Kleene fixed-point theorem
- Kleene Fixed-Point Theorem. Suppose is a directed-complete partial order (dcpo) with a least element, and let be a Scott-continuous (and therefore monotone) function. Then has a least fixed point, which is the supremum of the ascending Kleene chain of
The ascending Kleene chain of f is the chain
where denotes the least fixed point.
This result is often attributed to Alfred Tarski, but Tarski's fixed point theorem does not consider how fixed points can be computed by iterating f from some seed (also, it pertains to monotone functions on complete lattices).
We first have to show that the ascending Kleene chain of exists in . To show that, we prove the following:
- Lemma. If is a dcpo with a least element, and is Scott-continuous, then
- Proof. We use induction:
- Assume n = 0. Then since is the least element.
- Assume n > 0. Then we have to show that . By rearranging we get . By inductive assumption, we know that holds, 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:
From the definition of a dcpo it follows that has a supremum, call it What remains now is to show that is the least fixed-point.
First, we show that is a fixed point, i.e. that . Because is Scott-continuous, , that is . Also, since and because has no influence in determining the supremum we have: . It follows that , making a fixed-point of .
The proof that is in fact the least fixed point can be done by showing that any element in is smaller than any fixed-point of (because by property of supremum, if all elements of a set are smaller than an element of then also is smaller than that same element of ). This is done by induction: Assume is some fixed-point of . We now prove by induction over that . The base of the induction obviously holds: since is the least element of . As the induction hypothesis, we may assume that . We now do the induction step: From the induction hypothesis and the monotonicity of (again, implied by the Scott-continuity of ), we may conclude the following: Now, by the assumption that is a fixed-point of we know that and from that we get
- Stoltenberg-Hansen, V.; Lindstrom, I.; Griffor, E. R. (1994). Mathematical Theory of Domains by V. Stoltenberg-Hansen. Cambridge University Press. p. 24. doi:10.1017/cbo9781139166386. ISBN 0521383447.