The effect of forcing axioms on the tightness of the $G_\delta$ modification

Abstract

We show that $\mathsf{PFA}$ implies that the tightness $t(X_\delta)$ of the $G_\delta$-modification of a Fr\'echet $\alpha_1$-space $X$ is at most $\omega_1$, while $\Box(\kappa)$ implies that there is a Fr\'echet $\alpha_1$-space with $G_\delta$-tightness equal to $\kappa$. We use the example constructed from $\Box(\kappa)$ to show that a local version of the bound $t(X_\delta)\le 2^{t(X)L(X)}$ does not hold. We also construct, assuming $\mathsf{MA}$, an example of a Fr\'echet space whose $G_\delta$-tightness is larger than $\omega_1$.