Generic orbits and type isolation in the Gurarij space

TL;DR

This paper investigates the conditions under which the space of embeddings of a separable Banach space into the Gurarij space has a generic orbit, using model theory and convex analysis to characterize isolated types and their density.

Contribution

It characterizes isolated types over Banach spaces in the Gurarij space and links type isolation to the existence of dense orbits, advancing the understanding of the space's model-theoretic properties.

Findings

Dense isolated types imply a dense G_delta orbit.

If isolated types are not dense, all orbits are meagre.

The Gurarij space models an -categorical theory with quantifier elimination.

Abstract

We study the question of when the space of embeddings of a separable Banach space $E$ into the separable Gurarij space $\mathbf G$ admits a generic orbit under the action of the linear isometry group of $\mathbf G$. The question is recast in model-theoretic terms, namely type isolation and the existence of prime models. We characterise isolated types over $E$ using tools from convex analysis. We show that if the set of isolated types over $E$ is dense, then a dense $G\_\delta$ orbit exists, and otherwise all orbits are meagre. We then study some (families of) examples with respect to this dichotomy. We also point out that the class of Gurarij spaces is the class of models of an $\aleph\_0$-categorical theory with quantifier elimination, and calculate the density character of the space of types over $E$, answering a question of Avil{\'e}s et al.