least fixed point

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least fixed point

A function f may have many fixed points (x such that f x = x). For example, any value is a fixed point of the identity function, (\ x . x).

If f is recursive, we can represent it as

f = fix F

where F is some higher-order function and

fix F = F (fix F).

The standard denotational semantics of f is then given by the least fixed point of F. This is the least upper bound of the infinite sequence (the ascending Kleene chain) obtained by repeatedly applying F to the totally undefined value, bottom. I.e.

fix F = LUB bottom, F bottom, F.

The least fixed point is guaranteed to exist for a continuous function over a cpo.
This article is provided by FOLDOC - Free Online Dictionary of Computing (foldoc.org)
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