On ordinal VC-dimension and some notions of complexity

  • Authors:
  • Eric Martin;Arun Sharma;Frank Stephan

  • Affiliations:
  • School of Computer Science and Engineering, UNSW Sydney, NSW, Australia;Division of Research and Commercialisation, Queensland University of Technology, Brisbane, QLD, Australia;School of Computing, National University of Singapore, Singapore, Singapore

  • Venue:
  • Theoretical Computer Science - Algorithmic learning theory
  • Year:
  • 2006

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Abstract

We generalize the classical notion of Vapnik-Chernovenkis (VC) dimension to ordinal VC-dimension, in the context of logical learning paradigms. Logical learning paradigms encompass the numerical learning paradigms commonly studied in Inductive Inference. A logical learning paradigm is defined as a set W of structures over some vocabulary, and a set D of first-order formulas that represent data. The sets of models of ϕ in W, where ϕ varies over D, generate a natural topology W over W.We show that if D is closed under boolean operators, then the notion of ordinal VC-dimension offers a perfect characterization for the problem of predicting the truth of the members of D in a member of W, with an ordinal bound on the number of mistakes. This shows that the notion of VC-dimension has a natural interpretation in Inductive Inference, when cast into a logical setting. We also study the relationships between predictive complexity, selective complexity--a variation on predictive complexity--and mind change complexity. The assumptions that D is closed under boolean operators and that W is compact often play a crucial role to establish connections between these concepts.We then consider a computable setting with effective versions of the complexity measures, and show that the equivalence between ordinal VC-dimension and predictive complexity fails. More precisely, we prove that the effective ordinal VC-dimension of a paradigm can be defined when all other effective notions of complexity are undefined. On a better note, when W is compact, all effective notions of complexity are defined, though they are not related as in the noncomputable version of the framework.