Definable completeness of $P$-minimal fields and applications

TL;DR

This paper proves a key completeness property for $P$-minimal fields, demonstrating their polynomial boundedness and answering several open questions about their structure and definable sets.

Contribution

It establishes the definable completeness of $P$-minimal fields, showing they satisfy the extreme value property and characterizing those with cell preparation as having Skolem functions.

Findings

Every definable nested family of closed and bounded sets has a non-empty intersection.

$P$-minimal fields satisfy the extreme value property for interpretable functions.

Interpretable subsets of $K\times\Gamma_K^n$ are already interpretable in the language of rings.

Abstract

We show that every definable nested family of closed and bounded subsets of a $P$-minimal field $K$ has non-empty intersection. As an application we answer a question of Darni\`ere and Halupczok showing that $P$-minimal fields satisfy the "extreme value property": for every closed and bounded subset $U\subseteq K$ and every interpretable continuous function $f\colon U \to \Gamma_K$ (where $\Gamma_K$ denotes the value group), $f(U)$ admits a maximal value. Two further corollaries are obtained as a consequence of their work. The first one shows that every interpretable subset of $K\times\Gamma_K^n$ is already interpretable in the language of rings, answering a question of Cluckers and Halupczok. This implies in particular that every $P$-minimal field is polynomially bounded. The second one characterizes those $P$-minimal fields satisfying a classical cell preparation theorem as those having definable Skolem functions, generalizing a result of Mourgues.