Valued Modules over Skew Polynomial Rings 2

Abstract

Following our first article, we continue to investigate ultrametic modules over a ring of twisted polynomials of the form $[K;\vfi]$, where $\vfi$ is a ring endomorphism of $K$. The main motivation comes from the the theory of valued difference fields (including characteristic $p>0$ valued fields equipped with the Frobenius endomorphism). We introduce the class of modules, that we call, affinely maximal and residually divisible and we prove (relative -) quantifier elimination results. Ax-Kochen \& Erhov type theorems follows. As an application, we axiomatize, as a valued module, any ultraproduct of algebraically closed valued fields $(\mathbb{F}_{p^n}(t)^{alg})_{n\in \mathbb{N}}$, of fixed characteristic $p>0$, each equipped with the morphism $x\mapsto x^{p^n}$ and with the $t$-adic valuation.