Boundedness and absoluteness of some dynamical invariants in model theory

Abstract

Let ${\mathfrak C}$ be a monster model of an arbitrary theory $T$, $\bar \alpha$ any tuple of bounded length of elements of ${\mathfrak C}$, and $\bar c$ an enumeration of all elements of ${\mathfrak C}$. By $S_{\bar \alpha}({\mathfrak C})$ denote the compact space of all complete types over ${\mathfrak C}$ extending $tp(\bar \alpha/\emptyset)$, and $S_{\bar c}({\mathfrak C})$ is defined analogously. Then $S_{\bar \alpha}({\mathfrak C})$ and $S_{\bar c}({\mathfrak C})$ are naturally $Aut({\mathfrak C})$-flows. We show that the Ellis groups of both these flows are of bounded size (i.e. smaller than the degree of saturation of ${\mathfrak C}$), providing an explicit bound on this size. Next, we prove that these Ellis groups do not depend on the choice of the monster model ${\mathfrak C}$; thus, we say that they are absolute. We also study minimal left ideals (equivalently subflows) of the Ellis semigroups of the flows $S_{\bar \alpha}({\mathfrak C})$ and $S_{\bar c}({\mathfrak C})$. We give an example of a NIP theory in which the minimal left ideals are of unbounded size. We show that in each of these two cases, boundedness of a minimal left ideal is an absolute property (i.e. it does not depend on the choice of ${\mathfrak C}$) and that whenever such an ideal is bounded, then its isomorphism type is also absolute. Assuming NIP, we give characterizations of when a minimal left ideal of the Ellis semigroup of $S_{\bar c}({\mathfrak C})$ is bounded. Then we adapt a proof of Chernikov and Simon to show that whenever such an ideal is bounded, the natural epimorphism (described by Krupinski, Pillay and Rzepecki) from the Ellis group of the flow $S_{\bar c}({\mathfrak C})$ to the Kim-Pillay Galois group $Gal_{KP}(T)$ is an isomorphism (in particular, $T$ is G-compact). We provide some counter-examples for $S_{\bar \alpha}({\mathfrak C})$ in place of $S_{\bar c}({\mathfrak C})$.