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arXiv math/0006222math.AG
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分歧情形下的局部模型I。EL案件

Local models in the ramified case I. The EL-case

G. Pappas, M. Rapoport

摘要:

局部模型是用线性代数术语定义的方案,它描述了Shimura簇和其他模空间的积分模型的标准局部结构。我们指出,平坦性猜想的Rapoport-Zink局部模型失败的存在下的分歧,在这种情况下,必须修改其定义。我们详细研究了G= R_{E/F} GL(n)的局部模型的某些修改,其中E/F是完全分歧的扩展,并且针对最大parahoric水平子群。这些模型的特殊纤维是仿射格拉斯曼的子方案。我们表明,新的本地模型是顺利相当于“秩品种”的矩阵,是平坦的,正常的,合理的奇异性,其特殊的纤维包含预期的舒伯特地层。一个推论是,舒伯特品种在仿射格拉斯曼光滑等价于幂零轨道闭包,是正常的理性奇点,甚至在积极的特点。我们给出了一些应用程序的计算层附近的周期和描述的关系与几何卷积。最后,在一般EL的情况下,我们取代Rapoport-Zink的平坦性猜想的猜想修改后的本地模型。

英文摘要:

Local models are schemes defined in linear algebra terms that describe the 'etale local structure of integral models for Shimura varieties and other moduli spaces. We point out that the flatness conjecture of Rapoport-Zink on local models fails in the presence of ramification and that in that case one has to modify their definition. We study in detail certain modifications of the local models for G=R_{E/F}GL(n), with E/F a totally ramified extension, and for a maximal parahoric level subgroup. The special fibers of these models are subschemes of the affine Grassmannian. We show that the new local models are smoothly equivalent to "rank varieties" of matrices, are flat, normal, with rational singularities and that their special fibers contain the expected Schubert strata. A corollary is that Schubert varieties in the affine Grassmannian are smoothly equivalent to nilpotent orbit closures and are normal with rational singularities, even in positive characteristics. We give some applications to the calculation of sheaves of nearby cycles and describe a relation with geometric convolution. Finally, in the general EL case, we replace the flatness conjecture of Rapoport-Zink with a conjecture about the modified local models.

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