Serre-Tate theory for Calabi-Yau varieties

TL;DR

This paper extends Serre-Tate theory to Calabi-Yau varieties, constructing canonical liftings modulo p^2 and establishing Frobenius liftings on their moduli spaces, with implications for understanding their deformation theory.

Contribution

It develops a Serre-Tate type theory for Calabi-Yau varieties, generalizing previous results from abelian varieties and K3 surfaces, and introduces explicit constructions of Frobenius liftings.

Findings

Constructed canonical liftings modulo p^2 for Calabi-Yau varieties.

Established Frobenius liftings on the moduli space of such varieties.

Showed the crystalline Frobenius preserves the Hodge filtration.

Abstract

Classical Serre-Tate theory describes deformations of ordinary abelian varieties. It implies that every such variety has a canonical lift to characteristic zero and equips its local moduli space with a Frobenius lifting and canonical multiplicative coordinates. A variant of this theory has been obtained for ordinary K3 surfaces by Nygaard and Ogus. In this paper, we construct canonical liftings modulo $p^2$ of varieties with trivial canonical class which are ordinary in the weak sense that the Frobenius acts bijectively on the top cohomology of the structure sheaf. Consequently, we obtain a Frobenius lifting on the moduli space of such varieties. The quite explicit construction uses Frobenius splittings and a relative version of Witt vectors of length two. If the variety has a smooth deformation space and bijective first higher Hasse-Witt operation, the Frobenius lifting gives rise to canonical coordinates. One of the key features of our liftings is that the crystalline Frobenius preserves the Hodge filtration. We also extend Nygaard's approach from K3 surfaces to higher dimensions, and show that no nontrivial families of such varieties exist over simply connected bases with no global one-forms.