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arXiv 1004.3695math.AG
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特征为2的超奇椭圆曲线的一个奇异性

A singular property of the supersingular elliptic curve in characteristic 2

Leonardo Zapponi

摘要:

设E是定义在k上的超奇椭圆曲线,它是二元有限域的代数闭包,在k-同构下是唯一的.用0表示它的单位元,设C是E-{0}在群Isom(E)(它是非阿贝尔的,阶数为24)作用下的商。本文的主要结果是:集合C(k)自然地参数化Lamé覆盖的k-同构类,Lamé覆盖是在具有特定分歧基准的三点之外未分歧的射影线的平凡分歧覆盖.这一事实是令人惊讶的原因有两个:首先,它是第一个非平凡的例子,一个家庭的覆盖的射影线unramified以外的三个点,这是参数化的几何点的曲线。此外,当考虑任意特征时,Lamé覆盖的显式构造非常复杂,并且它们的算术性质仍然神秘。当结合从正特性到特性0的提升技术时,特性2中的问题的简单性具有许多深刻的后果。作为一个例子,我们得到了一些关于(局部)伽罗瓦作用的尖锐陈述,例如,在一个固定数域上定义的Lamé覆盖的同构类的数量上的界,仅取决于在2处的剩余扩张的程度。最后,在附录中我们给出了这些结果的部分推广,证明了对任意正整数g,亏格g超椭圆曲线的适当商的k-有理点点参数化了在三点外未分歧的射影线的tamely分歧覆盖的k-同构类。

英文摘要:

Let E be the supersingular elliptic curve defined over k, the algebraic closure of the finite field with two elements, which is unique up to k-isomorphism. Denote by 0 its identity element and let C be the quotient of E-{0} under the action of the group Isom(E) (which is non-abelian, of order 24). The main result of this paper asserts that the set C(k) naturally parametrizes k-isomorphism classes of Lamé covers, which are tamely ramified covers of the projective line unramified outside three points having a particular ramification datum. This fact is surprising for two reasons: first of all, it is the first non-trivial example of a family of covers of the projective line unramified outside three points which is parametrized by the geometric points of a curve. Moreover, when considered in arbitrary characteristic, the explicit construction of Lamé covers is quite involved and their arithmetic properties still remain misterious. The simplicity of the problem in characteristic 2 has many deep consequences when combined with lifting techniques from positive characteristic to characteristic 0. As an illustration, we obtain some sharp statements concerning the (local) Galois action, as, for example, a bound on the number of isomorphism classes of Lamé covers defined over a fixed number field, only depending on the degree of the residual extension at 2. Finally, in the appendix we give a partial generalization of these results by showing that, for any positive integer g, the k-rational points of a suitable quotient of a genus g hyperelliptic curve parametrize k-isomorphism classes of tamely ramified covers of the projective line unramified outside three points.

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