$Ψ$-Spaces and Semi-Proximality

Abstract

We discuss the proximal game and semi-proximality in $Ψ$-spaces of almost disjoint families over an infinite countable set and $Ψ$-spaces of ladder systems on $ω_1$. We show that a semi-proximal almost disjoint families must be nowhere MAD, anti-Luzin and characterize semi-proximality for a class of ${\mathbb R}$-embeddable almost disjoint families. We show that a $Ψ$-spaces defined from a uniformizable ladder system is semi-proximal and a $Ψ$-space defined on a $\clubsuit^*$ sequence is not semi-proximal. Thus the existence of non-semi-proximal $Ψ$-space over a ladder system is independent of ZFC.