Crystal (mathematics)


In mathematics, crystals are Cartesian sections of certain fibered categories. They were introduced by, who named them crystals because in some sense they are "rigid" and "grow". In particular quasicoherent crystals over the crystalline site are analogous to quasicoherent modules over a scheme.
An isocrystal is a crystal up to isogeny. They are p-adic analogues of Ql-adic étale sheaves, introduced by and . Convergent isocrystals are a variation of isocrystals that work better over non-perfect fields, and overconvergent isocrystals are another variation related to overconvergent cohomology theories.
A Dieudonné crystal is a crystal with Verschiebung and Frobenius maps. An F-crystal is a structure in semilinear algebra somewhat related to crystals.

Crystals over the infinitesimal and crystalline sites

The infinitesimal site Inf has as objects the infinitesimal extensions of open sets of X.
If X is a scheme over S then the sheaf OX/S is defined by
OX/S = coordinate ring of T, where we write T as an abbreviation for
an object U → T of Inf. Sheaves on this site grow in the sense that they can be extended from open sets to infinitesimal extensions of open sets.
A crystal on the site Inf is a sheaf F of OX/S modules that is rigid in the following sense:
This is similar to the definition of a quasicoherent sheaf of modules in the Zariski topology.
An example of a crystal is the sheaf OX/S.
Crystals on the crystalline site are defined in a similar way.

Crystals in fibered categories

In general, if E is a fibered category over F, then a crystal is a cartesian section of the fibered category. In the special case when F is the category of infinitesimal extensions of a scheme X and E the category of quasicoherent modules over objects of F, then crystals of this fibered category are the same as crystals of the infinitesimal site.