$p$-class groups in the cyclotomic ${\mathbb{Z}}_p$-extension of imaginary quadratic fields

Abstract

We study the structure of the $p$-class group of the first layer of the cyclotomic ${\mathbb{Z}}_p$-extension of an imaginary quadratic field where $p$ is non-split. We obtain restrictions on the growth of the cyclic factors from the base layer to the first layer; in particular, when the $p$-rank remains unchanged, each cyclic factor grows by exactly one power of $p$. We show that in a large number of cases the Iwasawa $\lambda$-invariant is completely determined by the $p$-rank of the first layer itself. Finally we use a Cohen-Lenstra-Martinet type philosophy to study the probability distribution of Iwasawa lambda invariants of the first layer when the base layer has cyclic $p$-class group.