diff --git a/content/set-theory/z/pairs.tex b/content/set-theory/z/pairs.tex index 49aa019d..6f894b81 100644 --- a/content/set-theory/z/pairs.tex +++ b/content/set-theory/z/pairs.tex @@ -8,11 +8,13 @@ The next axiom to consider is the following: \begin{axiom}[Pairs] -For any sets $a, b$, the set $\{a, b\}$ exists.\\ - $\forall a \forall b \exists P \forall x (x \in P \liff (x = a \lor x = b))$ +For any sets $a, b$, the set $\{a, b\}$ exists. +\[ + \forall a \forall b \exists P \forall x (x \in P \liff (x = a \lor x = b)) +\] \end{axiom} -Here is how to justify this axiom, using the iterative conception. Suppose $a$ is available at stage $S$, and $b$ is available at stage $T$. Let $M$ be whichever of stages $S$ and $T$ comes later. Then since $a$ and $b$ are both available at stage $M$, the set $\{a,b\}$ is a possible collection available at any stage after $M$ (whichever is the greater). +Here is how to justify this axiom, using the iterative conception. Suppose $a$ is available at stage $S$, and $b$ is available at stage $T$. Let $M$ be whichever of stages $S$ and $T$ comes later. Then since $a$ and $b$ are both available at stage $M$, the set $\{a,b\}$ is a possible collection available at any stage after $M$ (whichever is the greater). But hold on!{} Why assume that there \emph{are} any stages after $M$? If there are none, then our justification will fail. So, to justify Pairs, we will have to add another principle to the story we told in \olref[sth][z][story]{sec}, namely: \begin{enumerate} @@ -37,10 +39,10 @@ \begin{proof} \olref{singleton}. By Pairs, $\{a, a\}$ exists, which is $\{a\}$ by -Extensionality. +Extensionality. \olref{binunion}. By Pairs, $\{a, b\}$ exists. Now $a \cup b = \bigcup -\{a, b\}$ exists by Union. +\{a, b\}$ exists by Union. \olref{tuples}. By \olref{singleton}, $\{a\}$ exists. By Pairs, $\{a, b\}$ exists. Now $\{\{a\}, \{a, b\}\} = \tuple{a, b}$ exists, by Pairs @@ -61,4 +63,4 @@ % a_2\}, \{a_1, a_3\}\} = \{a_1, a_2, a_3\}$ exists. Repeat this % trick as often as necessary. \end{proof} -\end{document} \ No newline at end of file +\end{document} diff --git a/content/set-theory/z/powerset.tex b/content/set-theory/z/powerset.tex index 5cc61840..51a5d4eb 100644 --- a/content/set-theory/z/powerset.tex +++ b/content/set-theory/z/powerset.tex @@ -8,8 +8,10 @@ We will proceed with another axiom: \begin{axiom}[Powersets] -For any set $A$, the set $\Pow{A} = \Setabs{x}{x \subseteq A}$ exists.\\ - $\forall A \exists P \forall x(x \in P \liff (\forall z \in x)z \in A)$ +For any set $A$, the set $\Pow{A} = \Setabs{x}{x \subseteq A}$ exists. +\[ + \forall A \exists P \forall x(x \in P \liff (\forall z \in x)z \in A) +\] \end{axiom} Our justification for this is pretty straightforward. Suppose $A$ is @@ -19,7 +21,7 @@ all available, to be formed into a single set, at any stage after $S$. And we know that there is some such stage, since $S$ is not the last stage (by \stagessucc). So $\Pow{A}$ exists. - + Here is a nice consequence of Powersets: \begin{prop}\label{thm:Products} @@ -43,7 +45,7 @@ surprise. Without Separation, Powersets wouldn't be a very \emph{powerful} principle. After all, Separation tells us which subsets of a set exist, and hence determines just how ``fat'' each -Powerset is. +Powerset is. \begin{prob} Show that, for any sets $A, B$: (i) the set of all relations with @@ -57,4 +59,4 @@ $\equivclass{A}{\sim}$, exists. \end{prob} -\end{document} \ No newline at end of file +\end{document} diff --git a/content/set-theory/z/separation.tex b/content/set-theory/z/separation.tex index b383e84a..04bb853a 100644 --- a/content/set-theory/z/separation.tex +++ b/content/set-theory/z/separation.tex @@ -37,14 +37,14 @@ (by \stagesacc). Now in particular, consider all the sets which are members of $A$ and which also satisfy $\phi$; clearly all of these sets, too, were formed before stage~$S$. So they are formed into a set -$\Setabs{x \in A}{\phi(x)}$ at stage~$S$ too (by \stagesacc). +$\Setabs{x \in A}{\phi(x)}$ at stage~$S$ too (by \stagesacc). Unlike Na\"ive Comprehension, this avoid Russell's Paradox. For we cannot simply assert the existence of the set $\Setabs{x}{x \notin x}$. Rather, \emph{given} some set~$A$, we can assert the existence of the set $R_A = \Setabs{x \in A}{x \notin x}$. But all this proves is that $R_A \notin R_A$ and $R_A \notin A$, none of which is very -worrying. +worrying. However, Separation has an immediate and striking consequence: @@ -99,11 +99,11 @@ \begin{proof} Let $A \neq \emptyset$, so there is some $c \in A$. Then $\bigcap A = \Setabs{x}{(\forall y \in A)x \in y} = \Setabs{x \in c}{(\forall y \in -A)x \in y}$, which exists by Separation. +A)x \in y}$, which exists by Separation. \end{proof} Note the condition that $A \neq \emptyset$, though; for $\bigcap \emptyset$ would be the universal set, vacuously, contradicting \olref{thm:NoUniversalSet}. -\end{document} \ No newline at end of file +\end{document} diff --git a/content/set-theory/z/union.tex b/content/set-theory/z/union.tex index 39c59a58..97be1a85 100644 --- a/content/set-theory/z/union.tex +++ b/content/set-theory/z/union.tex @@ -10,8 +10,10 @@ axiom: \begin{axiom}[Union] For any set $A$, the set $\bigcup A = -\Setabs{x}{(\exists b \in A) x \in b}$ exists.\\ - $\forall A \exists U \forall x(x \in U \liff (\exists b \in A)x \in b)$ +\Setabs{x}{(\exists b \in A) x \in b}$ exists. +\[ + \forall A \exists U \forall x(x \in U \liff (\exists b \in A)x \in b) +\] \end{axiom} This axiom is also justified by the cumulative-iterative conception. @@ -21,4 +23,4 @@ before $S$. Thus all of \emph{those} sets are available before $S$, to be formed into a set at $S$. And that set is just $\bigcup A$. -\end{document} \ No newline at end of file +\end{document}