# Kleene fixed-point theorem Kleene fixed-point theorem This article is about Kleene's fixed-point theorem in lattice theory. For the fixed-point theorem in computability theory, see Kleene's recursion theorem. Computation of the least fixpoint of f(x) = 1 / 10 x2+atan(x)+1 using Kleene's theorem in the real interval [0,7] with the usual order In the mathematical areas of order and lattice theory, the Kleene fixed-point theorem, named after American mathematician Stephen Cole Kleene, states the following: Kleene Fixed-Point Theorem. Vermuten {Anzeigestil (L,sqsubseteq )} is a directed-complete partial order (dcpo) with a least element, und lass {Anzeigestil f:Lto L} be a Scott-continuous (and therefore monotone) Funktion. Dann {Anzeigestil f} has a least fixed point, which is the supremum of the ascending Kleene chain of {displaystyle f.} The ascending Kleene chain of f is the chain {displaystyle bot sqsubseteq f(bot )sqsubseteq f(f(bot ))sqsubseteq cdots sqsubseteq f^{n}(bot )sqsubseteq cdots } obtained by iterating f on the least element ⊥ of L. Expressed in a formula, der Satz besagt das {Anzeigestil {textrm {lfp}}(f)=sup left(links{f^{n}(bot )mid nin mathbb {N} Rechts}Rechts)} wo {Anzeigestil {textrm {lfp}}} denotes the least fixed point.

Although Tarski's fixed point theorem does not consider how fixed points can be computed by iterating f from some seed (Auch, it pertains to monotone functions on complete lattices), this result is often attributed to Alfred Tarski who proves it for additive functions  Darüber hinaus, Kleene Fixed-Point Theorem can be extended to monotone functions using transfinite iterations. Nachweisen We first have to show that the ascending Kleene chain of {Anzeigestil f} exists in {Anzeigestil L} . To show that, we prove the following: Lemma. Wenn {Anzeigestil L} is a dcpo with a least element, und {Anzeigestil f:Lto L} is Scott-continuous, dann {Anzeigestil f^{n}(bot )sqsubseteq f^{n+1}(bot ),nin mathbb {N} _{0}} Nachweisen. We use induction: Assume n = 0. Dann {Anzeigestil f^{0}(bot )=bot sqsubseteq f^{1}(bot ),} seit {displaystyle bot } is the least element. Assume n > 0. Then we have to show that {Anzeigestil f^{n}(bot )sqsubseteq f^{n+1}(bot )} . By rearranging we get {Anzeigestil f(f^{n-1}(bot ))sqsubseteq f(f^{n}(bot ))} . By inductive assumption, we know that {Anzeigestil f^{n-1}(bot )sqsubseteq f^{n}(bot )} hält, and because f is monotone (property of Scott-continuous functions), the result holds as well.

As a corollary of the Lemma we have the following directed ω-chain: {Anzeigestil mathbb {M} ={bot ,f(bot ),f(f(bot )),Punkte }.} From the definition of a dcpo it follows that {Anzeigestil mathbb {M} } has a supremum, nennen {displaystyle m.} What remains now is to show that {Anzeigestil m} is the least fixed-point.

Zuerst, das zeigen wir {Anzeigestil m} is a fixed point, d.h. das {Anzeigestil f(m)=m} . Da {Anzeigestil f} is Scott-continuous, {Anzeigestil f(sup(mathbb {M} ))=sup(f(mathbb {M} ))} , das ist {Anzeigestil f(m)=sup(f(mathbb {M} ))} . Ebenfalls, seit {Anzeigestil mathbb {M} = f(mathbb {M} )Tasse {bot }} and because {displaystyle bot } has no influence in determining the supremum we have: {displaystyle sup(f(mathbb {M} ))=sup(mathbb {M} )} . Es folgt dem {Anzeigestil f(m)=m} , Herstellung {Anzeigestil m} a fixed-point of {Anzeigestil f} .

The proof that {Anzeigestil m} is in fact the least fixed point can be done by showing that any element in {Anzeigestil mathbb {M} } is smaller than any fixed-point of {Anzeigestil f} (because by property of supremum, if all elements of a set {displaystyle Dsubseteq L} are smaller than an element of {Anzeigestil L} then also {displaystyle sup(D)} is smaller than that same element of {Anzeigestil L} ). This is done by induction: Davon ausgehen {Anzeigestil k} is some fixed-point of {Anzeigestil f} . We now prove by induction over {Anzeigestil i} das {displaystyle forall iin mathbb {N} :f^{ich}(bot )sqsubseteq k} . The base of the induction {Anzeigestil (ich=0)} obviously holds: {Anzeigestil f^{0}(bot )=bot sqsubseteq k,} seit {displaystyle bot } is the least element of {Anzeigestil L} . As the induction hypothesis, davon dürfen wir ausgehen {Anzeigestil f^{ich}(bot )sqsubseteq k} . We now do the induction step: From the induction hypothesis and the monotonicity of {Anzeigestil f} (wieder, implied by the Scott-continuity of {Anzeigestil f} ), we may conclude the following: {Anzeigestil f^{ich}(bot )sqsubseteq k~implies ~f^{i+1}(bot )sqsubseteq f(k).} Jetzt, by the assumption that {Anzeigestil k} is a fixed-point of {Anzeigestil f,} we know that {Anzeigestil f(k)=k,} and from that we get {Anzeigestil f^{i+1}(bot )sqsubseteq k.} See also Other fixed-point theorems References ^ Alfred Tarski (1955). "A lattice-theoretical fixpoint theorem and its applications". Pacific Journal of Mathematics. 5:2: 285–309., Seite 305. ^ Patrick Cousot and Radhia Cousot (1979). "Constructive versions of Tarski's fixed point theorems". Pacific Journal of Mathematics. 82:1: 43–57. ^ Stoltenberg-Hansen, v.; Lindstrom, ICH.; Griffor, E. R. (1994). Mathematical Theory of Domains by V. Stoltenberg-Hansen. Cambridge University Press. pp. 24. doi:10.1017/cbo9781139166386. ISBN 0521383447. Kategorien: Order theoryFixed-point theorems

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