Higher indescribability and derived topologies

TL;DR

This paper explores advanced reflection properties of cardinals, introduces new notions of indescribability, and extends topological sequences on regular cardinals, revealing deep connections between set-theoretic reflection principles and topology.

Contribution

It defines generalized indescribability notions for cardinals, constructs a hierarchy of derived topologies, and links indescribability with topological properties of stationary sets.

Findings

Existence of a hierarchy of $ au_\xi$ topologies on regular cardinals.

Stationary sets with high indescribability degrees contain nonisolated points.

Universal $ ext{Pi}^1_\xi$ formulas and associated ideals are established.

Abstract

We introduce reflection properties of cardinals in which the attributes that reflect are expressible by infinitary formulas whose lengths can be strictly larger than the cardinal under consideration. This kind of generalized reflection principle leads to the definitions of $L_{\kappa^+,\kappa^+}$-indescribability and $\Pi^1_\xi$-indescribability of a cardinal $\kappa$ for all $\xi<\kappa^+$. In this context, universal $\Pi^1_\xi$ formulas exist, there is a normal ideal associated to $\Pi^1_\xi$-indescribability and the notions of $\Pi^1_\xi$-indescribability yield a strict hierarchy below a measurable cardinal. Additionally, given a regular cardinal $\mu$, we introduce a diagonal version of Cantor's derivative operator and use it to extend Bagaria's \cite{MR3894041} sequence $langle\tau_\xi:\xi<\mu\rangle$ of derived topologies on $\mu$ to $\langle\tau_\xi:\xi<\mu^+\rangle$. Finally, we prove that for all $\xi<\mu^+$, if there is a stationary set of $\alpha<\mu$ that have a high enough degree of indescribability, then there are stationarily-many $\alpha<\mu$ that are nonisolated points in the space $(\mu,\tau_{\xi+1})$.