Strong Completeness of Provability Logic for Uncountable Languages

Abstract

For an ordinal $λ>0$, we use the Erdős--Rado partition theorem to prove the failure of strong completeness of $\mathsf{GL}$ for modal languages of cardinality $(2^{|λ|+\aleph_0})^{+}$ with respect to models on ordinals equipped with the generalized Icard topologies $\mathcal{I}_λ$ and ${τ_{c}}_{+λ}$. Specifically, we show that for such languages there exists a $\mathsf{GL}$-consistent set of formulas having neither $(Θ, \mathcal{I}_λ)$-model nor $(Θ, {τ_{c}}_{+λ})$-model. We also introduce two kinds of natural classes of topological spaces, called \emph{ $λ$-bouquet spaces} and \emph{ultralinear $λ$-bouquet spaces}, and prove that they yield strong completeness of $\mathsf{GL}$ and $\mathsf{GL}.3$ respectively for languages of cardinality $λ$.