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arXiv · 2610.10452

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

Somnath Jha·Debanjana Kundu·H. Laxmi·Lawrence C. Washington
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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 $λ$-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.

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Somnath Jha, Debanjana Kundu, H. Laxmi, Lawrence C. Washington. 2026-10-07. $p$-class groups in the cyclotomic $\mathbb{Z}_p$-extension of imaginary quadratic fields. https://arxiv.org/abs/2610.10452

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