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{{Short description|Theorem in order theory and lattice theory}}
{{otheruses4|Kleene's fixed-point theorem in lattice theory|the fixed-point theorem in computability theory|Kleene's recursion theorem}}
[[File:Kleene fixpoint svg.svg|thumb|Computation of the least fixpoint of ''f''(''x'') = {{sfrac|1|10}}''x''<sup>2</sup>+[[arctangent|atan]](''x'')+1 using Kleene's theorem in the real [[interval (mathematics)|interval]] [0,7] with the usual order]]
In the [[mathematics|mathematical]] areas of [[order theory|order]] and [[lattice theory]], the '''Kleene fixed-point theorem''', named after American mathematician [[Stephen Cole Kleene]], states the following:
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where <math>\textrm{lfp}</math> denotes the least fixed point.
does not consider how fixed points can be computed by iterating ''f'' from some seed (also, it pertains to [[monotone function]]s on [[complete lattices]]), this result is often attributed to [[Alfred Tarski]] who proves it for additive functions.<ref>{{cite journal | author=Alfred Tarski | url=http://projecteuclid.org/Dienst/UI/1.0/Summarize/euclid.pjm/1103044538 | title=A lattice-theoretical fixpoint theorem and its applications | journal = [[Pacific Journal of Mathematics]] | volume=5 | year=1955 | issue=2 | pages=285–309| doi=10.2140/pjm.1955.5.285 }}, page 305.</ref> Moreover, the Kleene fixed-point theorem can be extended to [[monotone function]]s using transfinite iterations.<ref>{{cite journal | author=Patrick Cousot and Radhia Cousot | url=https://projecteuclid.org/euclid.pjm/1102785059 | title=Constructive versions of Tarski's fixed point theorems | journal = Pacific Journal of Mathematics | volume=82 | year=1979 | issue=1 | pages=43–57| doi=10.2140/pjm.1979.82.43 }}</ref>
== Proof ==
We first have to show that the ascending Kleene chain of <math>f</math> exists in <math>L</math>. To show that, we prove the following:
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