Witt Vector Rings and the Relative de Rham Witt Complex

TL;DR

This paper introduces a new, more natural construction of Witt vector rings and the relative de Rham Witt complex that works for general commutative algebras, simplifying previous approaches and extending their applicability.

Contribution

The authors develop a presentation-based approach to Witt vectors and the de Rham Witt complex, removing the need for universal polynomials and generalizing to arbitrary algebras and truncation sets.

Findings

Ring structure is straightforward without universal polynomials.

Construction is independent of choices of presentations.

Extension of Witt vector description to all $ar{bF}_p$-algebras with injective Frobenius.

Abstract

In this paper we develop a novel approach to Witt vector rings and to the (relative) de Rham Witt complex. We do this in the generality of arbitrary commutative algebras and arbitrary truncation sets. In our construction of Witt vector rings the ring structure is obvious and there is no need for universal polynomials. Moreover a natural generalization of the construction easily leads to the relative de Rham Witt complex. Our approach is based on the use of free or at least torsion free presentations of a given commutative ring $R$ and it is an important fact that the resulting objects are independent of all choices. The approach via presentations also sheds new light on our previous description of the ring of $p$-typical Witt vectors of a perfect $\mathbb{F}_p$-algebra as a completion of a semigroup algebra. We develop this description in different directions. For example, we show that the semigroup algebra can be replaced by any free presentation of $R$ equipped with a linear lift of the Frobenius automorphism. Using the result in the appendix by Umberto Zannier we also extend the description of the Witt vector ring as a completion to all $\bar{\mathbb{F}}_p$-algebras with injective Frobenius map.