Delta Characters and Crystalline Cohomology

TL;DR

This work develops $m$-shifted $\pi$-typical Witt vectors and a delta-structure that interact canonically with the usual Witt-vector delta-structure, enabling a geometric interpretation of arithmetic jet spaces and their kernels. It provides a comprehensive framework for delta-characters and delta isocrystals, proving finiteness and freeness results for $\mathbf X_{prim}(A)$ and $\mathbf H_{\delta}(A)$ when $A$ is an abelian scheme, and establishing a weakly admissible filtered isocrystal structure for elliptic curves over $\mathbb Z_p$ with crystalline-cohomology comparisons depending on the lift of Frobenius. The paper also proves that generalized kernels are jet spaces of the first kernel and develops an integral-model viewpoint, including an isomorphism between $N^{[m]n}X$ and $J^{n-1}(N^{[m]1}X)$ and an alternative functor-of-points proof. In the elliptic setting, the arithmetic Picard–Fuchs operator ties the delta-geometry to classical crystalline cohomology, yielding a concrete bridge between delta-characters and cohomological invariants.

Abstract

The first part of the paper develops the theory of $m$-shifted $π$-typical Witt vectors which can be viewed as subobjects of the usual $π$-typical Witt vectors. We show that the shifted Witt vectors admit a delta structure that satisfy a canonical identity with the delta structure of the usual $π$-typical Witt vectors. Using this theory, we prove that the generalized kernels of arithmetic jet spaces are jet spaces of the kernel at the first level. This also allows us to interpret the arithmetic Picard-Fuchs operator geometrically. For a $π$-formal group scheme $G$, by a previous construction, one attaches a canonical filtered isocrystal $\mathbf{H}_δ(G)$ associated to the arithmetic jet spaces of $G$. In the second half of our paper, we show that $\mathbf{H}_δ(A)$ is of finite rank if $A$ is an abelian scheme. We also prove a strengthened version of a result of Buium on delta characters on abelian schemes. As an application, for an elliptic curve $A$ defined over $\mathbb{Z}_p$, we show that our canonical filtered isocrystal $\mathbf{H}_δ(A) \otimes \mathbb{Q}_p$ is weakly admissible. In particular, if $A$ does not admit a lift of Frobenius, we show that $\mathbf{H}_δ(A) \otimes \mathbb{Q}_p$ is isomorphic to the first crystalline cohomology $\mathbf{H}^1_{\mathrm{cris}}(A) \otimes \mathbb{Q}_p$ in the category of filtered isocrystals. On the other hand, if $A$ admits a lift of Frobenius, then $\mathbf{H}_δ(A) \otimes \mathbb{Q}_p$ is isomorphic to the sub-isocrystal $H^0(A,Ω_A) \otimes \mathbb{Q}_p$ of $\mathbf{H}^1_{\mathrm{cris}}(A) \otimes \mathbb{Q}_p$.

Theorems & Definitions (57)

• Theorem 1.1
• Theorem 1.2
• Theorem 1.3
• Theorem 1.4
• Theorem 1.5
• Theorem 1.6
• Theorem 1.7
• Theorem 5.1
• proof
• proof