Entailment systems for stably locally compact locales

  • Authors:
  • Steven Vickers

  • Affiliations:
  • School of Computer Science, University of Birmingham, Birmingham B15 2TT, UK

  • Venue:
  • Theoretical Computer Science - Logic, semantics and theory of programming
  • Year:
  • 2004

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Abstract

The category SCFrU of stably continuous frames and preframe homomorphisms (preserving finite meets and directed joins) is dual to the Karoubi envelope of a category Ent whose objects are sets and whose morphisms X → Y are upper closed relations between the finite powersets FX and FY. Composition of these morphisms is the "cut composition" of Jung et al. that interfaces disjunction in the codomains with conjunctions in the domains, and thereby relates to their multi-lingual sequent calculus. Thus stably locally compact locales are represented by "entailment systems" (X, ⊢) in which ⊢, a generalization of entailment relations, is idempotent for cut composition. Some constructions on stably locally compact locales are represented in terms of entailment systems: products, duality and powerlocales. Relational converse provides Ent with an involution, and this gives a simple treatment of the duality of stably locally compact locales. If A and B are stably continuous frames, then the internal preframe hom AψB is isomorphic to à ⊗ B where à is the Hofmann-Lawson dual. For a stably locally compact locale X, the lower powerlocale of X is shown to be the dual of the upper powerlocale of the dual of X.