Abstract
Kisin and Pappas (Publ Math Inst Hautes Études Sci, 2018) constructed integral models of Hodge-type Shimura varieties with parahoric level structure at \(p>2\), such that the formal neighbourhood of a mod p point can be interpreted as a deformation space of p-divisible group with some Tate cycles (generalising Faltings’ construction). In this paper, we study the central leaf and the closed Newton stratum in the formal neighbourhoods of mod p points of Kisin–Pappas integral models with parahoric level structure; namely, we obtain the dimension of central leaves and the almost product structure of Newton strata. In the case of hyperspecial level structure (i.e., in the good reduction case), our main results were already obtained by Hamacher (Math Z 287(3–4):1255–1277, 2017), and we show that the result of this paper holds for ramified groups as well.
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Notes
It is often convenient and natural to allow \(\mathfrak {X}\) to be an analytic space or a formal scheme. But it will be quite obvious how to adapt the subsequent discussion to these cases.
By replacing \(\nu _{\dot{\tilde{w}}}\) with \(r\nu _{\dot{\tilde{w}}}\) for some r, the claim reduces to a familiar statement on cocharacters of split tori.
We assume that \(p\not \mid |\pi _1(G^\mathrm {der})|\) to ensure that the local model \(\mathrm {M}^\mathrm {loc}_{\mathcal {G}^\circ ,\{\mu \}}\) is normal (so the “deformation ring” \(R_\mathcal {G}\) is normal). The rest of the assumptions are made because we need to have [24, Proposition 1.4.3] for the deformation theory, which is proved under the assumption that G splits after a tame extension and the adjoint group of G has no factor of type \(E_8\).
See also [10, §3.2] for the statement and argument which works for more general connected reductive groups over \(\mathbb {Z}_p\) instead of \(\mathrm {GL}(\Lambda )\).
Here, one can give an alternative construction of \(\jmath _\xi \) without using Proposition 5.2.3, as follows. Since \(X_\xi \) has constant Newton polygon, it is known that there exists a unique isomorphism of F-isocrystals \(M_\xi [\tfrac{1}{p}] \xrightarrow {\sim }W(R)\otimes _{\breve{\mathbb {Z}}_p}\varvec{\mathrm {M}}[\tfrac{1}{p}]\) that reduces to the identity map on \(\varvec{\mathrm {M}}[\tfrac{1}{p}]\) via \(W(R)\twoheadrightarrow W(\overline{\mathbb {F}}_p) = \breve{\mathbb {Z}}_p\). (The existence of such isomorphism is proved in [18, Theorem 2.7.4], and the uniqueness follows from [36, Lemma 3.9]). Now, by the Dieudonné theory over R (cf. [1, Corollaire 3.4.3]), the above isomorphism of F-isocrystals gives rise to a unique quasi-isogeny \(X_\xi \dashrightarrow \mathbb {X}_R\) of p-divisible groups over \(SpecR\), which should recover \(\jmath _\xi \) by uniqueness.
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Acknowledgements
The author thanks Paul Hamacher for many helpful discussions, and Julien Hauseux for his help with group theory used in the proof of Proposition 3.1.4. The author would also like to thank the anonymous referee whose comments were greatly helpful. This work was supported by the EPSRC (Engineering and Physical Sciences Research Council) in the form of EP/L025302/1.
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Kim, W. On central leaves of Hodge-type Shimura varieties with parahoric level structure. Math. Z. 291, 329–363 (2019). https://doi.org/10.1007/s00209-018-2086-1
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DOI: https://doi.org/10.1007/s00209-018-2086-1