Stability of 2-class groups in the ${\mathbb{Z}}_2$-extension of certain real biquadratic fields

Abstract

Greenberg's conjecture on the stability of $\ell$-class groups in the cyclotomic ${\mathbb{Z}}_{\ell }$-extension of a real field has been proven for various infinite families of real quadratic fields for the prime $\ell =2$. In this work, we consider an infinite family of real biquadratic fields $K$. With some extensive use of elementary group theoretic and class field theoretic arguments, we investigate the $2$-class groups of the $n$-th layers $K_n$ of the cyclotomic ${\mathbb{Z}}_2$-extension of $K$ and verify Greenberg's conjecture. We also relate capitulation of ideal classes of certain sub-extensions of $K_n$ to the relative sizes of the $2$-class groups.

18 pages