ITP-Course/lectures/09_induction.tex

\part{Induction Proofs}

\frame[plain]{\partpage}

\begin{frame}
\frametitle{Mathematical Induction}
\begin{itemize}
\item mathematical (\aka natural) induction principle:\\
  If a property $P$ holds for 0 and $P(n)$ implies $P(n+1)$ for all n,\\
  then $P(n)$ holds for all $n$.
\item HOL is expressive enough to encode this principle as a theorem.\\\medskip
\hol{|- !P. P 0 \holAnd{} (!n.\ P n ==> P (SUC n)) ==> !n.\ P n}\medskip
\item Performing mathematical induction in HOL means applying this theorem (\eg via \hol{HO\_MATCH\_MP\_TAC})

\item there are many similarish induction theorems in HOL
\item Example: complete induction principle\\\medskip
\hol{|- !P. (!n.\ (!m.\ m < n ==> P m) ==> P n) ==> !n.\ P n}
\end{itemize}
\end{frame}


\begin{frame}
\frametitle{Structural Induction Theorems}
\begin{itemize}
\item \emph{structural induction} theorems are an important special form of induction theorems 
\item they describe performing induction on the structure of a datatype
\item Example: \hol{\scriptsize|- !P.\ P [] \holAnd{} (!t.\ P t ==> !h.\ P (h::t)) ==> !l.\ P l}
\item structural induction is used very frequently in HOL
\item for each algabraic datatype, there is an induction theorem
\end{itemize}
\end{frame}


\begin{frame}
\frametitle{Other Induction Theorems}
\begin{itemize}
\item there are many induction theorems in HOL
\begin{itemize}
\item datatype definitions lead to induction theorems
\item recursive function definitions produce corresponding induction theorems
\item recursive relation definitions give rise to induction theorems
\item many are manually defined
\end{itemize}
\item Examples\\\bigskip{\scriptsize
\hol{|- !P.\ P [] \holAnd{} (!l.\ P l ==> !x.\ P (SNOC x l)) ==> !l.\ P l}\\\bigskip
\hol{|- !P.\ P FEMPTY \holAnd{} \\
\-\qquad\quad(!f.\ P f ==> !x y.\ x NOTIN FDOM f ==> P (f |+ (x,y))) ==> !f.\ P f}\\\bigskip
\hol{|- !P.\ P \{\} \holAnd{} \\
\-\qquad\quad(!s.\ FINITE s \holAnd{} P s ==> !e.\ e NOTIN s ==> P (e INSERT s)) ==> \\
\-\qquad\quad!s.\ FINITE s ==> P s}\\\bigskip
\hol{|- !R P.\ (!x y.\ R x y ==> P x y) \holAnd{} (!x y z.\ P x y \holAnd{} P y z ==> P x z) ==>\\
\-\qquad\quad!u v.\ R$^+$ u v ==> P u v}}

\end{itemize}
\end{frame}


\begin{frame}
\frametitle{Induction (and Case-Split) Tactics}
\begin{itemize}
\item the tactic \hol{Induct} (or \hol{Induct\_on}) is usually used to start induction proofs
\item it looks at the type of the quantifier (or its argument) and applies the default induction theorem for this type
\item this is usually what one needs
\item other (non default) induction theorems can be applied via \hol{INDUCT\_THEN} or \hol{HO\_MATCH\_MP\_TAC}
\item similarish \hol{Cases\_on} picks and applies default case-split theorems
\end{itemize}
\end{frame}


\begin{frame}[fragile]
\frametitle{Induction Proof - Example I - Slide 1}

\begin{itemize}
\item let's prove via induction\\
  \hol{!l1 l2.\ REVERSE (l1 ++ l2) = REVERSE l2 ++ REVERSE l1}
\item we set up the goal and start an induction proof on \hol{l1}
\end{itemize}

\begin{block}{Proof Script}
\begin{semiverbatim}\small
val REVERSE_APPEND = prove (
``!l1 l2.\ REVERSE (l1 ++ l2) = REVERSE l2 ++ REVERSE l1``,
Induct
\end{semiverbatim}
\end{block}

\end{frame}

\begin{frame}[fragile]
\frametitle{Induction Proof - Example I - Slide 2}

\begin{itemize}
\item the induction tactic produced two cases
\item base case:\\
{\scriptsize\hol{!l2.\ REVERSE ([] ++ l2) = REVERSE l2 ++ REVERSE []}}
\item induction step:\\
{\scriptsize
\begin{semiverbatim}
\hol{!h l2.\ REVERSE (h::l1 ++ l2) = REVERSE l2 ++ REVERSE (h::l1)}
-----------------------------------------------------------
\hol{!l2.\ REVERSE (l1 ++ l2) = REVERSE l2 ++ REVERSE l1}
\end{semiverbatim}}

\item both goals can be easily proved by rewriting
\end{itemize}

\begin{block}{Proof Script}
\begin{semiverbatim}\scriptsize
val REVERSE_APPEND = prove (``
!l1 l2.\ REVERSE (l1 ++ l2) = REVERSE l2 ++ REVERSE l1``,
Induct >| [
  REWRITE_TAC[REVERSE_DEF, APPEND, APPEND_NIL],
  ASM_REWRITE_TAC[REVERSE_DEF, APPEND, APPEND_ASSOC]
]);
\end{semiverbatim}
\end{block}

\end{frame}


\begin{frame}[fragile]
\frametitle{Induction Proof - Example II - Slide 2}

\begin{itemize}
\item let's prove via induction\\
  \hol{!l.\ REVERSE (REVERSE l) = l}
\item we set up the goal and start an induction proof on \hol{l}
\end{itemize}

\begin{block}{Proof Script}
\begin{semiverbatim}\small
val REVERSE_REVERSE = prove (
``!l.\ REVERSE (REVERSE l) = l``,
Induct
\end{semiverbatim}
\end{block}

\end{frame}

\begin{frame}[fragile]
\frametitle{Induction Proof - Example II - Slide 2}

\begin{itemize}
\item the induction tactic produced two cases
\item base case:\\
{\scriptsize\hol{REVERSE (REVERSE []) = []}}
\item induction step:\\
{\scriptsize
\begin{semiverbatim}
\hol{!h.\ REVERSE (REVERSE (h::l1)) = h::l1}
--------------------------------------------
\hol{REVERSE (REVERSE l) = l}
\end{semiverbatim}}

\item again both goals can be easily proved by rewriting
\end{itemize}

\begin{block}{Proof Script}
\begin{semiverbatim}\scriptsize
val REVERSE_REVERSE = prove (
``!l.\ REVERSE (REVERSE l) = l``,
Induct >| [
  REWRITE_TAC[REVERSE_DEF],
  ASM_REWRITE_TAC[REVERSE_DEF, REVERSE_APPEND, APPEND]
]);
\end{semiverbatim}
\end{block}

\end{frame}

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