CONTRACTING ENDOMORPHISMS OF VALUED FIELDS

We prove that the class of separably algebraically closed valued fields equipped with a distinguished Frobenius endomorphism x x^q is decidable, uniformly in q. The result is a simultaneous generalization of the work of Chatzidakis and Hrushovski (in the case of the trivial valuation) and the work of the first author and Hrushovski (in the case where the fields are algebraically closed). The logical setting for the proof is a model completeness result for valued fields equipped with an endomorphism σ which is locally infinitely contracting and fails to be onto. Namely, we prove the existence of a model complete theory VFE amalgamating the theories SCFE and VFA introduced in [5] and [11], respectively. In characteristic zero, we also prove that VFE is NTP2 and classify the stationary types: they are precisely those orthogonal to the fixed field and the value group.