when $\ell$ is odd and in particular when $\ell=3$. The main tools we use are class field theory, including Chevalley’s ambiguous class number formula and its generalization by Gras, and a stationary result about the $\ell$-class groups in the $2$-dimensional Kummer tower $\{K_{n,m}\}$.