On endomorphisms of the de Rham cohomology functor

Abstract

We compute the moduli of endomorphisms of the de Rham and crystalline cohomology functors, viewed as a cohomology theory on smooth schemes over truncated Witt vectors. As applications of our result, we deduce Drinfeld's refinement of the classical Deligne--Illusie decomposition result for de Rham cohomology of varieties in characteristic $p>0$ that are liftable to $W_2$, and prove further functorial improvements.

v1: 30 pages, comments welcome; v2: 33 pages, final version, to appear in Geometry and Topology, comments still welcome