Characterizations of the weakly compact ideal on $P_\kappa\lambda$

TL;DR

This paper generalizes Hellsten's characterization of $ ext{Pi}^1_n$-indescribability to subsets of $P_\u03bb$, explores associated ideals, introduces $n$-club sets, and connects these concepts to elementary embeddings and large cardinal principles.

Contribution

It extends the characterization of indescribability to $P_\u03bb$, identifies the corresponding ideals, and relates these to elementary embeddings and large cardinal properties.

Findings

The $ ext{Pi}^1_0$-indescribability ideal equals the minimal strongly normal ideal $ ext{NSS}_{,}$.

A set is $ ext{Pi}^1_n$-indescribable iff it intersects every $n$-club subset of $P_\u03bb$.

Connections established between elementary embeddings, ideals, and large cardinal hypotheses.

Abstract

Hellsten \cite{MR2026390} gave a characterization of $\Pi^1_n$-indescribable subsets of a $\Pi^1_n$-indescribable cardinal in terms of a natural filter base: when $\kappa$ is a $\Pi^1_n$-indescribable cardinal, a set $S\subseteq\kappa$ is $\Pi^1_n$-indescribable if and only if $S\cap C\neq\emptyset$ for every $n$-club $C\subseteq \kappa$. We generalize Hellsten's characterization to $\Pi^1_n$-indescribable subsets of $P_\kappa\lambda$, which were first defined by Baumgartner. After showing that under reasonable assumptions the $\Pi^1_0$-indescribability ideal on $P_\kappa\lambda$ equals the minimal \emph{strongly} normal ideal $\text{NSS}_{\kappa,\lambda}$ on $P_\kappa\lambda$, and is not equal to $\text{NS}_{\kappa,\lambda}$ as may be expected, we formulate a notion of $n$-club subset of $P_\kappa\lambda$ and prove that a set $S\subseteq P_\kappa\lambda$ is $\Pi^1_n$-indescribable if and only if $S\cap C\neq\emptyset$ for every $n$-club $C\subseteq P_\kappa\lambda$. We also prove that elementary embeddings considered by Schanker \cite{MR2989393} witnessing \emph{near supercompactness} lead to the definition of a normal ideal on $P_\kappa\lambda$, and indeed, this ideal is equal to Baumgartner's ideal of non--$\Pi^1_1$-indescribable subsets of $P_\kappa\lambda$. Additionally, as applications of these results we answer a question of Cox-L\"ucke \cite{MR3620068} about $\mathcal{F}$-layered posets, provide a characterization of $\Pi^m_n$-indescribable subsets of $P_\kappa\lambda$ in terms of generic elementary embeddings, prove several results involving a two-cardinal weakly compact diamond principle and observe that a result of Pereira \cite{MR3640048} yeilds the consistency of the existence of a $(\kappa,\kappa^+)$-semimorasses $\mu\subseteq P_\kappa\kappa^+$ which is $\Pi^1_n$-indescribable for all $n<\omega$.