diff --git a/iii/forcing/04_forcing_and_independence.tex b/iii/forcing/04_forcing_and_independence.tex index c07d602..e5be099 100644 --- a/iii/forcing/04_forcing_and_independence.tex +++ b/iii/forcing/04_forcing_and_independence.tex @@ -561,7 +561,7 @@ \subsection{The mixing lemma} For example, in \( p \Vdash \exists x.\, (\dot a \in x \wedge \dot b \in x) \), we can find a name \( \dot x = \operatorname{op}(\dot a, \dot b) \) without needing to know \( G \). Informally, the mixing lemma says that this is always the case, as long as \( M \) has \( \mathsf{AC} \). \begin{theorem}[the mixing lemma] - (\mathsf{ZFC}) + (\( \mathsf{ZFC} \)) Suppose that \( (p \Vdash \exists x.\, \varphi(x))^M \). Then there is a name \( \dot x \in M^{\mathbb P} \) such that \( (p \Vdash \varphi(\dot x))^M \). \end{theorem} diff --git a/iii/lc/02_measurable_cardinals.tex b/iii/lc/02_measurable_cardinals.tex index a1ce735..fa65a7a 100644 --- a/iii/lc/02_measurable_cardinals.tex +++ b/iii/lc/02_measurable_cardinals.tex @@ -545,7 +545,7 @@ \subsection{Ultrapowers of the universe} (c_\alpha) < (\id_\kappa) &\leftrightarrow \qty{\gamma \mid c_\alpha(\gamma) < \id_\kappa(\gamma)} \in U \\ &\leftrightarrow \qty{\gamma \mid \alpha < \gamma} \in U \end{align*} - But by a size argument, \( \qty{\gamma \mid \gamma \leq \alpha} \notin U \) as \( U \) is nonprincipal, so we must have \( \alpha < (id) \). + But by a size argument, \( \qty{\gamma \mid \gamma \leq \alpha} \notin U \) as \( U \) is nonprincipal, so we must have \( \alpha < (\id) \). Also, \begin{align*} (\id_\kappa) < (c_\kappa) &\leftrightarrow \qty{\gamma \mid \id_\kappa(\gamma) < c_\kappa(\gamma)} \in U \\ @@ -795,7 +795,7 @@ \subsection{The fundamental theorem on measurable cardinals} \end{proposition} This proposition provides an alternative view of reflection. Suppose that the ultrafilter \( U \) on \( \kappa \) is normal. -If \( M \vDash \Phi(\kappa) \), then \( M \vDash \Phi((id)) \). +If \( M \vDash \Phi(\kappa) \), then \( M \vDash \Phi((\id)) \). By \L{}o\'s' theorem, \[ \qty{\alpha < \kappa \mid \Phi(\id(\alpha))} \in U \] So \( \Phi \) reflects not only on a set of size \( \kappa \), but on an ultrafilter set. @@ -815,7 +815,7 @@ \subsection{The fundamental theorem on measurable cardinals} \end{cases} \] Thus the set of \( \alpha \) such that \( f(\alpha) \) is an \( \alpha \)-complete nonprincipal ultrafilter on \( \alpha \) is \( C \), so in \( U \). Equivalently, the set of \( \alpha \) such that \( f(\alpha) \) is an \( \id(\alpha) \)-complete nonprincipal ultrafilter on \( \id(\alpha) \) is in \( U \). - So by \L{}o\'s' theorem, \( M \) believes that \( (f) \) is an \( (id) \)-complete nonprincipal ultrafilter on \( (id) \). + So by \L{}o\'s' theorem, \( M \) believes that \( (f) \) is an \( (\id) \)-complete nonprincipal ultrafilter on \( (\id) \). So \( (f) \) witnesses that \( \kappa \) is measurable in \( M \). \end{proof} This shows that whether a cardinal \( \kappa \) is surviving depends only on \( \mathrm{V}_{\kappa + 2} \), and is therefore a 2-stable property.