We investigate approximating posets with projections (approximating pop's). These are triples (D; ; P) consisting of a poset (D; ) and a directed set P of projections with sup P = id D . They carry a canonical uniformity and thus a topology. We relate their properties such as completeness and compactness to properties of the poset and the projection set. We show that each monotone net in D is convergent if and only if (D; ) is an algebraic domain such that the images of the projections are precisely the compact elements of (D; ). We call these domains P-domains and characterize them as inverse limits of posets satisfying the ascending chain condition. Moreover, we describe P-domains by a certain system of so-called "complete" subsets. We prove that if the set of compact elements of an algebraic domain is mub-complete, then it is a P-domain if and only if the mub-closure of every finite set of compact elements fulfils the ascending chain condition. Furthermore, we characte...

This paper discusses paradox and imaginary values in relation to the mutuality of sign and space.

In this paper we give an outline of recent results concerning theories and models of the untyped lambda calculus. Algebraic and topological methods have been applied to study the structure of the lattice of λ-theories, the equational incompleteness of lambda calculus semantics, and the λ-theories induced by graph models of lambda calculus.

A longstanding open problem is whether there exists a model of the untyped λ-calculus in the category Cpo of complete partial orderings and Scott continuous functions, whose theory is exactly the least λ-theory λβ or the least extensional λ-theory λβη. In this paper we analyze the class of reflexive Scott domains, the models of λ-calculus living in the category of Scott domains (a full subcategory of Cpo). The following are the main results of the paper: (i) Extensional reflexive Scott domains are not complete for the λβη-calculus, i.e., there are equations not in λβη which hold in all extensional reflexive Scott domains. (ii) The order theory of an extensional reflexive Scott domain is never recursively enumerable. These results have been obtained by isolating among the reflexive Scott domains a class of webbed models arising from Scott’s information systems, called iweb-models. The class of iweb-models includes all extensional reflexive Scott domains, all preordered coherent models and all filter models living in Cpo. Based on a fine-grained study of an “effective” version of Scott’s information systems, we have shown that there are equations not in λβ (resp. λβη) which hold in all (extensional) iweb-models.

The invariance of the meaning of a #-term by reduction/expansion w.r.t. the considered computational rules is one of the minimal requirements one expects to hold for a #-model. Being the intersection type systems a general framework for the study of semantic domains for the Lambda-calculus, the present paper provides a characterisation of "meaning invariance" in terms of characterisation results for intersection type systems enabling typing invariance of terms w.r.t. various notions of reduction/expansion, like #, # and a number of relevant restrictions of theirs. 1.

In mathematical semantics, in the sense of Scott, the question arises of what domains of interpretation should be chosen. It has been felt by the author, and others, that lattices are the wrong choice and instead one should use complete partial orders (cpo’s), which do not necessarily have the embarrassing top element. So far, however, no mathematical theory as pleasant as that developed for Pw in the paper “Data Types as Lattices ” has been available. The present paper is intended to fill this gap and is a close analog of the Pw paper, replacing Pw by 8”‘, the w-power of the three-element truthvalue cpo, T. 1.

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 brevity). 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, 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 among the theories of graph models. Finally, we show that the class of graph models enjoys a kind of downwards Löwenheim-Skolem theorem.

We introduce a type assignment system which is parametric with respect to five families of trees obtained by evaluating λ-terms (Böhm trees, Levy-Longo trees, ...). Then we prove, in an (almost) uniform way, that each type assignment system fully describes the observational equivalences induced by the corresponding tree representation of terms. More precisely, for each family of trees, two terms have the same tree if and only if they get assigned the same types in the corresponding type assignment system.

Abstract. A closed λ-term M is easy if, for any other closed term N, the lambda theory generated by M = N is consistent, while it is simple easy if, given an arbitrary intersection type τ, one can find a suitable pre-order on types which allows to derive τ for M. Simple easiness implies easiness. The question whether easiness implies simple easiness constitutes Problem 19 in the TLCA list of open problems. In this paper we negatively answer the question providing a nonempty co-r.e. (complement of a recursively enumerable) set of easy, but non simple easy, λ-terms. Key words: Lambda calculus, easy terms, simple easy terms, filter models 1