Model theory on Hilbert spaces expanded by a representation of a group

Abstract

In this paper we study expansions of infinite dimensional Hilbert spaces with a unitary representation of a group. When the group is finite, we prove the theory of the corresponding expansion is $\aleph_0$-categorical, $\aleph_0$-stable and is SFB. On the other hand, when the group involved is a product of the form $H\times \mathbb{Z}^n$, where $H$ is a finite group and $n\geq 1$, the theory of the Hilbert space expanded by the representation of this group is, in general, stable not $\aleph_0$-stable, not $\aleph_0$-categorical, but it is $\aleph_0$-categorical up to perturbations and $\aleph_0$-stable up to perturbations.