Upper semilattice of binary strings with the relation "x is simple conditional to y"

Abstract

In this paper we construct a structure R that is a "finite version" of the semilattice of Turing degrees. Its elements are strings (technically, sequences of strings) and x/spl les/y means that K(x|)=(conditional Kolmogorov complexity of x relative to y) is small. We construct two elements in R that do not have greatest lower bound. We give a series of examples that show how natural algebraic constructions give two elements that have lower bound O (minimal element) but significant mutual information. (A first example of that kind was constructed by Gacs-Korner (1973) using completely different technique.) We define a notion of "complexity profile" of the pair of elements of R and give (exact) upper and lower bounds for it in a particular case.