least fixed point


least fixed point

(mathematics)A function f may have many fixed points (xsuch that f x = x). For example, any value is a fixed pointof 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 bythe least fixed point of F. This is the least upper boundof the infinite sequence (the ascending Kleene chain)obtained by repeatedly applying F to the totally undefinedvalue, bottom. I.e.

fix F = LUB bottom, F bottom, F.

The least fixed point is guaranteed to exist for acontinuous function over a cpo.