Mathematics_for_Physicists/chapters/sections/logic.tex

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\begin{document}
\section{Logic}
\begin{defi}[Statements]
A statement is a sentence (mathematical or colloquial) which can be either true or false.
\end{defi}

\begin{eg}
Statements are
\begin{itemize}
	\item Tomorrow is Monday
	\item $x > 1$ where $x$ is a natural number
	\item Green rabbits grow at full moon
\end{itemize}
No statements are
\begin{itemize}
	\item What is a statement?
	\item $x + 20y$ where $x, y$ are natural numbers
	\item This sentence is false
\end{itemize}
\end{eg}

\begin{defi}[Connectives]
When $\Phi, \Psi$ are statements, then
\begin{enumerate}[(i)]
	\item $\neg\Phi$ (not $\Phi$)
	\item $\Phi \wedge \Psi$ ($\Phi$ and $\Psi$)
	\item $\Phi \vee \Psi$ ($\Phi$ or $\Psi$)
	\item $\Phi \implies \Psi$ (if $\Phi$ then $\Psi$)
	\item $\Phi \iff \Psi$ ($\Phi$ if and only if (iff.) $\Psi$)
\end{enumerate}
are also statements. We can represent connectives with truth tables
\begin{center}
\begin{tabular}{ c|c||c|c|c|c|c }
 $\Phi$ & $\Psi$ & $\neg\Phi$ & $\Phi \wedge \Psi$ & $\Phi \vee \Psi$ & $\Phi \implies \Psi$ & $\Phi \iff \Psi$ \\
 \hline
 t & t & f & t & t & t & t\\
 t & f & f & f & t & f & f\\
 f & t & t & f & t & t & f\\
 f & f & t & f & f & t & t\\
\end{tabular}
\end{center}
\end{defi}

\begin{rem}\leavevmode
\begin{enumerate}[(i)]
	\item $\vee$ is inclusive
	\item $\Phi \implies \Psi$, $\Phi \impliedby \Psi$, $\Phi \iff \Psi$ are NOT the same
	\item $\Phi \implies \Psi$ is always true if $\Phi$ is false (ex falso quodlibet)
\end{enumerate}
\end{rem}

\begin{defi}[Hierarchy of logical operators]
$\neg$ binds stronger than $\wedge$ and $\vee$, which bind stronger than $\implies$ and $\iff$.
\end{defi}

\begin{eg}\leavevmode
\begin{align*}
	\neg\Phi \wedge \Psi ~&\cong~ (\neg\Phi) \wedge \Psi \\
	\neg\Phi \implies \Psi ~&\cong~ (\neg\Phi) \wedge \Psi \\
	\Phi \wedge \Psi \iff \Psi ~&\cong~ (\Phi \wedge \Psi) \iff \Psi \\
	\neg\Phi \vee \neg\Psi \implies \neg\Psi \wedge \Psi ~&\cong~ ((\neg\Phi) \vee (\neg\Psi)) \implies ((\neg\Psi) \wedge \Psi)
\end{align*}
We avoid writing statements like $\Phi \wedge \Psi \vee \Theta$. A statement that is always true is called a tautology. Some important equivalencies are
\begin{align*}
	\Phi ~&\text{equiv.}~ \neg(\neg\Phi)) \\
	\Phi \implies \Psi ~&\text{equiv.}~ \neg\Psi \implies \neg\Phi \\
	\Phi \iff \Psi ~&\text{equiv.}~ (\Phi \implies \Psi) \wedge (\Psi \implies \Phi) \\
	\Phi \vee \Psi ~&\text{equiv.}~ \neg(\neg\Phi \wedge \neg\Psi)
\end{align*}
Logical operators are commutative, associative and distributive. 
\end{eg}

\begin{defi}[Quantifiers]
Let $\Phi(x)$ be a statement depending on $x$. Then $\forall x ~\Phi(x)$ and $\exists x ~\Phi(x)$ are also statements. The interpretation of these statements is
\begin{itemize}
	\item $\forall x ~\Phi(x)$: "For all $x$, $\Phi(x)$ holds."
	\item $\exists x ~\Phi(x)$: "There is (at least one) $x$ s.t. $\Phi(x)$ holds."
\end{itemize}
\end{defi}

\begin{rem}\leavevmode
\begin{enumerate}[(i)]
	\item $\forall x ~x \ge 1$ is true for natural numbers, but not for integers. We must specify a domain.
	\item If the domain is infinite the truth value of $\forall x ~\Phi(x)$ cannot be algorithmically determined.
	\item $\forall x ~\Phi(x)$ and $\forall y ~\Phi(y)$ are equivalent.
	\item Same operators can be exchanged, different ones cannot.
	\item $\forall x ~\Phi(x)$ is equivalent to $\neg(\exists x ~\neg\Phi(x))$.
\end{enumerate}
\end{rem}

\end{document}