On the maximal unramified pro-2-extension of $\mathbb{Z}_2$-extension of certain real biquadratic fields

Abstract

For any positive integer $n$, we show that there exists a real number field $k$ (resp. $k'$) of degree $2^{n+2}$ whose $2$-class group is isomorphic to $\mathbb{Z}/2\mathbb{Z}\times \mathbb{Z}/2\mathbb{Z}$ such that the Galois group of the maximal unramified extension of $k$ (resp. $k'$) over $k$ (resp. $k'$) is abelian (resp. non abelian, more precisely isomorphic to $Q_8$ or $D_8$, the quaternion and the dihedral group of order $8$ respectively). In fact, we construct the first examples in the literature of families of real biquadratic fields for which the layers of the cyclotomic $\mathbb{Z}_2$-extension satisfy the previous conditions and whose unramified abelian $2$-Iwasawa modules are isomorphic to $\mathbb{Z}/2\mathbb{Z}\times \mathbb{Z}/2\mathbb{Z}$; hence these fields satisfy Greenberg's conjecture.