Upper semi-lattice of binary strings with the relation "x is simple conditional to y"

  • Authors:
  • Alexei Chernov;Andrej Muchnik;Andrei Romashchenko;Alexander Shen;Nikolai Vereshchagin

  • Affiliations:
  • Moscow State Univ., Moscow, Russia;Institute of New Technologies of Education, Moscow, Russia;Moscow State Univ., Moscow, Russia;Institute of Problems of Information transmission, Russia;Moscow State Univ., Moscow, Russia

  • Venue:
  • Theoretical Computer Science
  • Year:
  • 2002

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Abstract

In this paper we construct a structure R that is a "finite version" of the semi-lattice of Turing degrees. Its elements are strings (technically, sequences of strings) and xy means thatK(x|y)=(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 0 (minimal element) but significant mutual information. (A first example of that kind was constructed by Gács-Körner (Problems Control Inform. Theory 2 (1973) 149) using a 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.