diff --git a/content/blog/implications-prenex-normal-form.md b/content/blog/implications-prenex-normal-form.md index a6bad39..a8990b6 100644 --- a/content/blog/implications-prenex-normal-form.md +++ b/content/blog/implications-prenex-normal-form.md @@ -37,20 +37,11 @@ $$ \begin{align*} (\forall x \phi) \implies \psi &\iff \neg (\forall x \phi) \vee \psi \tag{0.1} \\\\ &\iff (\exists x \neg \phi) \vee \psi \tag{2.2}\\\\ -&\iff \exists x (\neg \phi \vee \psi) \tag{1.2}\\\\ -&\iff \exists x (\phi \implies \psi) \tag{0.1} +&\iff \exists x (\neg \phi \vee \psi) \tag{2.1}\\\\ +&\iff \exists x (\neg \phi \implies \psi) \tag{0.1} \end{align*} $$ -**2.** Show that $(\exists x \phi) \implies \psi$ is equivalent to $\forall x (\phi \implies \psi)$ -$$ -\begin{align*} -(\exists x \phi) \implies \psi &\iff \neg(\exists x \phi) \vee \psi \tag{0.1}\\\\ -&\iff \forall x (\neg \phi) \vee \psi \tag{2.1}\\\\ -&\iff \forall x (\neg \phi \vee \psi) \tag{1.1}\\\\ -&\iff \forall x (\phi \implies \psi) \tag{0.1} -\end{align*} -$$ -**3.** Show that $\phi \implies (\exists x \psi)$ is equivalent to $\exists x (\phi \implies \psi)$ +**2.** Show that $\phi \implies (\exists x \psi)$ is equivalent to $\exists x (\phi \implies \psi)$ $$ \begin{align*} \phi \implies (\exists x \psi) &\iff \neg \phi \vee (\exists x \psi) \tag{0.1}\\\\ @@ -60,13 +51,3 @@ $$ &\iff \exists x (\phi \implies \psi) \tag{0.1} \end{align*} $$ -**4.** Show that $\phi \implies (\forall x \psi)$ is equivalent to $\forall x (\phi \implies \psi)$ -$$ -\begin{align*} -\phi \implies (\forall x \psi) &\iff \neg \phi \vee (\forall x \psi) \tag{0.1}\\\\ -&\iff \forall x(\psi) \vee \neg \phi \tag{symmetry} \\\\ -&\iff \forall x (\psi \vee \neg \phi) \tag{1.1}\\\\ -&\iff \forall x (\neg \phi \vee \psi) \tag{symmetry} \\\\ -&\iff \forall x (\phi \implies \psi) \tag{0.1} -\end{align*} -$$