This paper is a continuation of our paper math.AG/0006222. We study the reduction of certain PEL Shimura varieties with parahoric level structure at primes p at which the group that defines the Shimura variety ramifies. We describe "good" $p$-adic integral models of these Shimura varieties and study their 'etale local structure. In particular, we exhibit a stratification of their (singular) special fibers and give a partial calculation of the sheaf of nearby cycles.
In this paper, we study special cycles on the Krämer model of $\mathrm{U}(1,1)(F/F_0)$-Rapoport-Zink spaces where $F/F_0$ is a ramified quadratic extension of $p$-adic number fields with the assumption that the $2$-dimensional hermitian space of special quasi-homomorphisms is anisotropic. We write down the decomposition of these special cycles and prove a version of Kudla-Rapoport conjecture in this case. We then apply the local results to compute the intersection numbers of special cycles on unitary Shimura curves and relate these intersection numbers to Fourier coefficients of central deriva…
Group schemes and local densities of ramified hermitian lattices in residue characteristic 2 Part I
对厄米特格(L,H)的局部-整体原理的阻碍可以通过计算(L,H)的质量来量化。质量公式将(L,H)的质量表示为局部因子的乘积,称为(L,H)的局部密度。局部密度公式是已知的,除了在一个分歧埃尔米特格的剩余特征2的情况下。设F是Q_2的有限非分歧域扩张。分支二次扩张E/F分为两种情况,我们称之为情况1和情况2。本文通过对一个合适的酉群构造一个光滑整群模型,得到了情形1中分歧厄米特格的局部密度公式。因此,本文结合W. T. GAN和J. - K. Yu,允许在情况1中计算厄米晶格(L,H)的质量公式。
Topological flatness of local models for ramified unitary groups. I. The odd dimensional case
局部模型是根据线性代数模问题定义的某些方案,其给出由Rapoport和Zink定义的某些p-adic PEL Shimura簇的积分模型的étale局部邻域。当定义志村簇的群在p处分歧时,最初定义的局部模型(以及志村模型)可能不是平坦的,并且需要修改它们的定义以获得平坦的方案。在酉相似群在Q_p处的局部化是分歧的情况下,准分裂GU_n,Pappas和Rapoport在定义原局部模型的模问题上增加了新的条件,即所谓的楔条件和旋条件,并证明了新的局部模型是平坦的。我们证明了他们猜想的一个初步形式,即他们的新模型是拓扑平坦的,在n是奇数的情况下。
Topological flatness of local models for ramified unitary groups. II. The even dimensional case
局部模型是根据线性代数模问题定义的方案,它给出了由Rapoport和Zink定义的某些p-adic PEL Shimura簇的积分模型的étale局部邻域。对于在Q_p上的局部化是分歧的么正相似群(准分裂GU_n),帕帕斯观察到原始局部模型通常不是平坦的,并且他和拉波波特为原始模问题引入了新的条件,他们推测这会产生平坦的方案。在以前的文件中,我们证明了他们的新的局部模型是拓扑平坦的,当n是奇数。本文证明了n为偶数时的拓扑平坦性。一路上,我们的特征的mu-容许集的某些cocharacters亩在类型B和D,我们表明,这些cocharacters的容许性可以在顶点的方式,证实了一个猜想的帕帕斯和Rapoport。
We study integral models, so-called Pappas-Rapoport or splitting models, of some PEL Shimura Varieties whose data are ramified at a prime p. We show that except in a specific case, these models are smooth when there is no level at p, and we study their special fiber modulo p. We show that interesting stratifications appear, and we show that the $μ$-ordinary locus is open and dense, following a strategy of Wedhorn in the unramified case.
The Deligne--Simpson problem is an existence problem for connections with specified local behavior. Almost all previous work on this problem has restricted attention to connections with regular or unramified singularities. Recently, the authors, together with Kulkarni and Matherne, formulated a version of the Deligne--Simpson problem where certain ramified singular points are allowed and solved it for the case of Coxeter connections, i.e., connections on the Riemann sphere with a maximally ramified singularity at zero and (possibly) an additional regular singular point at infinity. A certain m…
We study the geometry of equicharacteristic partial affine flag varieties associated to tamely ramified groups $G$ in characteristics $p>0$ dividing the order of the fundamental group $π_1(G_{\text{der}})$. We obtain that most Schubert varieties are not normal and provide an explicit criterion for when this happens. Apart from this, we show, on the one hand, that loop groups of semisimple groups satisfying $p \mid \lvert π_1(G_{\text{der}})\rvert$ are not reduced, and on the other hand, that their integral realizations are ind-flat. Our methods allow us to classify all tamely ramified Pappas-Z…
In their study of local models of Shimura varieties for totally ramified extensions, Pappas and Rapoport posed a conjecture about the reducedness of a certain subscheme of $n \times n$ matrices. We give a positive answer to their conjecture in full generality. Our main ideas follow naturally from two of our previous works. The first is our proof of a conjecture of Kreiman, Lakshmibai, Magyar, and Weyman on the equations defining type A affine Grassmannians. The second is the work of the first two authors and Kamnitzer on affine Grassmannian slices and their reduced scheme structure. We also pr…