On automorphisms of some semidirect product groups and ranks of Iwasawa modules

Abstract

Let $p$ be an odd prime number and $k$ an imaginary quadratic field in which $p$ does not split. Based on their heuristic, Kundu and Washington posed a question which asks whether $\lambda$- and $\mu$-invariant of the anti-cyclotomic ${\mathbb{Z}}_p$-extension $k_{\infty }^a$ of $k$ are always trivial. Also, if $k_{\infty }^a/k$ is totally ramified, for $n\ge 1$, they showed that the $p$-part of the ideal class group of the $n$th layer of the anti-cyclotomic ${\mathbb{Z}}_p$-extension of $k$ is not cyclic. In this article, inspired by their paper, we study anti-cyclotomic like ${\mathbb{Z}}_p$-extensions, extending both the above question and Kundu-Washington's result. We show that the values of $\lambda$ of certain anti-cyclotomic like ${\mathbb{Z}}_p$-extensions are always even. We also show the $p$-part of the ideal class groups of certain anti-cyclotomic like ${\mathbb{Z}}_p$-extensions of CM-fields are always not cyclic.