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\title{Mathematical Error Example Module}
\author{
Michael Meyling
}

\begin{document}

\maketitle

\setlength{\parskip}{5pt plus 2pt minus 1pt}
\mbox{}
\vfill

\par
The source for this document can be found here:
\par
\url{http://qedeq.org/0_03_07/sample/qedeq_sample5_error.xml}

\par
Copyright by the authors. All rights reserved.
\par
If you have any questions, suggestions or want to add something to the list of modules that use this one, please send an email to the address \url{mailto:mime@qedeq.org}

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\tableofcontents

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\chapter{Basics} \label{chapter0} \hypertarget{chapter0}{}

In this chapter we start with the very basic axioms and definitions of set theory. We shall make no attempt to introduce a formal language but shall be content with the common logical operators. To be more precise: precondition is a first-order predicate calculus with equality.

\section{Classes and Sets} \label{chapter0_section0} \hypertarget{chapter0_section0}{}
Although we want to speak about \emph{sets} at the very beginning we have \emph{classes}. No formal definition of a class will be given. Informally, a class is a collection of objects, the involved objects are called the elements or members of the class. 
Sets will be construed as a special kind of class. 
The following definitions and axioms are due to a strengthened version of \emph{von~Neumann-Bernays-G{\"o}del's} set theory (\emph{NBG}). This version is called \emph{MK} which is short for \emph{Morse-Kelley}.

\par
The theory of sets introduced here has initial objects, called \emph{classes}. Furthermore a binary relation called \emph{memberschip} is provided.

\begin{idefn}[Membership Operator]
\label{in} \hypertarget{in}{}
$$x \in y$$

\end{idefn}

We also say $x$ \emph{is element of} $y$.
Beside equalitiy this is the only predicate we start with. All other will be defined.\footnote{One could also define the equality predicate within the set theory, but then another axiom is needed and the theory presentation is not so smooth for technical reasons (derivation of the equality axioms).}


\par
Our first axiom states that, for any classes $x$ and $y$, if the membership of $x$ and $y$ are the same, then $x$ and $y$ are the same.\footnote{If equality were not part of our underlying logic, then we should need to take this as a definition of equality.}

\begin{ax}[Axiom of Extensionality]
\label{axiom:extensionality} \hypertarget{axiom:extensionality}{}
\mbox{}
\begin{longtable}{{@{\extracolsep{\fill}}p{\linewidth}}}
\centering $\forall x\ \forall x\ (\forall z\ (z \in x\ \leftrightarrow \ z \in y)\ \rightarrow \ x \ =  \ y)$
\end{longtable}

\end{ax}

The classes $x$ and $y$ may be defined in entirely different ways, for example:
\par
\begin{tabularx}{\linewidth}{rcX}
  $x$ & = & class of all nonnegative integers, \\
  $y$ & = & class of all integers, that can be written as sum of four squares,
\end{tabularx}
\par
but if they have the same members, they are the same class.


\par
Now we specify \emph{sets}.

\begin{defn}[Set Definition]
\label{isSet} \hypertarget{isSet}{}
$$\mathfrak{M}(x)\ :\leftrightarrow \ \exists x\ x \in y$$

\end{defn}

So sets are nothing else than special classes. A class is a set iff it is a member of any class.


\par
As a consequence of the axiom of extensionality we have the following.\footnote{The quantification over $z$ is restricted to sets.}

\begin{prop}
\label{module1:theorem} \hypertarget{module1:theorem}{}
\mbox{}
\begin{longtable}{{@{\extracolsep{\fill}}p{\linewidth}}}
\centering $\forall \ \mathfrak{M}(z)\ \ (z \in x\ \leftrightarrow \ z \in y)\ \rightarrow \ x \ =  \ y$
\end{longtable}

\end{prop}
\begin{proof}
Assume $\forall \ \mathfrak{M}(z) \ ( z \in x \ \leftrightarrow \ z \in y)$. Let $z$ be an arbitrary class. If $z \in x$ then $z$ is a set by definition~\ref{isSet}, and hence by the assumption, $z \in y$. Similarly $z \in y \ \rightarrow \ z \in x$. Since $z$ is arbitrary, it follows that $\forall z \ (z \in x \ \leftrightarrow \ z \in y)$. Thus by the axiom of extensionality~\ref{axiom:extensionality}, $x = y$.
\end{proof}





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