lecture 14
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@ -153,14 +153,14 @@ Relevant concepts to prove this theorem:
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\[
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\sup(A \cap \beta) \in A.
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\]
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\item $A$ is \vocab{club} (closed unbounded)
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\item $A$ is \vocab{club} (\emph{cl}osed \emph{un}bounded)
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iff it is closed and unbounded.
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\end{itemize}
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\end{definition}
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The interesting case is that $\alpha$ is a regular uncountable cardinal.
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\begin{fact}
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$A \subseteq \alpha$ being unbounded
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is equivalent to $f\colon \beta\to \alpha$ being cofinal,
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where
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$(\beta, \in ) \overset{f}{\cong} (A, \in )$.
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\end{fact}
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inputs/lecture_14.tex
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236
inputs/lecture_14.tex
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\lecture{14}{2023-12-04}{}
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\begin{abuse}
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Sometimes we say club
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instead of club in $\kappa$.
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\end{abuse}
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\begin{example}
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Let $\kappa$ be a regular uncountable cardinal.
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\begin{itemize}
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\item $\kappa$ is a club in $\kappa$.
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\item $\{\xi + 1 : \xi < \kappa\}$ is unbounded in $\kappa$,
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but not closed.
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\item For each $\alpha < \kappa$,
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the set $\alpha + 1 = \{\xi : \xi \le \alpha\}$
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is closed but not unbounded in $\kappa$.
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\item $\{\xi < \kappa : \xi \text{ is a limit ordinal}\} $
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is club in $\kappa$.
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\end{itemize}
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\end{example}
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\begin{lemma}
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\label{lem:clubintersection}
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Let $\kappa$ be regular and uncountable.
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Let $\alpha < \kappa$
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and let $\langle C_{\beta} : \beta < \alpha \rangle$
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be a sequence of subsets of $\kappa$ which are all club in $\kappa$.
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Then
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\[
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\bigcap_{\beta < \alpha} C_{\beta}
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\]
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is club in $\kappa$.
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\end{lemma}
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\begin{warning}
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This is false for $\alpha = \kappa$:
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Let $C_{\beta} \coloneqq \{\xi : \xi > \beta\}$.
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Clearly this is club
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but $\bigcap_{\beta < \kappa} C_\beta = \emptyset$.
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\end{warning}
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\begin{refproof}{lem:clubintersection}
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First let $\alpha = 2$.
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Let $C, D \subseteq \kappa$
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be a club.
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$C \cap D$ is trivially closed:
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Let $\beta < \kappa$. Suppose that $(C \cap D) \cap \beta$
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is unbounded in $\beta$, so $C \cap \beta$ and $D \cap \beta$
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are both unbounded in $\beta$,
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so $\beta \in C \cap D$.
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$C \cap D$ is unbounded:
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Take some $\gamma < \kappa$.
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Let $\gamma_0 = \gamma$
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and inductively define $\gamma_n$ :
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If $n$ is even, let $\gamma_n \coloneqq \min C \setminus (\gamma_{n-1}+1)$,
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otherwise $\gamma_n \coloneqq \min D \setminus (\gamma_{n-1}+1)$.
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Let $\xi = \sup \{\gamma_n : n < \omega\}$.
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Then $\xi = \sup \{\gamma_{2n + 2} : n < \omega\} \in D$
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and $\xi \in C$ by the same argument,
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so $\xi \in C \cap D$
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(here it is important, that $\cf(\kappa) > \omega$)
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and $\xi > \gamma$.
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The case $\alpha > 2$ is similar:
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The intersection is closed by exactly the same argument.%
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\footnote{``It is even more closed.''}
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Let's prove that $\bigcap \{C_{\beta} : \beta < \alpha\}$
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is unbounded in $\kappa$.
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We will define a sequence $\langle \gamma_i : i \le \alpha \cdot \omega \rangle$%
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\footnote{Ordinal multiplication, i.e.~$\alpha \cdot \omega = \sup_{n < \omega} \underbrace{\alpha + \ldots + \alpha}_{n \text{ times}}$.}
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as follows:
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Let $\gamma_0 \coloneqq \gamma$.
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Choose
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\[\gamma_{\alpha \cdot n + \beta + 1} = \min C_{\beta} \setminus (\gamma_{\alpha \cdot n + \beta} + 1)\]
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and at limits choose the supremum.
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Let $\xi = \sup_{i < \alpha \cdot \omega} \gamma_i
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= \sup_{i < \omega} \gamma_{\alpha \cdot n + \beta + 1} \in \bigcap_{\beta < \alpha} C_\beta$,
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where we have used that.
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$\cf(\kappa) > \alpha \cdot \omega$.
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\end{refproof}
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\begin{definition}
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$F \subseteq \cP(a)$ is a \vocab{filter}
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iff
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\begin{enumerate}[(a)]
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\item $X,Y \in F \implies X \cap Y \in F$,
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\item $X \in F \land X \subseteq Y \subseteq \kappa \implies Y \in F$,
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\item $\emptyset \not\in F$, $\kappa \in F$.
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\end{enumerate}
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Let $\alpha \le \kappa$.
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We call $F$ \vocab{$< \alpha$-closed}
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iff for all $\gamma < \alpha$ and $\{X_\beta : \beta < \gamma\} \subseteq F$
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then $\bigcap \{X_\beta : \beta < \gamma\} \in F$.
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\end{definition}
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Intuitively, a filter is a collection of ``big'' subsets of $a$.
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\begin{definition}
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Let $\kappa$ be regular and uncountable.
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The \vocab{club filter} is defined as
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\[
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\cF_{\kappa} \coloneqq \{X \subseteq \kappa : \exists \text{ club } C \subseteq \kappa .~ C \subseteq X\}.
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\]
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\end{definition}
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Clearly this is a filter.
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We have shown (assuming \AxC to choose contained clubs):
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\begin{theorem}
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If $\kappa$ is regular and uncountable.
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Then $\cF_\kappa$ is a $< \kappa$-closed filter.
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\end{theorem}
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\begin{definition}
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Let $\langle A_\beta : \beta < \alpha \rangle$
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be a sequence of sets.
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The \vocab{diagonal intersection},
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is defined to be
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\[
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\diagi_{\beta < \alpha} A_{\beta} \coloneqq
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\{\xi < \alpha : \xi \in \bigcap \{A_{\beta} : \beta < \xi\} \}.
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\]
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\end{definition}
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\begin{lemma}
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Let $\kappa$ be a regular, uncountable cardinal.
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If $\langle C_{\beta} : \beta < \kappa \rangle$
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is a sequence of club subsets of $\kappa$,
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then $\diagi_{\beta < \kappa} C_{\beta}$
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contains a club.
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\end{lemma}
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\begin{proof}
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Let us fix $\langle C_{\beta} : \beta < \alpha \rangle$.
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Write $D_{\beta} \coloneqq \bigcap \{C_{\gamma} : \gamma \le \beta\} $
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for $\beta < \kappa$.
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Each $D_{\beta}$ is a club,
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$D_{\beta} \subseteq C_{\beta}$
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and $D_{\beta} \supseteq D_{\beta'}$
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for $\beta \le \beta' < \kappa$.
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It suffices to show that $\diagi_{\beta < \kappa} D_{\beta}$
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contains a club.
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\begin{claim}
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$\diagi_{\beta < \kappa} D_{\beta}$ is closed in $\kappa$.
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\end{claim}
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\begin{subproof}
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Let $\gamma < \kappa$ be such that $\left( \diagi_{\beta < \kappa} D_{\beta} \right) \cap \gamma$
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is unbounded in $\gamma$.
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We aim to show that $\gamma \in \diagi_{\beta < \kappa} D_{\beta}$.
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Let $\beta_0 < \gamma$.
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We need to see $\gamma \in D_{\beta_0}$.
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For each $\beta'$ with $\beta < \beta' < \gamma$,
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there is some $\beta'' \in \diagi_{\beta < \kappa} D_\beta$
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with $\beta'' \ge \beta', \beta'' < \gamma$.
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In particular $\beta'' \in D_{\beta_0}$.
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We showed that $D_{\beta_0} \cap \gamma$
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is unbounded in $\gamma$,
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so $\gamma \in D_{\beta_0}$.
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As $\beta_0 < \gamma$ was arbitrary,
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this shows that $\gamma \in \diagi_{\beta < n} D_\beta$.
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\end{subproof}
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\begin{claim}
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$\diagi_{\beta < \kappa} D_{\beta}$
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is unbounded in $\kappa$.
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\end{claim}
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\begin{subproof}
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Fix $\gamma < \kappa$.
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We need to find $\delta > \gamma$
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with $\gamma \in \diagi_{\beta < \kappa} D_\beta$.
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Define $\langle \gamma_n : n < \omega \rangle$
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as follows:
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$\gamma_0 \coloneqq \gamma$
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and
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\[
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\gamma_{n+1} \coloneqq \min D_{\gamma_n} \setminus (\gamma_n + 1)
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\]
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We have $\delta \coloneqq \sup_{n < \omega} \gamma_n \in \kappa$
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by cofinality of $\kappa$.
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We need to show that $\delta \in D_{\overline{\gamma}}$
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for all $\overline{\gamma} < \delta$.
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If $\overline{\gamma} < \delta$, then $\overline{\gamma} \le \gamma_n$
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for some $n < \omega$.
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For $m \ge n$, $\gamma_{m+1} \in D_{\gamma_m} \subseteq D_{\gamma_n} \subseteq D_{\overline{\gamma}}$.
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So $D_{\overline{\gamma}} \cap \delta$ is unbounded
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in $\gamma$, hence $\delta \in D_{\overline{\gamma}}$.
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\end{subproof}
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\end{proof}
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\begin{definition}
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Let $\kappa$ be regular and uncountable.
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$S \subseteq \kappa$ is called \vocab{stationary} (in $\kappa$)
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iff $C \cap S \neq \emptyset$
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for every club $C \subseteq \kappa$.
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\end{definition}
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\begin{example}
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\begin{itemize}
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\item Every $D \subseteq \kappa$ which is club in $\kappa$
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is stationary in $\kappa$.
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\item There exist disjoint stationary sets:\footnote{Note that clubs can never be disjoint, since their intersection is a club.}
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Let $\kappa = \omega_2$.
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Let $S_0 \coloneqq \{\xi < \kappa : \cf(\xi) = \omega\}$
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and $S_1 \coloneqq \{\xi < \kappa : \cf(\xi) = \omega_1\}$.
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Clearly these are disjoint.
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They are both stationary:
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Let $c \subseteq \kappa$ be a club.
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Let $(\xi_i : i \le \omega_1)$
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be defined as follows:
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$\xi_0 \coloneqq \min C$,
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$\xi_i \coloneqq \min (C \setminus \sup_{j < i} \xi_j)$.
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For $i \le \omega_1$ we have that $\xi_i = \sup_{j < i} \xi_j$.
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In particular $\xi_\omega \in S_0 \cap C$
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and $\xi_{\omega_1} \in S_1 \cap C$.
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\end{itemize}
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\end{example}
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We will show later that if $ \kappa$ is a regular uncountable cardinal,
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then every stationary $S \subseteq \kappa$ can be written as
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$S = \bigcup_{i < \kappa} S_i$,
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where the $S_i$ are stationary and pairwise disjoint.
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@ -142,4 +142,6 @@
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\DeclareSimpleMathOperator{cf}
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\newcommand\lecture[3]{\hrule{\color{darkgray}\hfill{\tiny[Lecture #1, #2]}}}
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%\newcommand\diagi{\mathop{\large \Delta}\limits}
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\newcommand\diagi{\mathop{\large \Delta}}
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@ -37,6 +37,7 @@
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\input{inputs/lecture_11}
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\input{inputs/lecture_12}
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\input{inputs/lecture_13}
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\input{inputs/lecture_14}
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\cleardoublepage
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