doc-src/TutorialI/Sets/sets.tex
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   881 More general than these is induction over a well-founded relation.
   881 More general than these is induction over a well-founded relation.
   882 Such A relation expresses the notion of a terminating process.
   882 Such A relation expresses the notion of a terminating process.
   883 Intuitively, the relation~$\prec$ is \textbf{well-founded} if it admits no
   883 Intuitively, the relation~$\prec$ is \textbf{well-founded} if it admits no
   884 infinite  descending chains
   884 infinite  descending chains
   885 $\cdots \prec a@2 \prec a@1 \prec a@0.$
   885 $\cdots \prec a@2 \prec a@1 \prec a@0.$
   886 If $\prec$ is well-founded then it can be used with the well-founded
   886 If $\prec$ is well-founded then it can be used with the \textbf{well-founded
   887 induction rule:
   887 induction}\indexbold{induction!well-founded}\index{well-founded

   888 induction|see{induction, well-founded}} rule:
   888 $\infer{P(a)}{\infer*{P(x)}{[\forall y.\, y\prec x \imp P(y)]}}$
   889 $\infer{P(a)}{\infer*{P(x)}{[\forall y.\, y\prec x \imp P(y)]}}$
   889 To show $P(a)$ for a particular term~$a$, it suffices to show $P(x)$ for
   890 To show $P(a)$ for a particular term~$a$, it suffices to show $P(x)$ for
   890 arbitrary~$x$ under the assumption that $P(y)$ holds for $y\prec x$.
   891 arbitrary~$x$ under the assumption that $P(y)$ holds for $y\prec x$.
   891 Intuitively, the well-foundedness of $\prec$ ensures that the chains of
   892 Intuitively, the well-foundedness of $\prec$ ensures that the chains of
   892 reasoning are finite.
   893 reasoning are finite.  For a fuller account of well-founded relations and

   894 induction see, for example, \cite{Baader-Nipkow}.
   893
   895
   894 In Isabelle, the induction rule is expressed like this:
   896 In Isabelle, the induction rule is expressed like this:
   895 \begin{isabelle}
   897 \begin{isabelle}
   896 {\isasymlbrakk}wf\ r;\
   898 {\isasymlbrakk}wf\ r;\
   897   {\isasymAnd}x.\ {\isasymforall}y.\ (y,x)\ \isasymin\ r\
   899   {\isasymAnd}x.\ {\isasymforall}y.\ (y,x)\ \isasymin\ r\