Extending p-divisible Groups and Barsotti–Tate Deformation Ring in the Relative Case

Abstract

Abstract Let $k$ be a perfect field of characteristic $p> 2$, and let $K$ be a finite totally ramified extension of $W(k)\big[\frac{1}{p}\big]$ of ramification degree $e$. We consider an unramified base ring $R_0$ over $W(k)$ satisfying certain conditions, and let $R = R_0\otimes _{W(k)}\mathcal{O}_K$. Examples of such $R$ include $R = \mathcal{O}_K[\![s_1, \ldots , s_d]\!]$ and $R = \mathcal{O}_K\langle t_1^{\pm 1}, \ldots , t_d^{\pm 1}\rangle $. We show that the generalization of Raynaud’s theorem on extending $p$-divisible groups holds over the base ring $R$ when $e < p-1$, whereas it does not hold when $R = \mathcal{O}_K[\![s]\!]$ with $e \geq p$. As an application, we prove that if $R$ has Krull dimension $2$ and $e < p-1$, then the locus of Barsotti–Tate representations of $\textrm{Gal}(\overline{R}\big[\frac{1}{p}\big]/R\big[\frac{1}{p}\big])$ cuts out a closed subscheme of the universal deformation scheme. If $R = \mathcal{O}_K[\![s]\!]$ with $e \geq p$, we prove that such a locus is not $p$-adically closed.