Approximation and algebraicity in positive characteristic Hahn fields

Abstract

We study the relative algebraic closure $K$ of $\bar{\mathbb{F}}_p((t))$ inside $\bar{\mathbb{F}}((t^{\mathbb{Q}}))$. We show that the supports of elements in $K$ have order type strictly less than $\omega^\omega$. We also recover a theorem by Rayner giving a bound to the ramification away from $p$ in the support of elements in $K$, and an analogue of Rayner's result for the residue field. This work has applications to the decidability of the first order theory of $\mathbb{F}_p((t^{\mathbb{Q}}))$, and other tame fields, in the language of valued fields with a constant symbol for $t$.