Chia-Fu Yu
We describe the supersingular locus of the Siegel 3-fold with a parahoric level structure. We also study its higher dimensional generalization. Using this correspondence and a deep result of Li and Oort, we evaluate the number of irreducible components of the supersingular locus of the Siegel moduli space $\calA_{g,1,N}$ for arbitrary $g$.
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Recently Ibukiyama proves an explicit formula for the number of certain non-principal polarizations on a superspecial abelian surface, extending his earlier work with Katsura for principal polarizations [Compos. Math. 1994]. As a consequence of Ibukiyama's formula, there exists a geometrically irreducible component of the Siegel supersingular locus which is defined over the prime finite field. In this note we give a direct proof of this result.
On the supersingular locus of the Siegel modular variety of genus 3 or 4
本文研究亏格3或4的Siegel模簇的超奇异轨迹，在perfection后将其分解为Coxeter型Deligne-Lusztig簇与有限维仿射空间乘积的不交并。
A classical observation of Deligne shows that, for any prime $p \geq 5$, the divisor polynomial of the Eisenstein series $E_{p-1}(z)$ mod $p$ is closely related to the supersingular polynomial at $p$, $$S_p(x) := \prod_{E/\bar{\mathbb{F}}_p \text{ supersingular}}(x-j(E)) \in \mathbb{F}_p[x].$$ Deuring, Hasse, and Kaneko and Zagier found other families of modular forms which also give the supersingular polynomial at $p$. In a new approach, we prove an analogue of Deligne's result for the Hecke trace forms $T_k(z)$ defined by the Hecke action on the space of cusp forms $S_k$. We use the Eichler-…
The number of Ekedahl-Oort strata intersecting the supersingular locus in the Siegel modular variety
本文证明了 Siegel 模簇中与超奇异轨迹相交的 Ekedahl-Oort 层数 $N_g$ 在 $g$ 趋于无穷时渐近等于 $2^{g-1}$。
In this note, we study Shimura varieties for the groups $\mathrm{GU}(V)$, where $V$ is a Hermitian space relative to a CM extension $E/E^+$. We give a description of the supersingular locus of the fiber at a prime $ν$ over $p$ of such a Shimura variety, under the assumptions that $\mathrm{dim}_E(V) \leq 4$ and that the prime $p$ splits completely in $E^+$ and is unramified in $E$.
We study moduli spaces of abelian varieties in positive characteristic, more specifically the moduli space of principally polarized abelian varieties on the one hand, and the analogous space with Iwahori type level structure, on the other hand. We investigate the Ekedahl-Oort stratification on the former, the Kottwitz-Rapoport stratification on the latter, and their relationship. In this way, we obtain structural results about the supersingular locus in the case of Iwahori level structure, for instance a formula for its dimension in case $g$ is even.
We study the structure of the supersingular locus of the Rapoport--Zink integral model of the Shimura variety for $\mathrm{GU}(2,2)$ over a ramified odd prime with the special maximal parahoric level. We prove that the supersingular locus equals the disjoint union of two basic loci, one of which is contained in the flat locus, and the other is not. We also describe explicitly the structures of basic loci. More precisely, the former one is purely $2$-dimensional, and each irreducible component is birational to the Fermat surface. On the other hand, the latter one is purely $1$-dimensional, and …
We describe the structure of the supersingular locus of a Shimura variety for a quaternionic unitary similitude group of degree $2$ over a ramified odd prime $p$ if the level at $p$ is given by a special maximal compact open subgroup. More precisely, we show that such a locus has purely $2$-dimensional, and every irreducible component is birational to the Fermat surface. Furthermore, we have an estimation of the numbers of connected and irreducible components. To prove these assertions, we completely determine the structure of the underlying reduced scheme of the Rapoport--Zink space for the q…
Let $\mathcal Q_D$ be the set of moduli points on Siegel's modular threefold whose corresponding principally polarized abelian surfaces have quaternionic multiplication by a maximal order $\mathcal O$ in an indefinite quaternion algebra of discriminant $D$ over $\mathbb Q$ such that the Rosati involution coincides with a positive involution of the form $α\mapstoμ^{-1}\overlineαμ$ on $\mathcal O$ for some $μ\in\mathcal O$ with $μ^2+D=0$. In this paper, we first give a formula for the number of irreducible components in $\mathcal Q_D$, strengthening an earlier result of Rotger. Then for each irr…
On the non-Transversality of the Hyperelliptic Locus and the Supersingular Locus for $g=3$
本文研究亏格3时西格尔模叠中超椭圆轨迹与超奇异轨迹的交点性质，给出非横截交点的判别准则，并证明对无穷多个素数p存在此类交点。
We investigate Siegel modular varieties in positive characteristic with Iwahori level structure. On these spaces, we have the Newton stratification, and the Kottwitz-Rapoport stratification; one would like to understand how these stratifications are related to each other. We give a simple description of all KR strata which are entirely contained in the supersingular locus as disjoint unions of Deligne-Lusztig varieties. We also give an explicit numerical description of the KR stratification in terms of abelian varieties.
On the Supersingular Locus of the $\mathrm{GU}(2,n-2)$ Shimura Variety
本文研究$\mathrm{GU}(2,n-2)$ Shimura簇在惰性素数处约化的超奇异轨迹，通过完美化将其分解为经典Deligne-Lusztig簇上的迭代纤维积的不交并，并当$p\geq3$时证明了Oort猜想在Siegel模簇上的类比。
In this article we prove several level lowering results for cuspidal automorphic representations occurring in the cohomology of the Siegel modular threefold with paramodular level structure by adapting a method of Ribet in his proof of the Serre's epsilon conjecture. The proof is purely geometric and relies on the description of the supersingular locus of certain quaternionic unitary Shimura variety and an arithmetic level raising result on this Shimura variety. The heart of the proof is a comparison of the dimension of the space of vanishing cycles on the paramodular Siegel modular threefold …
In a letter to Tate, Serre proves that the systems of Hecke eigenvalues given by modular forms (mod p) are the same as the ones given by locally constant functions on an adelic double coset space constructed from the endomorphism algebra of a supersingular elliptic curve. We generalize this result to Siegel modular forms (mod p), proving that the systems of Hecke eigenvalues obtained from them are the same as the ones given by algebraic modular forms (mod p) on a quaternionic unitary group, as defined by Gross. The correspondence is obtained by restricting to the superspecial locus of the modu…
Under the axiom of He and Rapoport for the stratifications of Shimura varieties, we explain a result of Görtz, He and Nie that the EKOR strata contained in the basic loci can be described as a disjoint union of Deligne-Lusztig varieties. In the special case of Siegel modular varieties, we compare their descriptions to that of Görtz and Yu for the supersingular Kottwitz-Rapoport strata and to the descriptions of Harashita, Hoeve for the supersingular Ekedahl-Oort strata.
In a letter to Tate (published in Israel J. Math. in 1996), J.-P. Serre proves that the systems of Hecke eigenvalues given by modular forms (mod p) are the same as the ones given by locally constant functions on an adelic double coset space constructed from the endomorphism algebra of a supersingular elliptic curve. We generalize this result to Siegel modular forms, proving that the systems of Hecke eigenvalues given by Siegel modular forms (mod p) are the same as the ones given by algebraic modular forms (mod p) on a quaternionic unitary group, as defined by Gross in Israel J. Math. in 1999. …
Hilbert surfaces, modular forms, and Siegel-Veech constants
本文研究了翻译表面中普林姆特征形式定位的希伯特-维克常数，证明其在不同定位间一致，并揭示了相关希伯特模曲面的欧拉特征关系。
A geometric Jacquet-Langlands correspondence for paramodular Siegel threefolds
本文研究仿模层级西格尔模三重fold的 Picard-Lefschetz 公式，证明权-单值猜想，并借助 Rapoport-Zink 一致化构造 GSp₄ 与定内形式间的几何 Jacquet-Langlands 对应，同时得到整数值结果与降层级定理。
$p$-adic Equidistribution of Special Loci in a Product of Modular Curves
该论文研究定义在ℂₚ上的光滑曲线C与模子簇Y₀(n)交轨迹的p进等分布性，给出等分布成立的条件，证明对应交集合非离散，并给出等分布不成立的典型例子。
Some generalizations of Oort's conjecture
该研究针对素数p≥5，推广Oort猜想，证明g≥4时主极化超奇异阿贝尔簇模空间中自同构群非$\boldsymbol{\boldsymbol{\times1}}$的补集余维数至少为2，通过仿射Deligne--Lusztig簇相关命题完成证明。
We study the intersection of the Torelli locus with the Newton polygon stratification of the modulo $p$ reduction of certain PEL-type Shimura varieties. We develop a clutching method to show that the intersection of the open Torelli locus with some Newton polygon strata is non-empty. This allows us to give a positive answer, under some compatibility conditions, to a question of Oort about smooth curves in characteristic $p$ whose Newton polygons are an amalgamate sum. As an application, we produce infinitely many new examples of Newton polygons that occur for smooth curves that are cyclic cove…
Degrees of Genus-Two Split-Jacobian Loci and Humbert-Form Reconstruction
该研究推导了二次亏格分裂雅可比轨迹相关西格尔模形式的次数关系，确定洪伯特曲面相关模形式的限制与系数，将多项式计算转化为线性问题并完成n=5的实例计算。
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