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arXiv 2302.06200math.NT
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某些实二次域的分圆$\mathbb{Z}_{2}$ -扩张中$2$-类群的秩的结构和稳定性

On the structure and stability of ranks of $2$-class groups in cyclotomic $\mathbb{Z}_{2}$-extensions of certain real quadratic fields

Jaitra Chattopadhyay, H Laxmi, Anupam Saikia

摘要:

对于判别式$D_{K}$具有四个不同素因子的实二次域$K= \mathbb{Q}(\sqrt{d})$,我们研究了$K$的分圆$\mathbb{Z}_{2}$ -扩张的第一层$K_{1} = \mathbb{Q}(\sqrt{2},\sqrt{d})$的$2$-类群$A(K_{1})$的结构.通过对$A(K_{1})$的秩和阶的一些适当的假设,刻画了$K$的特征,即$2$类群$A(K)$同构于$\mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z}$ .我们推断,$2$ -秩的类组在每一层中的稳定凭借福田的结果。这也提供了另一种方法来建立$K$的Iwasawa $μ$ -不变量消失。在某些情况下,我们还提供了关于$D_{K}$的构成素因子的充分条件,这些构成素因子包含$A(K) \simeq \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z}$,$A(K_{1}) \simeq \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/4\mathbb{Z}$和$A(K^{\prime}) \simeq \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z}$,其中$K^{\prime} = \mathbb{Q}(\sqrt{2d})$。这推广了Mizusawa的一些结果。

英文摘要:

For a real quadratic field $K= \mathbb{Q}(\sqrt{d})$ with discriminant $D_{K}$ having four distinct prime factors, we study the structure of the $2$-class group $A(K_{1})$ of the first layer $K_{1} = \mathbb{Q}(\sqrt{2},\sqrt{d})$ of the cyclotomic $\mathbb{Z}_{2}$-extension of $K$. With some suitably convenient assumptions on the rank and the order of $A(K_{1})$, we characterize $K$ for which the $2$-class group $A(K)$ is isomorphic to $\mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z}$. We infer that the $2$-ranks of the class groups in each layer stabilizes by virtue of a result of Fukuda. This also provides an alternate way to establish that the Iwasawa $μ$-invariant of $K$ vanishes. In some cases, we also provide sufficient conditions on the constituent prime factors of $D_{K}$ that imply $A(K) \simeq \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z}$, $A(K_{1}) \simeq \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/4\mathbb{Z}$ and $A(K^{\prime}) \simeq \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z} \oplus \mathbb{Z}/2\mathbb{Z}$, where $K^{\prime} = \mathbb{Q}(\sqrt{2d})$. This extends some results obtained by Mizusawa.

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