The continuum as a final coalgebra

  • Authors:
  • D. Pavlović;V. Pratt

  • Affiliations:
  • Kestrel Institute, Palo Alto, CA;Stanford Univ., Stanford, CA

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

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Abstract

We define the continuum up to order isomorphism, and hence up to homeomorphism via the order topology, in terms of the final coalgebra of either the functor NX, product with the set of natural numbers, or the functor 1+NX. This makes an attractive analogy with the definition of N itself as the initial algebra of the functor 1+X, disjoint union with a singleton. We similarly specify Baire space and Cantor space in terms of these final coalgebras. We identify two variants of this approach, a coinductive definition based on final coalgebraic structure in the category of sets, and a direct definition as a final coalgebra in the category of posets. We conclude with some paradoxical discrepancies between continuity and constructiveness in this setting.