Learning classes of approximations to non-recursive functions

  • Authors:
  • Frank Stephan;Thomas Zeugmann

  • Affiliations:
  • Mathematisches Institut, Universität Heidelberg, Im Neuenheimer Feld 294, 69120 Heidelberg, Germany;Medizinische Universität Lübeck, Institut für Theoretische Informatik, Wallstraße 40, 23560 Lübeck, Germany

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

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Abstract

Blum and Blum (Inform. and Control 28 (1975) 125-155) showed that a class B of suitable recursive approximations to the halting problem K is reliably EX-learnable but left it open whether or not B is in NUM. By showing B to be not in NUM we resolve this old problem.Moreover, variants of this problem obtained by approximating any given recursively enumerable set A instead of the halting problem K are studied. All corresponding function classes U(A) are still EX-inferable but may fail to be reliably EX-learnable, for example if A is non-high and hypersimple.Blum and Blum (1975) considered only approximations to K defined by monotone complexity functions. We prove this condition to be necessary for making learnability independent of the underlying complexity measure. The class B of all recursive approximations to K generated by all total complexity functions is shown to be not even behaviorally correct learnable for a class of natural complexity measures. On the other hand, there are complexity measures such that B is EX-learnable. A similar result is obtained for all classes U(A).For natural complexity measures, B is shown to be not robustly learnable, but again there are complexity measures such that B and, more generally, every class U(A) is robustly EX-learnable. This result extends the criticism of Jain et al. (J. Comput. System Sci. 62(1) (2001) 178-212), since the classes defined by artificial complexity measures turn out to be robustly learnable while those defined by natural complexity measures are not robustly learnable.