English

Super Black Boxes Revisited

Logic 2026-02-11 v1

Abstract

Let κ,θ<λ \kappa , \theta < \lambda be cardinals, with λ\lambda and κ\kappa regular. Concentrating on a simple case, we say that the triple (λ,κ,θ)(\lambda,\kappa,\theta) has a Super Black Box when the following holds. For some stationary S{δ<λ:cf(δ)=κ}S \subseteq \{\delta < \lambda : cf(\delta) = \kappa\} and C=Cδ:δS\overline C = \langle C_\delta : \delta \in S \rangle, where CδC_\delta is a club of δ\delta of order type κ\kappa, for every coloring F=Fδ:δS\overline F = \langle F_\delta : \delta \in S \rangle with Fδ:CδλθF_\delta : {}^{C_\delta}\lambda \to \theta, there exists cδ:δSS ⁣θ\langle c_\delta : \delta \in S\rangle \in {}^S\!\theta such that for every f:λθf : \lambda \to \theta, for stationarily many δS\delta \in S, we have Fδ(fCδ)=cδF_\delta(f \upharpoonright C_\delta) = c_\delta. In an earlier work, it was proved (along with much more) that for a class of cardinals λ\lambda this holds for many pairs (κ,θ)(\kappa,\theta). E.g.~κ<ω\kappa < \aleph_\omega is large enough, and ω(θ)<λ\beth_\omega(\theta) < \lambda. However, the most interesting cases (at least with regards to Abelian groups) are κ=0,1\kappa = \aleph_0,\aleph_1 (which have not been covered yet). Here we restrict ourselves to the case where F\overline F is a {so-called} \emph{continuous coloring}, which includes the case where FδF_\delta is computed from some Fδ,β(f(Cδβ)):βCδ. \big\langle F_{\delta,\beta}'(f \upharpoonright (C_\delta \cap \beta)) : \beta \in C_\delta \big\rangle. This covers the cases we have in mind. We mainly prove results without any other caveats: e.g. For every regular κ\kappa and θ\theta there exists such a λ\lambda. We also deal with having multiple {Cˉ\bar C-s}, and the existence of quite free subsets of κμ{}^\kappa\mu.

Cite

@article{arxiv.2602.09592,
  title  = {Super Black Boxes Revisited},
  author = {Saharon Shelah},
  journal= {arXiv preprint arXiv:2602.09592},
  year   = {2026}
}
R2 v1 2026-07-01T10:29:25.985Z