Some basic thoughts on the cofiality of Chang structures with an application to forcing

Abstract

Consider $(\kappa^{+++},\kappa^{++}) \twoheadrightarrow (\kappa^+,\kappa)$ where $\kappa$ is an uncountable regular cardinal. By a result of Shelah's we have $\operatorname{cof}(X \cap \kappa^{++}) = \kappa$ for almost all $X \subset \kappa^{+++}$ witnessing this. Here we consider the question if there could be a similar result for $X \cap \kappa^+$. We use this discussion to give an interesting example of a pseudo Prikry forcing answering a question of Sinapova.