Contents

cohomology

# Contents

## Idea

Crystalline cohomology is the abelian sheaf cohomology with respect to the crystalline site of a scheme. Hence, put more generally, it is the cohomology of de Rham spaces/coreduced objects.

Crystalline cohomology serves to refine the notion of de Rham cohomology for schemes.

Crystalline cohomology is in particular a Weil cohomology and is generalized by the notion of rigid cohomology.

## Definition

Let $X$ be a scheme over a base $S$. The crystalline site $Cris(X/S)$ of $X$ is

• the category whose objects are all nilpotent $S$-immersions $U \hookrightarrow T$, where $U$ is an open set of $X$ and and the ideal on $T$ defining this immersion being endowed with a nilpotent divided power structure (…details…).;

• the Grothendieck topology on this category is the Zariski topology.

If $S$ is of characteristic 0, then $Cris(X/S)$ coincides with the infinitesimal site of $X$. (…details…).

## Properties

### Relation to de Rham space

Crystalline cohomology of $X$ is the cohomology of the de Rham space of $X$. See there for more.

### Relation to differential homotopy type theory

In differential homotopy type theory the infinitesimal flat modality sends coefficients to coefficients for crystalline cohomology.

## References

An original account of the definition of the crystalline topos is section 7, page 299 of

• Alexander Grothendieck, Crystals and de Rham cohomology of schemes , chapter IX in Dix Exposes sur la cohomologie des schemas (pdf)

A more recent account is

• Luc Illusie, Crystalline cohomology, in Motives, Proc. Sympos. Pure Math., vol. 55, part 1, Amer. Math. Soc. Providence, RI, 1994, 43–70.

Discussion of this in the modern context of higher geometry/D-geometry is in

A p-adic cohomology for varieties in characteristic $p$ it was it was discussed in

• Pierre Berthelot, Cohomologie cristalline des schémas de caractéristique $p \gt 0$, Lecture Notes in Mathematics, Vol. 407, Springer- Verlag, Berlin, 1974. (doi:10.1007/BFb0068636, MR 0384804)

• Pierre Berthelot, Arthur Ogus, $F$-Isocrystals and de Rham Cohomology, I, Invent. math. 72, 1983, pp. 159-199 (eudml:143016)

Review:

Discussion of this in terms of Cech cohomology is in