Automorphisms of valued fields: amalgamation and existential closedness

We study valued fields equipped with an automorphism. We prove that all of them have an extension admitting an equivariant cross-section of the valuation. In residual characteristic zero, and in the presence of such a cross-section, we show that amalgamation problems are solvable precisely when the induced residual problem is, characterise the existentially closed objects of this category, and prove that its positive theory does not have the tree property of the second kind. We prove analogous results with cross-sections replaced by angular components. Along the way, we show that array modelling does not require thickness.

💡 Research Summary

The paper investigates valued fields equipped with an automorphism, i.e., valued difference fields (K, v, σ). Its main contributions are fourfold.

First, it proves a “purity” theorem (Theorem A, 2.14): if (K, v, σ) is existentially closed as a valued difference field, then the multiplicative group of the valuation ring O× is a pure Z