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arXiv 2407.01276math.AG
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三维簇的诺特不等式及三个极小体积模空间

The Noether inequality for threefolds and three moduli spaces with minimal volumes

Meng Chen, Yong Hu, Chen Jiang

AI总结:

本文建立了几何亏格5≤p_g≤10的一般型射影三维簇的诺特不等式,解决三维簇诺特不等式所有剩余情形,并证明三个特定极小体积典范三维簇的粗模空间是不可约单有理簇且成员单连通。

摘要:

我们建立了诺特不等式\textrm{Vol}(X)\u2265 \frac{4}{3}p_g(X)-\frac{10}{3},适用于所有几何亏格5\u2264p_g(X)\u226410的一般型射影三维簇X,其中\textrm{Vol}(X)是典范体积。该结果解决了三维簇诺特不等式的所有剩余情形。我们进一步研究了小亏格且极小体积的典范三维簇的模空间:对于几何亏格2、极小典范体积为\frac{1}{3}的一般型三维簇,证明其典范模型是\bbP(1,1,2,3,8)中16次超曲面,明确描述了其典范环;这表明参数化所有典范体积为\frac{1}{3}、几何亏格2的典范三维簇的粗模空间\uc2ac_M_{\frac{1}{3}, 2}是189维不可约单有理簇。类似研究显示\uc2ac_M_{1, 3}是236维不可约单有理簇,\uc2ac_M_{2, 4}是270维不可约单有理簇,且这三个家族中的每个成员都是单连通的。

英文摘要:

We establish the Noether inequality \[\textrm{Vol}(X)\geq \frac{4}{3}p_g(X)-\frac{10}{3}\] for all projective $3$-folds $X$ of general type with geometric genus $5\leq p_g(X)\leq 10$ where $\textrm{Vol}(X)$ is the canonical volume. This result resolves all remaining cases of the Noether inequality for $3$-folds. We further investigate the moduli spaces of canonical $3$-folds with small genera and minimal volumes. For a $3$-fold of general type with geometric genus $2$ and with minimal canonical volume $\frac{1}{3}$, we prove that its canonical model is a hypersurface of degree $16$ in $\mathbb{P}(1,1,2,3,8)$, which gives an explicit description of its canonical ring. This implies that the coarse moduli space $\mathcal{M}_{\frac{1}{3}, 2}$, parametrizing all canonical $3$-folds with canonical volume $\frac{1}{3}$ and geometric genus $2$, is an irreducible unirational variety of dimension $189$. Parallel studies show that $\mathcal{M}_{1, 3}$ is irreducible, unirational, and $236$-dimensional, and that $\mathcal{M}_{2, 4}$ is irreducible, unirational, and $270$-dimensional. As being conceived, every member in these 3 families is simply-connected.

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