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Notions of computability at higher types I
 In Logic Colloquium 2000
, 2005
"... We discuss the conceptual problem of identifying the natural notions of computability at higher types (over the natural numbers). We argue for an eclectic approach, in which one considers a wide range of possible approaches to defining higher type computability and then looks for regularities. As a ..."
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We discuss the conceptual problem of identifying the natural notions of computability at higher types (over the natural numbers). We argue for an eclectic approach, in which one considers a wide range of possible approaches to defining higher type computability and then looks for regularities. As a first step in this programme, we give an extended survey of the di#erent strands of research on higher type computability to date, bringing together material from recursion theory, constructive logic and computer science. The paper thus serves as a reasonably complete overview of the literature on higher type computability. Two sequel papers will be devoted to developing a more systematic account of the material reviewed here.
Lambda theories of effective lambda models
 In 16th EACSL Annual Conference on Computer Science and Logic (CSL’07), LNCS
, 2007
"... Abstract. A longstanding open problem is whether there exists a nonsyntactical model of the untyped λcalculus whose theory is exactly the least λtheory λβ. In this paper we investigate the more general question of whether the equational/order theory of a model of the untyped λcalculus can be recu ..."
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Cited by 9 (5 self)
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Abstract. A longstanding open problem is whether there exists a nonsyntactical model of the untyped λcalculus whose theory is exactly the least λtheory λβ. In this paper we investigate the more general question of whether the equational/order theory of a model of the untyped λcalculus can be recursively enumerable (r.e. for brevity). We introduce a notion of effective model of λcalculus, 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, λβη. We then show that no effective model living in the stable or strongly stable semantics has an r.e. equational theory. Concerning 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 one. Finally, we show that the class of graph models enjoys a kind of downwards LöwenheimSkolem theorem.
Representations versus Numberings: On the Relationship of Two Computability Notions
 THEORETICAL COMPUTER SCIENCE
, 2001
"... This paper gives an answer to Weihrauch's question [31] whether and, if not always, when an effective map between the computable elements of two represented sets can be extended to a (partial) computable map between the represented sets. Examples are known showing that this is not possible in genera ..."
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This paper gives an answer to Weihrauch's question [31] whether and, if not always, when an effective map between the computable elements of two represented sets can be extended to a (partial) computable map between the represented sets. Examples are known showing that this is not possible in general. A condition is introduced and for countably based topological T 0 spaces it is shown that exactly the (partial) effective maps meeting the requirement are extendable. For total effective maps the extra condition is satisfied in the standard cases of effectively given separable metric spaces and continuous directedcomplete partial orders, in which the extendability is already known. In the first case a similar result holds also for partial effective maps, but not in the second.
A Categorytheoretic characterization of functional completeness
, 1990
"... . Functional languages are based on the notion of application: programs may be applied to data or programs. By application one may define algebraic functions; and a programming language is functionally complete when any algebraic function f(x 1 ,...,x n ) is representable (i.e. there is a constant a ..."
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. Functional languages are based on the notion of application: programs may be applied to data or programs. By application one may define algebraic functions; and a programming language is functionally complete when any algebraic function f(x 1 ,...,x n ) is representable (i.e. there is a constant a such that f(x 1 ,...,x n ) = (a . x 1 . ... . x n ). Combinatory Logic is the simplest typefree language which is functionally complete. In a sound categorytheoretic framework the constant a above may be considered as an "abstract gödelnumber" for f, when gödelnumberings are generalized to "principal morphisms", in suitable categories. By this, models of Combinatory Logic are categorically characterized and their relation is given to lambdacalculus models within Cartesian Closed Categories. Finally, the partial recursive functionals in any finite higher type are shown to yield models of Combinatory Logic. ________________ (+) Theoretical Computer Science, 70 (2), 1990, pp.193211. A p...
The Visser topology of lambda calculus
"... A longstanding open problem in lambda calculus is whether there exists a nonsyntactical model of the untyped lambda calculus whose theory is exactly the least λtheory λβ. In this paper we make use of the Visser topology for investigating the related question of whether the equational theory of a m ..."
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A longstanding open problem in lambda calculus is whether there exists a nonsyntactical model of the untyped lambda calculus whose theory is exactly the least λtheory λβ. In this paper we make use of the Visser topology for investigating the related question of whether the equational theory of a model can be recursively enumerable (r.e. for brevity). We introduce the notion of an effective model of lambda calculus and prove the following results: (i) The equational theory of an effective model cannot be λβ, λβη; (ii) The order theory of an effective model cannot be r.e.; (iii) No effective model living in the stable or strongly stable semantics has an r.e. equational theory. Concerning Scott’s semantics, we investigate the class of graph models and prove the following, where “graph theory ” is a shortcut for “theory of a graph model”: (iv) There exists a minimum order graph theory (for equational graph theories this was proved in [9, 10]). (v) The minimum equational/order graph theory is the theory of an effective graph model. (vi) No order graph theory can be r.e. (vii) Every equational/order graph theory is the theory of a graph model having a countable web. This last result proves that the class of graph models enjoys a kind of (downwards) LöwenheimSkolem theorem, and it answers positively Question 3 in [4, Section 6.3] for the class of graph models. 1.
Interactive Computation: Stepping Stone in the Pathway From Classical to Developmental Computation ∗
"... This paper reviews and extends previous work on the domaintheoretic notion of Machine Development. It summarizes the concept of Developmental Computation and shows how Interactive Computation can be understood as a stepping stone in the pathway from Classical to Developmental Computation. A critica ..."
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This paper reviews and extends previous work on the domaintheoretic notion of Machine Development. It summarizes the concept of Developmental Computation and shows how Interactive Computation can be understood as a stepping stone in the pathway from Classical to Developmental Computation. A critical appraisal is given of Classical Computation, showing in which ways its shortcomings tend to restrict the possible evolution of real computers, and how Interactive and Developmental Computation overcome such shortcomings. A formal conceptual framework is sketched, in order to frame the future development of the formal theory of Developmental Computation. Finally, the current frontier of the work on Developmental Computation is briefly exposed. 1
The Continuum: Foundations and Applications
, 1997
"... Most of the structures one deals with in Computer Science have a discrete and effective character: (finite) graphs, nets, trees in programming and formal languages, algorithms over finite strings or natural numbers, circuits etc... By continuous structures we mean "smooth" spaces, usually of cardina ..."
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Most of the structures one deals with in Computer Science have a discrete and effective character: (finite) graphs, nets, trees in programming and formal languages, algorithms over finite strings or natural numbers, circuits etc... By continuous structures we mean "smooth" spaces, usually of cardinality not less than continuum, where interesting topological or order properties give some information on, say, classes of functions over them. By discussing results from various areas of Mathematical Computer Science, we stress the role of continuous structures as tools for proving results about discrete or even finite structures. In particular we overview results concerning functionals in computability theory, trees in lambda calculus, boolean circuits in complexity theory and relate the finitary/combinatorial nature of the problems with their continuous solutions. We mostly focus on the methodology, and just hint to the technical aspects of the results presented. 2.1 Introduction In order...