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Finitary PCF is not decidable
 Theoretical Computer Science
, 1996
"... The question of the decidability of the observational ordering of finitary PCF was raised [5] to give mathematical content to the full abstraction problem for PCF [9, 14]. We show that the ordering is in fact undecidable. This result places limits on how explicit a representation of the fully abstra ..."
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Cited by 33 (1 self)
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The question of the decidability of the observational ordering of finitary PCF was raised [5] to give mathematical content to the full abstraction problem for PCF [9, 14]. We show that the ordering is in fact undecidable. This result places limits on how explicit a representation of the fully abstract model can be. It also gives a slight strengthening of the author’s earlier result on typed λdefinability [6].
Socially Responsive, Environmentally Friendly Logic
 in Truth and Games: Essays in Honour of Gabriel Sandu, Aho, Tuomo and AhtiVeikko Pietarinen, eds., Acta Philosophica Fennica
, 2006
"... We consider the following questions: What kind of logic has a natural semantics in multiplayer (rather than 2player) games? How can we express branching quantifiers, and other partialinformation constructs, with a properly compositional syntax and semantics? We develop a logic in answer to these ..."
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Cited by 7 (0 self)
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We consider the following questions: What kind of logic has a natural semantics in multiplayer (rather than 2player) games? How can we express branching quantifiers, and other partialinformation constructs, with a properly compositional syntax and semantics? We develop a logic in answer to these questions, with a formal semantics based on multiple concurrent strategies, formalized as closure operators on KahnPlotkin concrete domains. Partial information constraints are represented as coclosure operators. We address the syntactic issues by treating syntactic constituents, including quantifiers, as arrows in a category, with arities and coarities. This enables a fully compositional account of a wide
Order algebras: a quantitative model of interaction
, 2010
"... Abstract. A quantitative model of concurrent interaction in introduced. The basic objects are linear combinations of partial order relations, acted upon by a group of permutations that represents potential nondeterminism in synchronisation. This algebraic structure is shown to provide faithful inte ..."
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Abstract. A quantitative model of concurrent interaction in introduced. The basic objects are linear combinations of partial order relations, acted upon by a group of permutations that represents potential nondeterminism in synchronisation. This algebraic structure is shown to provide faithful interpretations of finitary process algebras, for an extension of the standard notion of testing semantics, leading to a model that is both denotational (in the sense that the internal workings of processes are ignored) and noninterleaving. Constructions on algebras and their subspaces enjoy a good structure that make them (nearly) a model of differential linear logic, showing that the underlying approach to the representation of nondeterminism as linear