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arXiv 1012.1838math.NTmath.AG
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关于三次曲面的Mordell-Weil生成元个数

On the number of Mordell-Weil generators for cubic surfaces

Samir Siksek

摘要:

设S是域K上的光滑三次曲面。众所周知,新的K-有理点可以通过割线和切线构造从旧的K-有理点获得。Mordell-Weil生成集是S(K)的最小基数子集B,它通过连续的割线和切线构造生成S(K)。设r(S,K)是这样一个Mordell-Weil生成集的基数。马宁提出了被称为三次曲面的莫德尔-韦伊问题:如果K是在其素子域上生成的,那么r(S,K)是有限的。本文证明了这一猜想的一个特例。也就是说,如果S包含两条定义在K上的斜直线,则r(S,K)= 1。研究三次曲面上的割线和切线过程的困难之一是它不像椭圆曲线那样导致结合的二元运算。作为一种部分的补救,我们引入了一个阿贝尔群H_S(K),它与一个由K-有理点自然生成的三次曲面S/K相联系,它保留了许多关于割线和切线过程的信息。特别地,只要H_S(K)的最小生成元数大,则r(S,K)也大。在弱近似成立的情况下,H_S具有良好的局部到全局性质。我们利用这些构造一族有理数上的光滑三次曲面,使得r(S,K)在这个族中是无界的.这是椭圆曲线秩猜想无界性的三次曲面模拟。

英文摘要:

Let S be a smooth cubic surface over a field K. It is well-known that new K-rational points may be obtained from old ones by secant and tangent constructions. A Mordell-Weil generating set is a subset B of S(K) of minimal cardinality which generates S(K) via successive secant and tangent constructions. Let r(S,K) be the cardinality of such a Mordell-Weil generating set. Manin posed what is known as the Mordell-Weil problem for cubic surfaces: if K is finitely generated over its prime subfield then r(S,K) is finite. In this paper, we prove a special case of this conjecture. Namely, if S contains two skew lines both defined over K then r(S,K) = 1. One of the difficulties in studying the secant and tangent process on cubic surfaces is that it does not lead to an associative binary operation as in the case of elliptic curves. As a partial remedy we introduce an abelian group H_S(K) associated to a cubic surface S/K, naturally generated by the K-rational points, which retains much information about the secant and tangent process. In particular, r(S, K) is large as soon as the minimal number of generators of H_S(K) is large. In situations where weak approximation holds, H_S has nice local-to-global properties. We use these to construct a family of smooth cubic surfaces over the rationals such that r(S,K) is unbounded in this family. This is the cubic surface analogue of the unboundedness of ranks conjecture for elliptic curves.

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