$\mu$-Clubs OF $P_\kappa (\lambda)$ : Paradise on earth

Abstract

If $V = L$, and $\mu$, $\kappa$ and $\lambda$ are three infinite cardinals with $\mu = {\rm cf} (\mu) < \kappa = {\rm cf}(\kappa) \leq \lambda$, then, as shown in \cite{Heaven}, the $\mu$-club filters on $P_\kappa (\lambda)$ and $P_\kappa (\lambda^{< \kappa})$ are isomorphic if and only if ${\rm cf} (\lambda) \not= \mu$. Now in $L$, $\lambda^{< \kappa}$ equals $u (\kappa, \lambda)$ (the least size of a cofinal subset in $(P_\kappa (\lambda), \subseteq)$) equals $\lambda$ if ${\rm cf} (\lambda) \geq \kappa$, and $\lambda^+$ otherwise. We show that, in ZFC, there are many triples $(\mu, \kappa, \lambda)$ for which ($u (\kappa, \lambda) > \lambda$ and) the $\mu$-club filters on $P_\kappa (\lambda)$ and $P_\kappa (u (\kappa, \lambda))$ are isomorphic.