The Gauss-Manin connection on the periodic cyclic homology

TL;DR

This paper extends Kaledin's theory of periodic cyclic homology in positive characteristic to a relative setting, revealing the Gauss-Manin connection's structure and its monodromy properties for DG algebras.

Contribution

It develops a relative version of Kaledin's theory, connecting the Gauss-Manin connection with Hochschild homology and the Kodaira-Spencer operator.

Findings

The Gauss-Manin connection can be recovered from Hochschild homology with Kodaira-Spencer action.

Under certain conditions, the monodromy of the Gauss-Manin connection is quasi-unipotent.

The theory applies to DG algebras over complex punctured disks, linking characteristic p techniques to complex geometry.

Abstract

It is expected that the periodic cyclic homology of a DG algebra over the field of complex numbers (and, more generally, the periodic cyclic homology of a DG category) carries a lot of additional structure similar to the mixed Hodge structure on the de Rham cohomology of algebraic varieties. Whereas a construction of such a structure seems to be out of reach at the moment its counterpart in finite characteristic is much better understood thanks to recent groundbreaking works of Kaledin. In particular, it is proven by Kaledin that under some assumptions on a DG algebra $A$ over a perfect field $k$ of characteristic $p$, a lifting of $A$ over the ring of second Witt vectors $W_2(k)$ specifies the structure of a Fontaine-Laffaille module on the periodic cyclic homology of $A$. The purpose of this paper is to develop a relative version of Kaledin's theory for DG algebras over a base $k$-algebra $R$ incorporating in the picture the Gauss-Manin connection on the relative periodic cyclic homology constructed by Getzler. Our main result asserts that, under some assumptions on $A$, the Gauss-Manin connection on its periodic cyclic homology can be recovered from the Hochschild homology of $A$ equipped with the action of the Kodaira-Spencer operator as the inverse Cartier transform (in the sense of Ogus-Vologodsky). As an application, we prove, using the reduction modulo $p$ technique, that, for a smooth and proper DG algebra over a complex punctured disk, the monodromy of the Gauss-Manin connection on its periodic cyclic homology is quasi-unipotent.