On the $p$-adic limit of class numbers along a pro-$p$-extension

TL;DR

This paper proves that the non-$p$-part of class numbers in certain pro-$p$-extensions converges $p$-adically and explores deep relationships between various arithmetic invariants in cyclotomic $Z_p$-extensions.

Contribution

It establishes the $p$-adic convergence of class number parts using representation theory and uncovers relationships between arithmetic invariants in cyclotomic extensions.

Findings

Non-$p$-parts of class numbers converge $p$-adically.

Limit of class number parts is independent of the intermediate fields chosen.

Identifies relationships between class number limits, regulators, discriminants, and algebraic $K_2$-groups.

Abstract

Let $K/k$ be a pro-$p$-extension over a number field $k$ whose Galois group is finitely generated and $k_0\subseteq k_1\subseteq\cdots\subseteq k_n\subseteq\cdots$ an ascending sequence of intermediate fields of $K/k$ such that $k_n/k$ is normal, $[k_n:k]<\infty$ and $\bigcup_{n\ge 0} k_n=K$. We will show by using representation theory of finite groups that the non-$p$-part $h_n(p')$ of the class number of $k_n$ converges $p$-adically as $n\rightarrow\infty$, and the limit is independent to the choice of $k_n$'s. Also, in the case where $K/k$ is the cyclotomic $\mathbb{Z}_p$-extension over an abelian number field $k$, we will take an analytic approach and obtain certain enigmatic relationships between the $p$-adic limits of vaious arithmetic invariants along $K/k$, namely, the class number, the ratio of $p$-adic regulator and the square root of the discriminant, and the order of the algebraic $K_2$-group of the ring of integers.