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arXiv 2009.13262math.NT
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关于二次域的交换$2$ -分歧扭模

On abelian $2$-ramification torsion modules of quadratic fields

Jianing Li, Yi Ouyang, Yue Xu

摘要:

对于数域$F$和素数$p$,$F$在$F$上的非分歧的极大交换pro-$p$扩张的Galois群的$\mathbb{Z}_p$ -扭模(记作$\mathcal{T}_p(F)$)是交换$p$ -分歧理论中的一个重要课题.本文研究二次域$F=\mathbb{Q}(\sqrt{ m})$的群$\mathcal{T}_2(F)=\mathcal{T}_2(m)$.首先,假设$m>0$,我们证明了一个明确的$4$ -秩公式$\mathcal{T}_2(-m)$。应用这个公式,我们还得到了虚二次域的$\mathcal{T}_2$ -群的$4$ -秩密度.其次,对于奇素数$l$,我们得到了$\mathcal{T}_2(\pm l)$和$\mathcal{T}_2(\pm 2l)$的阶的$2$ -整除性的结果.特别地,我们发现,如果$l\equiv 7\bmod{8}$,则$\#\mathcal{T}_2(l)\equiv 2\# \mathcal{T}_2(2l)\equiv h_2(-2l)\bmod{16}$,其中$h_2(-2l)$是$\mathbb{Q}(\sqrt{-2l})$的$2$类编号。然后,我们获得$\mathcal{T}_2(\pm l)$和$\mathcal{T}_2(\pm 2l)$的密度结果。最后,基于我们的密度结果和数值数据,我们提出了关于$\mathcal{T}_p(F)$的分布图时,$F$变化的实或虚二次域的任何总理$p$,以及关于$\mathcal{T}_2(\pm l)$和$\mathcal{T}_2(\pm 2 l)$时,$l$变化,在科恩-Lenstra数学的精神。我们的猜想在$\mathcal{T}_2(l)$的情况下是密切相关的桑克斯-西姆-华盛顿的猜测$2$ -adic $L$ -函数的零点的分布和基本单位的分布。

英文摘要:

For a number field $F$ and a prime number $p$, the $\mathbb{Z}_p$-torsion module of the Galois group of the maximal abelian pro-$p$ extension of $F$ unramified outside $p$ over $F$, denoted as $\mathcal{T}_p(F)$, is an important subject in abelian $p$-ramification theory. In this paper we study the group $\mathcal{T}_2(F)=\mathcal{T}_2(m)$ of the quadratic field $F=\mathbb{Q}(\sqrt{ m})$. Firstly, assuming $m>0$, we prove an explicit $4$-rank formula for $\mathcal{T}_2(-m)$. Furthermore, applying this formula, we obtain the $4$-rank density of $\mathcal{T}_2$-groups of imaginary quadratic fields. Secondly, for $l$ an odd prime, we obtain results about the $2$-divisibility of orders of $\mathcal{T}_2(\pm l)$ and $\mathcal{T}_2(\pm 2l)$. In particular we find that $\#\mathcal{T}_2(l)\equiv 2\# \mathcal{T}_2(2l)\equiv h_2(-2l)\bmod{16}$ if $l\equiv 7\bmod{8}$ where $h_2(-2l)$ is the $2$-class number of $\mathbb{Q}(\sqrt{-2l})$. We then obtain density results for $\mathcal{T}_2(\pm l)$ and $\mathcal{T}_2(\pm 2l)$. Finally, based on our density results and numerical data, we propose distribution conjectures about $\mathcal{T}_p(F)$ when $F$ varies over real or imaginary quadratic fields for any prime $p$, and about $\mathcal{T}_2(\pm l)$ and $\mathcal{T}_2(\pm 2 l)$ when $l$ varies, in the spirit of Cohen-Lenstra heuristics. Our conjecture in the $\mathcal{T}_2(l)$ case is closely connected to Shanks-Sime-Washington's speculation on the distributions of the zeros of $2$-adic $L$-functions and to the distributions of the fundamental units.

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