English

Maximal WAP and tame quotients of type spaces

Logic 2025-01-28 v1

Abstract

We study maximal WAP and tame (in the sense of topological dynamics) quotients of SX(C)S_X(\mathfrak{C}), where C\mathfrak{C} is a sufficiently saturated (called monster) model of a complete theory TT, XX is a \emptyset-type-definable set, and SX(C)S_X(\mathfrak{C}) is the space of complete types over C\mathfrak{C} concentrated on XX. Namely, let FWAPSX(C)×SX(C)F_{\textrm{WAP}}\subseteq S_X(\mathfrak{C})\times S_X(\mathfrak{C}) be the finest closed, aut(C)aut(\mathfrak{C})-invariant equivalence relation on SX(C)S_X(\mathfrak{C}) such that the flow (aut(C),SX(C)/FWAP)( aut(\mathfrak{C}), S_X(\mathfrak{C})/F_{\textrm{WAP}} ) is WAP, and let FTameSX(C)×SX(C)F_{\textrm{Tame}}\subseteq S_X(\mathfrak{C})\times S_X(\mathfrak{C}) be the finest closed, aut(C)aut(\mathfrak{C})-invariant equivalence relation on SX(C)S_X(\mathfrak{C}) such that the flow (aut(C),SX(C)/FTame)( aut(\mathfrak{C}), S_X(\mathfrak{C})/F_{\textrm{Tame}} ) is tame. We show good behaviour of FWAPF_{\textrm{WAP}} and FTameF_{\textrm{Tame}} under changing the monster model C\mathfrak{C}. Namely, we prove that if CC\mathfrak{C}'\succ \mathfrak{C} is a bigger monster model, FWAPF'_{\textrm{WAP}} and FTameF'_{\textrm{Tame}} are the counterparts of FWAPF_{\textrm{WAP}} and FTameF_{\textrm{Tame}} computed for C\mathfrak{C}', and r ⁣:SX(C)SX(C)r\colon S_X(\mathfrak{C}')\to S_X(\mathfrak{C}) is the restriction map, then r[FWAP]=FWAPr[F'_{\textrm{WAP}}]=F_{\textrm{WAP}} and r[FTame]=FTamer[F'_{\textrm{Tame}}]=F_{\textrm{Tame}}. Using these results, we show that the Ellis (or ideal) groups of (aut(C),SX(C)/FWAP)( aut(\mathfrak{C}), S_X(\mathfrak{C})/F_{\textrm{WAP}} ) and (aut(C),SX(C)/FTame)(aut(\mathfrak{C}), S_X(\mathfrak{C})/F_{\textrm{Tame}}) do not depend on the choice of the monster model C\mathfrak{C}.

Keywords

Cite

@article{arxiv.2501.15632,
  title  = {Maximal WAP and tame quotients of type spaces},
  author = {Krzysztof Krupiński and Adrián Portillo},
  journal= {arXiv preprint arXiv:2501.15632},
  year   = {2025}
}