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arXiv 0905.4798math.NTmath.AG
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投射同余群的提升

Lifts of projective congruence groups

Ian Kiming, Matthias Schuett, Helena Verrill

摘要:

我们证明了SL_2(Z)的投射等价于同余子群的非同余子群是普遍存在的。更确切地说,如果所讨论的同余子群是水平N>2的主同余子群Gamma(N),则它们总是存在,并且它们在许多情况下也存在于Gamma_0(N)。提出这个问题的动机与模形式有关:射影等价群对所有偶数权具有相同的尖点形式空间,而奇数权的尖点形式空间一般是不同的。我们通过对所附椭圆模曲面的几何考虑,对重量为3的这种现象进行了一些初步观察。我们还开发了算法,构造所有的子群投影等价于一个给定的同余子群,并决定其中的同余。这方面的一个关键工具是沃尔法特的广义层次概念。

英文摘要:

We show that noncongruence subgroups of SL_2(Z) projectively equivalent to congruence subgroups are ubiquitous. More precisely, they always exist if the congruence subgroup in question is a principal congruence subgroup Gamma(N) of level N>2, and they exist in many cases also for Gamma_0(N). The motivation for asking this question is related to modular forms: projectively equivalent groups have the same spaces of cusp forms for all even weights whereas the spaces of cusp forms of odd weights are distinct in general. We make some initial observations on this phenomenon for weight 3 via geometric considerations of the attached elliptic modular surfaces. We also develop algorithms that construct all subgroups projectively equivalent to a given congruence subgroup and decides which of them are congruence. A crucial tool in this is the generalized level concept of Wohlfahrt.

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