Delta Characters in Positive Characteristic and Galois Representations

Abstract

In this article we develop the theory of $\delta$-characters of Anderson modules. Given any Anderson module $E$ (satisfying certain conditions), using the theory of $\delta$-geometry, we construct a canonical $z$-isocrystal ${\mathbf{H}_{\delta}(E)}$ with a Hodge-Pink structure. As an application, we show that when $E$ is a Drinfeld module, our constructed $z$-isocrystal ${\mathbf{H}_{\delta}(E)}$ is weakly admissible given that a $\delta$-parameter is non-zero. Therefore the equal characteristic analogue of the Fontaine functor associates a local shtuka and hence a crystalline $z$-adic Galois representation to the $\delta$-geometric object ${\mathbf{H}_{\delta}(E)}$. It is also well known that there is a natural local shtuka attached to $E$. In the case of Carlitz modules, we show that the Galois representation associated to ${\mathbf{H}_{\delta}(E)}$ is indeed the usual one coming from the Tate module. Hence this article further raises the question of how the above two apparently different Galois representations compare with each other for a Drinfeld module of arbitrary rank.