Merge branch 'master' of https://git.abstractnonsen.se/josia-notes/w23-logic-2
This commit is contained in:
commit
c03b6fc0ef
15 changed files with 152 additions and 27 deletions
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@ -12,7 +12,7 @@ jobs:
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- name: Prepare pages
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- name: Prepare pages
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run: |
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run: |
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mkdir public
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mkdir public
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mv build/logic2.pdf build/logic2.log README.md public
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mv build/*.pdf build/*.log README.md public
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- name: Deploy to pages
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- name: Deploy to pages
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uses: actions/pages@v1
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uses: actions/pages@v1
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with:
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with:
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|
|
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@ -109,6 +109,7 @@
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If $A \sim B$, there is a bijection $h\colon A \to B$.
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If $A \sim B$, there is a bijection $h\colon A \to B$.
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\end{theorem}
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\end{theorem}
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||||||
\begin{proof}
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\begin{proof}
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||||||
|
\gist{%
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||||||
Let $f\colon A \hookrightarrow B$ and $g\colon B \hookrightarrow A$
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Let $f\colon A \hookrightarrow B$ and $g\colon B \hookrightarrow A$
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||||||
be injective.
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be injective.
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We need to define a bijection $h\colon A \to B$.
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We need to define a bijection $h\colon A \to B$.
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@ -146,6 +147,7 @@
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It is clear that this is bijective.
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It is clear that this is bijective.
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|
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\todo{missing picture $f(A^{\text{odd}}) \subseteq B^{\text{even}}$, $f(A^\infty) = B^\infty$}.
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\todo{missing picture $f(A^{\text{odd}}) \subseteq B^{\text{even}}$, $f(A^\infty) = B^\infty$}.
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}{Preimage sequence}
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\end{proof}
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\end{proof}
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||||||
|
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\begin{definition}
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\begin{definition}
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|
|
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@ -1,4 +1,5 @@
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\lecture{02}{2023-10-19}{Topology on $\R$}
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\lecture{02}{2023-10-19}{Topology on $\R$}
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\gist{%
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\begin{definition}
|
\begin{definition}
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A set $O \subseteq \R$ is called \vocab{open} in $\R$ iff it is the union
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A set $O \subseteq \R$ is called \vocab{open} in $\R$ iff it is the union
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of a set of open intervals.
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of a set of open intervals.
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@ -20,6 +21,7 @@
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\begin{remark}+
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\begin{remark}+
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$\{O \subseteq \R\} \sim 2^{\aleph_0} < \cP(\R)$.
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$\{O \subseteq \R\} \sim 2^{\aleph_0} < \cP(\R)$.
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\end{remark}
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\end{remark}
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||||||
|
}{}
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|
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||||||
\begin{definition}
|
\begin{definition}
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||||||
We call $x \in \R$
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We call $x \in \R$
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@ -28,9 +30,11 @@
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there is some $y \in A$, $y \in (a,b)$, $y \neq x$.
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there is some $y \in A$, $y \in (a,b)$, $y \neq x$.
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We write \vocab{$A'$} for the set of all accumulation points of $A$.
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We write \vocab{$A'$} for the set of all accumulation points of $A$.
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\end{definition}
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\end{definition}
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||||||
|
\gist{%
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\begin{example}
|
\begin{example}
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$\{\frac{1}{n+1} | n \in \N\}' = \{0\}$.
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$\{\frac{1}{n+1} | n \in \N\}' = \{0\}$.
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\end{example}
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\end{example}
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||||||
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}{}
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||||||
|
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||||||
\begin{lemma}
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\begin{lemma}
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\label{lem:closedaccumulation}
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\label{lem:closedaccumulation}
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@ -38,9 +42,11 @@
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\end{lemma}
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\end{lemma}
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\begin{refproof}{lem:closedaccumulation}
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\begin{refproof}{lem:closedaccumulation}
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``$\implies$''
|
``$\implies$''
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|
\gist{%
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||||||
Let $A$ be closed. Suppose that $x \in A' \setminus A$.
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Let $A$ be closed. Suppose that $x \in A' \setminus A$.
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Then there exists $(a,b) \ni x$
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Then there exists $(a,b) \ni x$
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disjoint from $A$. Hence $x \not\in A' \lightning$
|
disjoint from $A$. Hence $x \not\in A' \lightning$
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|
}{trivial.}
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|
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||||||
``$\impliedby$''
|
``$\impliedby$''
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Suppose $A' \subseteq A$.
|
Suppose $A' \subseteq A$.
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|
@ -48,6 +54,7 @@
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$A \subseteq \R$ is closed iff all Cauchy sequences
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$A \subseteq \R$ is closed iff all Cauchy sequences
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||||||
in $A$ converge in $A$.
|
in $A$ converge in $A$.
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\end{claim}
|
\end{claim}
|
||||||
|
\gist{%
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||||||
\begin{subproof}
|
\begin{subproof}
|
||||||
Let $A$ be closed and $\langle x_n : n \in \omega \rangle$
|
Let $A$ be closed and $\langle x_n : n \in \omega \rangle$
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a Cauchy sequence in $A$.
|
a Cauchy sequence in $A$.
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|
@ -62,6 +69,7 @@
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we may pick $x_n \in (x - \frac{1}{n+1}, x + \frac{1}{n+1}) \cap A$
|
we may pick $x_n \in (x - \frac{1}{n+1}, x + \frac{1}{n+1}) \cap A$
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for all $n < \omega$.
|
for all $n < \omega$.
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||||||
\end{subproof}
|
\end{subproof}
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||||||
|
}{}
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||||||
|
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||||||
Now if $A' \subseteq A$ and $A$ were not closed,
|
Now if $A' \subseteq A$ and $A$ were not closed,
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there would be some Cauchy-sequence $(x_n)$
|
there would be some Cauchy-sequence $(x_n)$
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|
@ -92,6 +100,7 @@ We want to prove two things:
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Then $P \sim \R$.
|
Then $P \sim \R$.
|
||||||
\end{lemma}
|
\end{lemma}
|
||||||
\begin{proof}
|
\begin{proof}
|
||||||
|
\gist{%
|
||||||
It suffices to find an injection $f\colon \R \hookrightarrow P$.
|
It suffices to find an injection $f\colon \R \hookrightarrow P$.
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||||||
We have $\underbrace{\{0,1\}^{\omega}}_{\text{infinite 0-1-sequences}} \sim \R$,
|
We have $\underbrace{\{0,1\}^{\omega}}_{\text{infinite 0-1-sequences}} \sim \R$,
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||||||
hence it suffices to construct $f\colon \{0,1\}^\omega\hookrightarrow P$.
|
hence it suffices to construct $f\colon \{0,1\}^\omega\hookrightarrow P$.
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||||||
|
@ -132,4 +141,5 @@ We want to prove two things:
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and $f(t') \in [a_{t'\defon{n}}, b_{t'\defon{n}}]$
|
and $f(t') \in [a_{t'\defon{n}}, b_{t'\defon{n}}]$
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which are disjoint.
|
which are disjoint.
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Thus $f(t) \neq f(t')$, i.e.~$f$ is injective.
|
Thus $f(t) \neq f(t')$, i.e.~$f$ is injective.
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||||||
|
}{Cantor scheme.}
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||||||
\end{proof}
|
\end{proof}
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||||||
|
|
|
@ -77,7 +77,7 @@ all condensation points are accumulation points.
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||||||
\begin{subproof}
|
\begin{subproof}
|
||||||
$P \neq \emptyset$: $\checkmark$
|
$P \neq \emptyset$: $\checkmark$
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||||||
|
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||||||
$P \subseteq P'$ (i.e. $P$ is closed):
|
$P \subseteq P'$:
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||||||
% \begin{IEEEeqnarray*}{rCl}
|
% \begin{IEEEeqnarray*}{rCl}
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||||||
% P &=& \{x \in A | \text{every open neighbourhood of $x$ is uncountable}\}\\
|
% P &=& \{x \in A | \text{every open neighbourhood of $x$ is uncountable}\}\\
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||||||
% &\subseteq & \{x \in A | \text{every open neighbourhood of $x$ is at least countable}\} = P'.
|
% &\subseteq & \{x \in A | \text{every open neighbourhood of $x$ is at least countable}\} = P'.
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|
@ -97,7 +97,7 @@ all condensation points are accumulation points.
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||||||
But then $(a,b) \cap A$ is at most countable
|
But then $(a,b) \cap A$ is at most countable
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contradicting $ x \in P$.
|
contradicting $ x \in P$.
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|
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||||||
$P' \subseteq P$ :
|
$P' \subseteq P$ (i.e.~$P$ is closed):
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||||||
Let $x \in P'$.
|
Let $x \in P'$.
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||||||
Then for $a < x < b$ the set
|
Then for $a < x < b$ the set
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||||||
$(a,b) \cap P$
|
$(a,b) \cap P$
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||||||
|
@ -111,7 +111,7 @@ all condensation points are accumulation points.
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||||||
\]
|
\]
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||||||
\end{refproof}
|
\end{refproof}
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||||||
|
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||||||
\todo{Alternative proof of Cantor-Bendixson}
|
\gist{\todo{Alternative proof of Cantor-Bendixson}}{}
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||||||
% \begin{remark}
|
% \begin{remark}
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||||||
% There is an alternative proof of Cantor-Bendixson, going as follows:
|
% There is an alternative proof of Cantor-Bendixson, going as follows:
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% Fix $A \subseteq \R$ closed.
|
% Fix $A \subseteq \R$ closed.
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||||||
|
|
|
@ -5,13 +5,14 @@
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||||||
\section{\texorpdfstring{$\ZFC$}{ZFC}}
|
\section{\texorpdfstring{$\ZFC$}{ZFC}}
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||||||
|
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||||||
% 1900, Russel's paradox
|
% 1900, Russel's paradox
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\todo{Russel's Paradox}
|
% \todo{Russel's Paradox}
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||||||
$\ZFC$ stands for
|
$\ZFC$ stands for
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||||||
\begin{itemize}
|
\begin{itemize}
|
||||||
\item \textsc{Zermelo}’s axioms (1905), % crises around 19000
|
\item \textsc{Zermelo}’s axioms (1905), % crises around 19000
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||||||
\item \textsc{Fraenkel}'s axioms,
|
\item \textsc{Fraenkel}'s axioms,
|
||||||
\item the \yaref{ax:c}.
|
\item the \yaref{ax:c}.
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
|
\gist{
|
||||||
\begin{notation}
|
\begin{notation}
|
||||||
We write $x \subseteq y$ as a shorthand
|
We write $x \subseteq y$ as a shorthand
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||||||
for $\forall z.~(z \in x \implies z \in y)$.
|
for $\forall z.~(z \in x \implies z \in y)$.
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||||||
|
@ -45,6 +46,7 @@ $\ZFC$ stands for
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||||||
\forall u.~((u \in z) \iff (u \in x \land u \not\in y)).
|
\forall u.~((u \in z) \iff (u \in x \land u \not\in y)).
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||||||
\]
|
\]
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||||||
\end{notation}
|
\end{notation}
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||||||
|
}{Trivial, boring notation.}
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||||||
$\ZFC$ consists of the following axioms:
|
$\ZFC$ consists of the following axioms:
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||||||
\begin{axiom}[\vocab{Extensionality}]
|
\begin{axiom}[\vocab{Extensionality}]
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||||||
\yalabel{Axiom of Extensionality}{(Ext)}{ax:ext} % (AoE)
|
\yalabel{Axiom of Extensionality}{(Ext)}{ax:ext} % (AoE)
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||||||
|
@ -148,13 +150,15 @@ $\ZFC$ consists of the following axioms:
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||||||
\begin{axiomschema}[\vocab{Replacement} (Fraenkel)]
|
\begin{axiomschema}[\vocab{Replacement} (Fraenkel)]
|
||||||
\yalabel{Axiom of Replacement}{(Rep)}{ax:rep}
|
\yalabel{Axiom of Replacement}{(Rep)}{ax:rep}
|
||||||
Let $\phi$ be some $\cL_{\in }$ formula
|
Let $\phi$ be some $\cL_{\in }$ formula
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||||||
with free variables $x, y$.\todo{Allow more variables}
|
with free variables $x, y$.
|
||||||
Then
|
Then
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||||||
\[
|
\begin{IEEEeqnarray*}{l}
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||||||
\forall x \in a.~\exists !y \phi(x,y) \implies \exists b.~\forall x \in a.~\exists y \in b.~\phi(x,y).
|
\forall v_1 \ldots \forall v_p.~\\
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|
\left[\left( \forall x \exists! y.~\phi(x,y,\overline{v})\right) \to \forall a .~\exists b .~\forall y.~(y \in b \leftrightarrow \exists x (x \in a \land \phi(x,y, \overline{v}))\right]
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||||||
|
\end{IEEEeqnarray*}
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||||||
|
%\forall x \in a.~\exists !y \phi(x,y) \implies \exists b.~\forall x \in a.~\exists y \in b.~\phi(x,y).
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% \forall v_1 \ldots \forall v_p .~\forall x.~ \exists y'.~\forall y.~(y = y' \iff \phi(x))
|
% \forall v_1 \ldots \forall v_p .~\forall x.~ \exists y'.~\forall y.~(y = y' \iff \phi(x))
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||||||
% \implies \forall a.~\exists b.~\forall y.~(y \in b \iff \exists x.~(x \in a \land \phi(x))
|
% \implies \forall a.~\exists b.~\forall y.~(y \in b \iff \exists x.~(x \in a \land \phi(x))
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||||||
\]
|
|
||||||
\end{axiomschema}
|
\end{axiomschema}
|
||||||
|
|
||||||
\begin{axiom}[\vocab{Choice}]
|
\begin{axiom}[\vocab{Choice}]
|
||||||
|
|
|
@ -23,6 +23,7 @@
|
||||||
\]
|
\]
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||||||
\end{definition}
|
\end{definition}
|
||||||
|
|
||||||
|
\gist{%
|
||||||
\begin{definition}
|
\begin{definition}
|
||||||
For sets $x, y$ we write
|
For sets $x, y$ we write
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||||||
$(x,y)$ for $\{\{x\}, \{x,y\}\}$.
|
$(x,y)$ for $\{\{x\}, \{x,y\}\}$.
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||||||
|
@ -130,7 +131,7 @@
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||||||
(In other mathematical fields, this is sometimes
|
(In other mathematical fields, this is sometimes
|
||||||
denoted as $f(a)$. We don't do that here.)
|
denoted as $f(a)$. We don't do that here.)
|
||||||
\end{notation}
|
\end{notation}
|
||||||
|
}{[Some boring definitions omitted.]}
|
||||||
\begin{definition}
|
\begin{definition}
|
||||||
A binary relation $\le $ on a set $a$
|
A binary relation $\le $ on a set $a$
|
||||||
is a \vocab{partial order}
|
is a \vocab{partial order}
|
||||||
|
@ -168,7 +169,6 @@
|
||||||
x \in b \land \forall y \in b.~y \le x.
|
x \in b \land \forall y \in b.~y \le x.
|
||||||
\]
|
\]
|
||||||
|
|
||||||
|
|
||||||
In a similar way we define \vocab[Minimal element]{minimal elements}
|
In a similar way we define \vocab[Minimal element]{minimal elements}
|
||||||
and the \vocab{minimum} of $b$.
|
and the \vocab{minimum} of $b$.
|
||||||
We say that $x $ is an \vocab{upper bound}
|
We say that $x $ is an \vocab{upper bound}
|
||||||
|
@ -181,6 +181,7 @@
|
||||||
(This does not necessarily exist.)
|
(This does not necessarily exist.)
|
||||||
Similarly $\text{\vocab{$\inf$}}(b)$ is defined.
|
Similarly $\text{\vocab{$\inf$}}(b)$ is defined.
|
||||||
\end{definition}
|
\end{definition}
|
||||||
|
\gist{
|
||||||
\begin{remark}+
|
\begin{remark}+
|
||||||
Note that in a partial order,
|
Note that in a partial order,
|
||||||
a maximal element is not necessarily a maximum.
|
a maximal element is not necessarily a maximum.
|
||||||
|
@ -200,6 +201,7 @@
|
||||||
We write $(a,\le_a) \cong (b, \le_b)$
|
We write $(a,\le_a) \cong (b, \le_b)$
|
||||||
if they are isomorphic.
|
if they are isomorphic.
|
||||||
\end{definition}
|
\end{definition}
|
||||||
|
}{}
|
||||||
\begin{definition}
|
\begin{definition}
|
||||||
Let $(a,\le)$ be a partial order.
|
Let $(a,\le)$ be a partial order.
|
||||||
Then $(a,\le)$ is
|
Then $(a,\le)$ is
|
||||||
|
|
|
@ -8,6 +8,7 @@
|
||||||
Then $a$ has a maximal element.
|
Then $a$ has a maximal element.
|
||||||
\end{theorem}
|
\end{theorem}
|
||||||
\begin{refproof}{thm:zorn}
|
\begin{refproof}{thm:zorn}
|
||||||
|
\gist{%
|
||||||
Fix $(a, \le )$ as in the hypothesis.
|
Fix $(a, \le )$ as in the hypothesis.
|
||||||
Let $A \coloneqq \{ \{(b,x) : x \in b\} : b \subseteq a, b \neq \emptyset\}$.
|
Let $A \coloneqq \{ \{(b,x) : x \in b\} : b \subseteq a, b \neq \emptyset\}$.
|
||||||
Note that $A$ is a set (use separation on $\cP(\cP(a) \times \bigcup \cP(a))$).
|
Note that $A$ is a set (use separation on $\cP(\cP(a) \times \bigcup \cP(a))$).
|
||||||
|
@ -65,6 +66,36 @@
|
||||||
Then $B = B_{u_0}^{\le^{\ast\ast}}$.
|
Then $B = B_{u_0}^{\le^{\ast\ast}}$.
|
||||||
So $\le^{\ast\ast} \in W$, but now $u_0 \in b$.
|
So $\le^{\ast\ast} \in W$, but now $u_0 \in b$.
|
||||||
So $b$ must have a maximum.
|
So $b$ must have a maximum.
|
||||||
|
\todo{Why does this prove the lemma?}
|
||||||
|
}{
|
||||||
|
\begin{itemize}
|
||||||
|
\item $A \coloneqq \{\{(b,x) : x \in b\}, b \subseteq a, b \neq \emptyset\}$.
|
||||||
|
\item \AxC $\leadsto$ choice function on $A$,
|
||||||
|
$f\colon \cP(a) \setminus \{\emptyset\} \to a$, $f(b) \in b$.
|
||||||
|
\item $\le^\ast$ on $a$:
|
||||||
|
\begin{itemize}
|
||||||
|
\item $W \coloneqq \{\le' \text{wo on} b \subseteq a : \forall u,b \in b.~u \le' v \implies u \le v, B_u^{\le '} \neq \emptyset, u = f(B^{\le '}_u)\}$ where
|
||||||
|
\item $B_u^{\le'} = \{w \in a : w \text{ $\le $-upper bound of } \{v \in b : v <' u\} \}$.
|
||||||
|
\end{itemize}
|
||||||
|
\item $\le', \le '' \in W \implies \le' \substack{\subseteq\\\supseteq} \le''$:
|
||||||
|
\begin{itemize}
|
||||||
|
\item $(b, \le') \overset{g}{\cong} (c, \le'') (\defon{v})$.
|
||||||
|
\item $g = \id_b$:
|
||||||
|
\begin{itemize}
|
||||||
|
\item $u_0$ $\le'$ minimal with $g(u_0) \neq u_0$.
|
||||||
|
\item $\{w \in b : w <' u_0\} \overset{g \defon{\ldots}}{=} \{w \in c : w <'' g(u_0)\}$.
|
||||||
|
\item $B^{\le '}_{u_0} = B_{g(u_0)}^{\le ''} \neq \emptyset$,
|
||||||
|
so $u_0 = f(B^{\le '}_{u_0}) = f(B^{\le ''}_{g(u_0)}) = g(u_0) \lightning$
|
||||||
|
\end{itemize}
|
||||||
|
\end{itemize}
|
||||||
|
\item $\le^\ast \coloneqq \bigcup W$ is wo on $b \subseteq a$.
|
||||||
|
\item Suppose $b$ has no maximum. Then $B \cap b = \emptyset$.
|
||||||
|
\item $u_0 \coloneqq f(B)$, $\le^{\ast\ast} = \le^\ast \cup \{(u,u_0) | u \in b\} \cup \{(u_0,u_0)\}$.
|
||||||
|
\item $B = B_{u_0}^{\le^{\ast\ast}}$, so $\le^{\ast\ast} \in W$,
|
||||||
|
but $u_0 \in b \lightning$. ?
|
||||||
|
\end{itemize}
|
||||||
|
}
|
||||||
|
|
||||||
\end{refproof}
|
\end{refproof}
|
||||||
|
|
||||||
\begin{remark}
|
\begin{remark}
|
||||||
|
@ -82,6 +113,7 @@
|
||||||
Then $A$ contains a $\subseteq$-maximal element.
|
Then $A$ contains a $\subseteq$-maximal element.
|
||||||
\end{corollary}
|
\end{corollary}
|
||||||
|
|
||||||
|
\gist{%
|
||||||
\begin{remark}[Cultural enrichment]
|
\begin{remark}[Cultural enrichment]
|
||||||
Other assertions which are equivalent
|
Other assertions which are equivalent
|
||||||
to the \yaref{ax:c}:
|
to the \yaref{ax:c}:
|
||||||
|
@ -96,12 +128,14 @@
|
||||||
\item Every set can be well-ordered.%\footnote{This is clearly false.}
|
\item Every set can be well-ordered.%\footnote{This is clearly false.}
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
\end{remark}
|
\end{remark}
|
||||||
|
}{}
|
||||||
% \begin{remark}
|
% \begin{remark}
|
||||||
% The axiom of choice is true.
|
% The axiom of choice is true.
|
||||||
% \end{remark}
|
% \end{remark}
|
||||||
|
|
||||||
\pagebreak
|
\pagebreak
|
||||||
\subsection{The Ordinals}
|
\subsection{The Ordinals}
|
||||||
|
\gist{
|
||||||
\begin{goal}
|
\begin{goal}
|
||||||
We want to define nice representatives of the equivalence classes
|
We want to define nice representatives of the equivalence classes
|
||||||
of well-orders.
|
of well-orders.
|
||||||
|
@ -115,7 +149,7 @@ We can hence form the smallest inductive set
|
||||||
Note that $\omega$ exists, as it is a subset of the inductive
|
Note that $\omega$ exists, as it is a subset of the inductive
|
||||||
set given by \AxInf.
|
set given by \AxInf.
|
||||||
We call $\omega$ the set of \vocab{natural numbers}.
|
We call $\omega$ the set of \vocab{natural numbers}.
|
||||||
|
}{}
|
||||||
\begin{notation}
|
\begin{notation}
|
||||||
We write $0$ for $\emptyset$,
|
We write $0$ for $\emptyset$,
|
||||||
and $y + 1$ for $y \cup \{y\}$.
|
and $y + 1$ for $y \cup \{y\}$.
|
||||||
|
@ -147,16 +181,19 @@ We have the following principle of induction:
|
||||||
and for all $y, z \in x$,
|
and for all $y, z \in x$,
|
||||||
we have that $y = z$, $y \in z$ or $y \ni z$.
|
we have that $y = z$, $y \in z$ or $y \ni z$.
|
||||||
\end{definition}
|
\end{definition}
|
||||||
|
\gist{
|
||||||
Clearly, the $\in$-relation is a well-order on an ordinal $x$.
|
Clearly, the $\in$-relation is a well-order on an ordinal $x$.
|
||||||
\begin{remark}
|
\begin{remark}
|
||||||
This definition is due to \textsc{John von Neumann}.
|
This definition is due to \textsc{John von Neumann}.
|
||||||
\end{remark}
|
\end{remark}
|
||||||
|
}{}
|
||||||
|
|
||||||
\begin{lemma}
|
\begin{lemma}
|
||||||
Each natural number (i.e.~element of $\omega$)
|
Each natural number (i.e.~element of $\omega$)
|
||||||
is an ordinal.
|
is an ordinal.
|
||||||
\end{lemma}
|
\end{lemma}
|
||||||
\begin{proof}
|
\begin{proof}
|
||||||
|
\gist{
|
||||||
We use \yaref{lem:induction}.
|
We use \yaref{lem:induction}.
|
||||||
Clearly $\emptyset$ is an ordinal.
|
Clearly $\emptyset$ is an ordinal.
|
||||||
Now let $\alpha$ be an ordinal.
|
Now let $\alpha$ be an ordinal.
|
||||||
|
@ -169,6 +206,7 @@ Clearly, the $\in$-relation is a well-order on an ordinal $x$.
|
||||||
since $\alpha$ is an ordinal.
|
since $\alpha$ is an ordinal.
|
||||||
Suppose $x = \alpha$.
|
Suppose $x = \alpha$.
|
||||||
Then either $y = x$ or $y \in \alpha = x$.
|
Then either $y = x$ or $y \in \alpha = x$.
|
||||||
|
}{Induction}
|
||||||
\end{proof}
|
\end{proof}
|
||||||
|
|
||||||
\begin{lemma}
|
\begin{lemma}
|
||||||
|
|
|
@ -20,6 +20,7 @@
|
||||||
\end{enumerate}
|
\end{enumerate}
|
||||||
\end{lemma}
|
\end{lemma}
|
||||||
\begin{refproof}{lem:7:ordinalfacts}
|
\begin{refproof}{lem:7:ordinalfacts}
|
||||||
|
\gist{
|
||||||
We have already proved (a) before.
|
We have already proved (a) before.
|
||||||
|
|
||||||
(b) Fix $x \in \alpha$. Then $x \subseteq \alpha$.
|
(b) Fix $x \in \alpha$. Then $x \subseteq \alpha$.
|
||||||
|
@ -113,6 +114,7 @@
|
||||||
But this violates \AxFund,
|
But this violates \AxFund,
|
||||||
as $\alpha_0 \in \beta_0 \in \alpha_0$.
|
as $\alpha_0 \in \beta_0 \in \alpha_0$.
|
||||||
\end{subproof}
|
\end{subproof}
|
||||||
|
}{Long and tedious, but not many ideas.}
|
||||||
\end{refproof}
|
\end{refproof}
|
||||||
|
|
||||||
\begin{lemma}
|
\begin{lemma}
|
||||||
|
@ -146,6 +148,7 @@ for example $\bigcup \omega = \omega$.
|
||||||
Otherwise $\alpha$ is called
|
Otherwise $\alpha$ is called
|
||||||
a \vocab[Ordinal!limit]{limit ordinal}.
|
a \vocab[Ordinal!limit]{limit ordinal}.
|
||||||
\end{definition}
|
\end{definition}
|
||||||
|
\gist{
|
||||||
\begin{observe}
|
\begin{observe}
|
||||||
Note that $\alpha$ is a limit ordinal iff for all
|
Note that $\alpha$ is a limit ordinal iff for all
|
||||||
$\beta \in \alpha$, $\beta + 1 \in \alpha$:
|
$\beta \in \alpha$, $\beta + 1 \in \alpha$:
|
||||||
|
@ -158,7 +161,8 @@ for example $\bigcup \omega = \omega$.
|
||||||
then by definition there is some $\beta \in \alpha$,
|
then by definition there is some $\beta \in \alpha$,
|
||||||
with $\beta + 1 = \alpha$, so $\beta + 1 \not\in \alpha$.
|
with $\beta + 1 = \alpha$, so $\beta + 1 \not\in \alpha$.
|
||||||
\end{observe}
|
\end{observe}
|
||||||
|
}{}
|
||||||
|
\gist{
|
||||||
\begin{notation}
|
\begin{notation}
|
||||||
If $\alpha, \beta$ are ordinals,
|
If $\alpha, \beta$ are ordinals,
|
||||||
we write $\alpha < \beta$ for $\alpha \in \beta$
|
we write $\alpha < \beta$ for $\alpha \in \beta$
|
||||||
|
@ -185,3 +189,4 @@ for example $\bigcup \omega = \omega$.
|
||||||
\item $\omega +1 = \omega \cup \{\omega\} , \omega + 2, \ldots$,
|
\item $\omega +1 = \omega \cup \{\omega\} , \omega + 2, \ldots$,
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
\end{example}
|
\end{example}
|
||||||
|
}{}
|
||||||
|
|
|
@ -1,6 +1,7 @@
|
||||||
\lecture{08}{2023-11-13}{Induction and recursion}
|
\lecture{08}{2023-11-13}{Induction and recursion}
|
||||||
|
|
||||||
\subsection{Classes}
|
\subsection{Classes}
|
||||||
|
\gist{
|
||||||
It is often very handy to work in a class theory rather than
|
It is often very handy to work in a class theory rather than
|
||||||
in set theory.
|
in set theory.
|
||||||
|
|
||||||
|
@ -11,10 +12,11 @@ sets (denoted by lower case letters)
|
||||||
and classes (denoted by capital letters),
|
and classes (denoted by capital letters),
|
||||||
as well as one binary relation symbol $\in$
|
as well as one binary relation symbol $\in$
|
||||||
for membership.
|
for membership.
|
||||||
|
}{}
|
||||||
|
|
||||||
\vocab{Bernays-Gödel class theory} (\vocab{BG})
|
\vocab{Bernays-Gödel class theory} (\vocab{BG})
|
||||||
has the following axioms:
|
has the following axioms:
|
||||||
|
\gist{
|
||||||
\begin{axiom}[Extensionality]
|
\begin{axiom}[Extensionality]
|
||||||
\yalabel{Axiom of Extensionality}{(Ext)}{ax:bg:ext}
|
\yalabel{Axiom of Extensionality}{(Ext)}{ax:bg:ext}
|
||||||
\[
|
\[
|
||||||
|
@ -54,6 +56,7 @@ has the following axioms:
|
||||||
\exists x .~(\emptyset \in x \land \left( \forall y \in x .~y \cup \{y\} \in x \right)).
|
\exists x .~(\emptyset \in x \land \left( \forall y \in x .~y \cup \{y\} \in x \right)).
|
||||||
\]
|
\]
|
||||||
\end{axiom}
|
\end{axiom}
|
||||||
|
}{\AxExt, \AxFund, \AxPair, \AxUnion, \AxPow, \AxInf as from $\ZF$.}
|
||||||
|
|
||||||
Together with the following axioms for classes:
|
Together with the following axioms for classes:
|
||||||
|
|
||||||
|
@ -108,10 +111,10 @@ Furthermore $F\,''a \coloneqq \{y : \exists x \in a .~(x,y) \in F\}$.
|
||||||
\end{axiom}
|
\end{axiom}
|
||||||
|
|
||||||
|
|
||||||
\todo{notation: $\emptyset, \cap$}
|
\gist{\todo{notation: $\emptyset, \cap$}}{}
|
||||||
|
|
||||||
|
|
||||||
\todo{the following was actually done in lecture 9}
|
\gist{(The following was actually done in lecture 9, but has been moved here for clarity.)}{}
|
||||||
|
|
||||||
$\BGC$ (in German often NBG\footnote{\vocab{Neumann-Bernays-Gödel}})
|
$\BGC$ (in German often NBG\footnote{\vocab{Neumann-Bernays-Gödel}})
|
||||||
is defined to be $\BG$
|
is defined to be $\BG$
|
||||||
|
|
|
@ -75,7 +75,7 @@ We often write $\kappa, \lambda, \ldots$ for cardinals.
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
}
|
}
|
||||||
\end{proof}
|
\end{proof}
|
||||||
We may now use the \yaref{lem:recursion}
|
We may now use the \yaref{thm:recursion}
|
||||||
to define a sequence $\langle \aleph_\alpha : \alpha \in \OR \rangle$
|
to define a sequence $\langle \aleph_\alpha : \alpha \in \OR \rangle$
|
||||||
with the following properties:
|
with the following properties:
|
||||||
\begin{IEEEeqnarray*}{rCl}
|
\begin{IEEEeqnarray*}{rCl}
|
||||||
|
|
|
@ -322,7 +322,7 @@ We have shown (assuming \AxC to choose contained clubs):
|
||||||
and $S_1 \coloneqq \{\xi < \kappa : \cf(\xi) = \omega_1\}$.
|
and $S_1 \coloneqq \{\xi < \kappa : \cf(\xi) = \omega_1\}$.
|
||||||
Clearly these are disjoint.
|
Clearly these are disjoint.
|
||||||
They are both stationary:
|
They are both stationary:
|
||||||
Let $c \subseteq \kappa$ be a club.
|
Let $C \subseteq \kappa$ be a club.
|
||||||
Let $(\xi_i : i \le \omega_1)$
|
Let $(\xi_i : i \le \omega_1)$
|
||||||
be defined as follows:
|
be defined as follows:
|
||||||
$\xi_0 \coloneqq \min C$,
|
$\xi_0 \coloneqq \min C$,
|
||||||
|
|
|
@ -60,7 +60,7 @@
|
||||||
Recall the following:
|
Recall the following:
|
||||||
|
|
||||||
\begin{definition}
|
\begin{definition}
|
||||||
A substructure $X \subseteq V_\theta$\todo{make this more general. Explain why $V_\theta$ is a model}
|
A substructure $X \subseteq V_\theta$\gist{\todo{make this more general. Explain why $V_\theta$ is a model}}{}
|
||||||
is an \vocab{elementary substructure}
|
is an \vocab{elementary substructure}
|
||||||
of $V_\theta$,
|
of $V_\theta$,
|
||||||
denoted $X \prec V_{\theta}$,\footnote{more formally $(X,\in ) \prec (V_{\theta})$}
|
denoted $X \prec V_{\theta}$,\footnote{more formally $(X,\in ) \prec (V_{\theta})$}
|
||||||
|
|
|
@ -236,8 +236,7 @@ We will only proof
|
||||||
\item $X \le Y :\iff \{\alpha < \omega_1 : f_X(\alpha) \le f_Y(\alpha)\}$ stationary.
|
\item $X \le Y :\iff \{\alpha < \omega_1 : f_X(\alpha) \le f_Y(\alpha)\}$ stationary.
|
||||||
\item (2) $X\le Y \lor Y \le X$:
|
\item (2) $X\le Y \lor Y \le X$:
|
||||||
Suppose $X \not\le Y, Y \not\le X$. Choose witnessing clubs $C, D$.
|
Suppose $X \not\le Y, Y \not\le X$. Choose witnessing clubs $C, D$.
|
||||||
$C \cap D$ is club, but then $f_X(\alpha) \le f_Y(\alpha)$ or
|
$C \cap D$ is club, so $C\cap D \ni \alpha \implies f_X(\alpha) \substack{\nleq\\\ngeq} f_Y(\alpha) \lightning$
|
||||||
$f_X(\alpha) \ge f_Y(\alpha)$ for $\alpha \in C \cap D$.
|
|
||||||
\item (3) $X \subseteq \aleph_{ \omega_1}$, then $|\underbrace{\{Y \subseteq \aleph_{ \omega_1} : Y \le X\}}_{A}| \le \aleph_{ \omega_1}$
|
\item (3) $X \subseteq \aleph_{ \omega_1}$, then $|\underbrace{\{Y \subseteq \aleph_{ \omega_1} : Y \le X\}}_{A}| \le \aleph_{ \omega_1}$
|
||||||
\begin{itemize}
|
\begin{itemize}
|
||||||
\item Suppose $|A| \ge \aleph_{ \omega_1 + 1}$.
|
\item Suppose $|A| \ge \aleph_{ \omega_1 + 1}$.
|
||||||
|
@ -245,9 +244,21 @@ We will only proof
|
||||||
$2^{\aleph_1} = \aleph_2 \implies$ at most $\aleph_2$ such $S_Y$.
|
$2^{\aleph_1} = \aleph_2 \implies$ at most $\aleph_2$ such $S_Y$.
|
||||||
\item If $\forall S \in \cP( \omega_1).~ |\underbrace{\{Y \in A : S_Y = S\}}_{A_S}| < \aleph_{ \omega_1 + 1}$,
|
\item If $\forall S \in \cP( \omega_1).~ |\underbrace{\{Y \in A : S_Y = S\}}_{A_S}| < \aleph_{ \omega_1 + 1}$,
|
||||||
then $|A| \le \aleph_2 \cdot <\aleph_{ \omega_1 + 1}$ $\lightning$ $\aleph_{ \omega_1 + 1}$ regular.
|
then $|A| \le \aleph_2 \cdot <\aleph_{ \omega_1 + 1}$ $\lightning$ $\aleph_{ \omega_1 + 1}$ regular.
|
||||||
\item So $\exists S \in \omega_1.~|A_S| = \aleph_{ \omega_1 + 1 + 1}$.
|
\item So $\exists S \in \omega_1.~|A_S| = \aleph_{ \omega_1 + 1}$.
|
||||||
% TODO TODO TODO hier weiter!
|
\item Fix surjection $\langle g_\alpha : \aleph_\alpha \twoheadrightarrow f_X(\alpha) + 1 : \alpha \in S \rangle$.
|
||||||
|
($f_Y(\alpha) \le f_X(\alpha) < \aleph_{\alpha+1}$)
|
||||||
|
\item $\forall Y \in A_S$ define $\overline{f}_Y \colon S \to \aleph_{ \omega_1}, \alpha \mapsto \min \{\xi : g_\alpha(\xi) = f_Y(\alpha)\}$.
|
||||||
|
\item $S^\ast$ ($S \cap$ limit ordinals) is stationary.
|
||||||
|
\item $\forall Y \in A$ define $h_Y\colon S^\ast \to \omega_1, \alpha \mapsto \min \{\beta < \alpha : \overline{f}_Y(\alpha) < \aleph_\beta\}$.
|
||||||
|
\item Apply \yaref{thm:fodor} to $h_Y, S^\ast$ to get $T_Y \subseteq S^\ast$ stationary with $h_Y\defon{T_Y}$ constant.
|
||||||
|
\item $\exists T.~|\underbrace{\{Y \in A_S : T_Y = T\}}_{A_{S,T}}| = \aleph_{ \omega_1 + 1}$.
|
||||||
|
\item Let $\{\beta\} = h_Y''T$, i.e.~$\overline{f}_Y(\alpha) < \aleph_{\beta}$ for $Y\in A_{S,T}, \alpha \in T$.
|
||||||
|
\item $\leftindex^T \aleph_\beta \le 2^{\aleph_\beta \cdot \aleph_1} = \aleph_{\beta+1} \aleph_2 < \aleph_{ \omega_1}$.
|
||||||
|
\item $\exists \tilde{f}\colon T \to \aleph_\beta .~ |\underbrace{\{Y \in A_{S,T} : \overline{f}_Y\defon{T} = \tilde{f}\} }_{A_{S,T,\tilde{f}}}| = \aleph_{ \omega_1 + 1}$.
|
||||||
|
\item $Y,Y' \in A_{S,T,\tilde{f}} \implies \forall \alpha \in T.~f_Y(\alpha) = f_{Y'}\left( \alpha \right) \overset{T \text{ unbounded}}{\implies} Y = Y'$
|
||||||
|
Thus $|A_{S,T, \tilde{f}} | \le 1 \lightning$.
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
|
|
||||||
\item Define sequence $\langle X_i : i < \aleph_{ \omega_1 + 1} \rangle$
|
\item Define sequence $\langle X_i : i < \aleph_{ \omega_1 + 1} \rangle$
|
||||||
of subsets of $\aleph_{\omega_1}$:
|
of subsets of $\aleph_{\omega_1}$:
|
||||||
\begin{itemize}
|
\begin{itemize}
|
||||||
|
@ -259,7 +270,7 @@ We will only proof
|
||||||
\item $P \coloneqq \{Y \subseteq \aleph_{ \omega_1} : \exists i < \aleph_{ \omega_1 + 1}.~Y \le X_i\}$
|
\item $P \coloneqq \{Y \subseteq \aleph_{ \omega_1} : \exists i < \aleph_{ \omega_1 + 1}.~Y \le X_i\}$
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
\item $|P| \overset{\text{in fact } =}{\le} \aleph_{ \omega_1 + 1} \aleph_{ \omega_1} = \aleph_{ \omega_1 + 1}$ by (3).
|
\item $|P| \overset{\text{in fact } =}{\le} \aleph_{ \omega_1 + 1} \aleph_{ \omega_1} = \aleph_{ \omega_1 + 1}$ by (3).
|
||||||
\item $X \in \cP(\aleph_{ \omega_1}) \setminus M \implies \forall i < \aleph_{ \omega_1 + 1}.~X_i \le X$
|
\item $X \in \cP(\aleph_{ \omega_1}) \setminus P \implies \forall i < \aleph_{ \omega_1 + 1}.~X_i \le X$
|
||||||
$\lightning$ (3).
|
$\lightning$ (3).
|
||||||
Thus $P = \cP(\aleph_{ \omega_1})$.
|
Thus $P = \cP(\aleph_{ \omega_1})$.
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
|
|
|
@ -77,6 +77,7 @@
|
||||||
\end{enumerate}
|
\end{enumerate}
|
||||||
\end{theorem}
|
\end{theorem}
|
||||||
\begin{proof}
|
\begin{proof}
|
||||||
|
\gist{%
|
||||||
2. $\implies$ 1.:
|
2. $\implies$ 1.:
|
||||||
Fix $j\colon V \to M$.
|
Fix $j\colon V \to M$.
|
||||||
Let $U = \{X \subseteq \kappa : \kappa \in j(X)\}$.
|
Let $U = \{X \subseteq \kappa : \kappa \in j(X)\}$.
|
||||||
|
@ -221,11 +222,60 @@ such that
|
||||||
as otherwise $\forall \delta < \alpha. ~ \kappa \setminus X_\delta \in U$,
|
as otherwise $\forall \delta < \alpha. ~ \kappa \setminus X_\delta \in U$,
|
||||||
i.e.~$\emptyset = (\bigcap_{\delta < \alpha} \kappa \setminus X_{\delta}) \cap X \in U \lightning$.
|
i.e.~$\emptyset = (\bigcap_{\delta < \alpha} \kappa \setminus X_{\delta}) \cap X \in U \lightning$.
|
||||||
We get $[f] = [c_{\delta}]$,
|
We get $[f] = [c_{\delta}]$,
|
||||||
so $\beta = \sigma([f]) = \delta([c_{\delta}]) = j(\delta) = \delta$,
|
so $\beta = \sigma([f]) = \sigma([c_{\delta}]) = j(\delta) = \delta$,
|
||||||
where for the last equality we have applied the induction hypothesis.
|
where for the last equality we have applied the induction hypothesis.
|
||||||
So $j(\alpha) \le \alpha$.
|
So $j(\alpha) \le \alpha$.
|
||||||
|
|
||||||
|
For all $\eta < \kappa$, we have $\eta = \sigma([c_{\eta}]) < \sigma([c_{\id}]) < \sigma([c_{\kappa}])$,
|
||||||
|
so $j(\kappa) > \kappa$.
|
||||||
|
|
||||||
% It is also easy to show $j(\kappa) > \kappa$.
|
% It is also easy to show $j(\kappa) > \kappa$.
|
||||||
|
}{%
|
||||||
|
$2 \implies 1$:
|
||||||
|
Fix $j\colon V \to M$. $U \coloneqq \{X \subseteq \kappa : \kappa \in j(X)\}$ is an UF:
|
||||||
|
\begin{itemize}
|
||||||
|
\item $M \models j(X \cap Y) = j(X) \cap j(Y)$,
|
||||||
|
hence $\kappa \in j(X) \cap j(Y) \implies \kappa \in j(X \cap Y)$
|
||||||
|
\item $M \ni X \subseteq Y \implies Y \in M$, $\emptyset \not\in M$ (same argument)
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\item $\kappa \in U$:
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\begin{itemize}
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\item $\forall \alpha \in \OR.~ j(\alpha) \in \Ord, j(\alpha) \ge \alpha$:
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\begin{itemize}
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\item Write $\alpha\in \OR$ and use $\alpha \in \OR \iff M \models j(\alpha) \in \OR$.
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\item Suppose $j(\alpha) < \alpha$, $\alpha$ minimal,
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but $M \models j(j(\alpha)) < j(\alpha) \implies j(j(\alpha)) < j(\alpha)$.
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\end{itemize}
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\item $j(\kappa) \neq \kappa \implies j(\kappa) > \kappa \implies \kappa \in j(\kappa)$.
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\end{itemize}
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\item Ultrafilter: $\kappa \in j(\kappa) = j(X \cup (\kappa \setminus X)) = j(X) \cup j(\kappa \setminus X)$.
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\item $< \kappa$ closed: For $\theta < \kappa$, $X_i \in U$:
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$\kappa \in \bigcap_{i < \theta} j(X_i) = j\left( \bigcap_{i < \theta} X_i \right) \in U$.
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($j(\theta) = \theta$, so $j\left( \langle X_i : i < \theta \rangle \right) = \langle j(X_i) : i < \theta \rangle$.
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\item Not principal:
|
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|
$\xi < \kappa \implies j(\{\xi\}) = \{\xi\} \not\ni \kappa$.
|
||||||
|
\end{itemize}
|
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|
$1 \implies 2$:
|
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|
\begin{itemize}
|
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\item Fix $U$. Consider $\leftindex^\kappa V$.
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|
\item $f \sim g :\iff \{\xi < \kappa: f(\xi) = g(\xi)\} \in U$.
|
||||||
|
\item $[f] \coloneqq \{g : g \sim f \land g \in V_\alpha \text{ for minimal } \alpha\}$ (Scott's Trick).
|
||||||
|
\item $[f] \tilde{\in } [g] :\iff \{\xi < \kappa: f(\xi ) \in g(\xi)\} \in U$.
|
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|
\item \yaref{thm:los}: $(\cF, \tilde{\in }) \models \phi([f_1], \ldots, [f_k]) \iff \{\xi < \kappa: (V, \in ) \models \phi(f_1(\xi), \ldots, f_k(\xi))\} \in U$.
|
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|
\item $\overline{j}(x) \coloneqq [ \xi \mapsto x]$.
|
||||||
|
\item $\tilde{\in }$ well-founded: lift decreasing sequences ($U$ is $<\kappa$ closed, $ \omega < \kappa$)
|
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|
\item $\tilde{\in }$ set-like $\overset{\yaref{thm:mostowksi}}{\leadsto}$ $(\cF, \tilde{\in }) \overset{\sigma}{\cong} (M, \in )$.
|
||||||
|
\item $j \coloneqq \sigma \circ \overline{j}$.
|
||||||
|
\item $\alpha < \kappa \implies j(\alpha) = \alpha$ (know $j(\alpha) \ge \alpha$):
|
||||||
|
\begin{itemize}
|
||||||
|
\item Induction for $j(\alpha) \le \alpha$: Fix $\alpha$, $\sigma([f]) = \beta \in j(\alpha)$.
|
||||||
|
\item $[f] \tilde{\in }[c_\alpha]$.
|
||||||
|
\item $\exists \delta < \alpha.~[f] = [c_\delta]$ ($U $ is $<\kappa$ closed)
|
||||||
|
\item $\beta = \sigma([f]) = \sigma([c_\delta]) = j(\delta) \overset{\text{IH}}{=} \delta \in \alpha$.
|
||||||
|
\end{itemize}
|
||||||
|
\item $j(\kappa) \neq \kappa$:
|
||||||
|
$\forall \eta < \kappa.~\eta = \sigma([c_{\eta}]) < \sigma([\id]) < \sigma([\kappa])$.
|
||||||
|
\end{itemize}
|
||||||
|
}
|
||||||
\end{proof}
|
\end{proof}
|
||||||
|
|
||||||
\begin{theorem}[\L o\'s]
|
\begin{theorem}[\L o\'s]
|
||||||
|
|
|
@ -137,7 +137,7 @@ is well founded.
|
||||||
so $M[g] \models \text{``$\{x,y\}$ is the pair of $x$ and $y$''}$.
|
so $M[g] \models \text{``$\{x,y\}$ is the pair of $x$ and $y$''}$.
|
||||||
Hence $M[g] \models \AxPair$.
|
Hence $M[g] \models \AxPair$.
|
||||||
\item \AxUnion:
|
\item \AxUnion:
|
||||||
Similar to \AxPair.\todo{Exercise}
|
Similar to \AxPair.\gist{\todo{Exercise}}{}
|
||||||
\end{itemize}
|
\end{itemize}
|
||||||
\end{proof}
|
\end{proof}
|
||||||
|
|
||||||
|
|
Loading…
Reference in a new issue