The Crystals Associated to Barsotti-Tate Groups

Me] W. Messing, The crystals associated to Barsotti-Tate groups with applications to abelian schemes. Lecture Notes in Math. 264, Springer-Verlag, 1972. 

The Crystals Associated to Barsotti-Tate Groups with Applications to Abelian Schemes... 

In this paper Barsotti-Tate groups over an arbitrary base scheme S (on which p is locally nilpotent) are studied. Intuitively we can think of these as nicely varying families of Barsotti-Tate groups parametrized by S. The basic theorems are those which allow us to associate various crystals to such groups. Actually in order to construct these crystals we assume that the Barsotti-Tate groups in question are, locally, liftable to infinitesimal neighborhoods. This restriction is surely unnecessary. 

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