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# Ekedahl-Oort strata for good reductions of Shimura varieties of Hodge type

Published:2017-12-19
Printed: Apr 2018
• Chao Zhang,
Yau Mathematical Sciences Center, Tsinghua University, Beijing
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## Abstract

For a Shimura variety of Hodge type with hyperspecial level structure at a prime~$p$, Vasiu and Kisin constructed a smooth integral model (namely the integral canonical model) uniquely determined by a certain extension property. We define and study the Ekedahl-Oort stratifications on the special fibers of those integral canonical models when $p\gt 2$. This generalizes Ekedahl-Oort stratifications defined and studied by Oort on moduli spaces of principally polarized abelian varieties and those defined and studied by Moonen, Wedhorn and Viehmann on good reductions of Shimura varieties of PEL type. We show that the Ekedahl-Oort strata are parameterized by certain elements $w$ in the Weyl group of the reductive group in the Shimura datum. We prove that the stratum corresponding to $w$ is smooth of dimension $l(w)$ (i.e. the length of $w$) if it is non-empty. We also determine the closure of each stratum.
 Keywords: Shimura variety, F-zip
 MSC Classifications: 14G35 - Modular and Shimura varieties [See also 11F41, 11F46, 11G18] 11G18 - Arithmetic aspects of modular and Shimura varieties [See also 14G35]

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