English

Martin's Maximum and tower forcing

Logic 2011-10-19 v3

Abstract

There are several examples in the literature showing that compactness-like properties of a cardinal κ\kappa cause poor behavior of some generic ultrapowers which have critical point κ\kappa (Burke \cite{MR1472122} when κ\kappa is a supercompact cardinal; Foreman-Magidor \cite{MR1359154} when κ=ω2\kappa = \omega_2 in the presence of strong forcing axioms). We prove more instances of this phenomenon. First, the Reflection Principle (RP) implies that if I\vec{\mathcal{I}} is a tower of ideals which concentrates on the class GICω1GIC_{\omega_1} of ω1\omega_1-guessing, internally club sets, then I\vec{\mathcal{I}} is not presaturated (a set is ω1\omega_1-guessing iff its transitive collapse has the ω1\omega_1-approximation property as defined in Hamkins \cite{MR2540935}). This theorem, combined with work from \cite{VW_ISP}, shows that if PFA+PFA^+ or MMMM holds and there is an inaccessible cardinal, then there is a tower with critical point ω2\omega_2 which is not presaturated; moreover this tower is significantly different from the non-presaturated tower already known (by Foreman-Magidor \cite{MR1359154}) to exist in all models of Martin's Maximum. The conjunction of the Strong Reflection Principle (SRP) and the Tree Property at ω2\omega_2 has similar implications for towers of ideals which concentrate on the wider class GISω1GIS_{\omega_1} of ω1\omega_1-guessing, internally stationary sets. Finally, we show that the word "presaturated" cannot be replaced by "precipitous" in the theorems above: Martin's Maximum (which implies SRP and the Tree Property at ω2\omega_2) is consistent with a precipitous tower on GICω1GIC_{\omega_1}.

Keywords

Cite

@article{arxiv.1110.1584,
  title  = {Martin's Maximum and tower forcing},
  author = {Sean Cox and Matteo Viale},
  journal= {arXiv preprint arXiv:1110.1584},
  year   = {2011}
}
R2 v1 2026-06-21T19:16:51.690Z