Effective λ-models versus recursively enumerable λ-theories

  • Authors:

  • Chantal Berline;Giulio Manzonetto;Antonino Salibra

  • Affiliations:

  • Cnrs, laboratoire pps, université paris 7, 2, place jussieu (case 7014), 75251 paris cedex 05, france email: chantal.berline@pps.jussieu.fr;Inria email: giulio.manzonetto@inria.fr;Università ca'foscari di venezia, dipartimento di informatica, via torino 155, 30172 venezia, italy email: salibra@dsi.unive.it

  • Venue:
  • Mathematical Structures in Computer Science
  • Year:
  • 2009

  • Scott is always simple

    MFCS'12 Proceedings of the 37th international conference on Mathematical Foundations of Computer Science

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Abstract

A longstanding open problem is whether there exists a non-syntactical model of the untyped λ-calculus whose theory is exactly the least λ-theory λβ. In this paper we investigate the more general question of whether the equational/order theory of a model of the untyped λ-calculus can be recursively enumerable (r.e. for short). We introduce a notion of effective model of λ-calculus, which covers, in particular, all the models individually introduced in the literature. We prove that the order theory of an effective model is never r.e.; from this it follows that its equational theory cannot be λβ or λβη. We then show that no effective model living in the stable or strongly stable semantics has an r.e. equational theory. For Scott's semantics, we investigate the class of graph models and prove that no order theory of a graph model can be r.e., and that there exists an effective graph model whose equational/order theory is the minimum among the theories of graph models. Finally, we show that the class of graph models enjoys a kind of downwards Löwenheim–Skolem theorem.