We give a first-order presentation of higher-order logic based on explicit substitutions. This presentation is intentionally equivalent to the usual presentation of higher-order logic based on λ-calculus, that is, a proposition can be proved without the extensionality axioms in one theory if and only if it can be in the other. We show that the Extended Narrowing and Resolution first-order proof-search method can be applied to this theory. In this way we get a step-by-step simulation of higher-order resolution. Hence, expressing higher-order logic as a first-order theory and applying a first-order proof search method is a relevant alternative to a direct implementation. In particular, the well-studied improvements of proof search for first-order logic could be reused at no cost for higher-order automated deduction. Moreover, as we stay in a first-order setting, extensions, such as equational higher-order resolution, may be easier to handle.

Session types allow high-level specifications of structured patterns of communication, such as client-server protocols, to be expressed as types and verified by static typechecking. In collaboration with Malcolm Hole, we previously introduced a notion of subtyping for session types, which was formulated for an extended pi calculus. Subtyping allows one part of a system, for example, a server, to be refined without invalidating type-correctness of other parts, for example, clients. In this paper we introduce bounded polymorphism, which is based on the same notion of subtyping, in order to support more precise and flexible specifications of protocols; in particular, a choice of type in one message may affect the types of future messages. We formalise the syntax, operational semantics and typing rules of an extended pi calculus, and prove that typechecking guarantees the absence of run-time communication errors. We study algorithms for checking instances of the subtype relation in two versions of our system, which we call Kernel S≤ and Full S≤, and establish that subtyping in Kernel S≤ is decidable, and that subtyping in Full S≤ is undecidable.

Mathematical modelling is one of the fundamental tools of science and engineering. Very often, models are required to be executable, as a simulation, on a computer. In this paper, we present some contributions to the process-theoretic and logical foundations of discrete-event modelling with resources and processes. We present a process calculus with an explicit representation of resources in which processes and resources co-evolve. The calculus is closely connected to a logic that may be used as a specification language for properties of models. The logic is strong enough to allow requirements that a system has a certain structure: for example, that it is a parallel composite of subsystems. This work consolidates, extends and improves upon aspects of earlier work of ours in this area. An extended example, consisting of a semantics for a simple parallel programming language, indicates a connection with separating logics for concurrency.

Place/transition (PT) Petri nets are one of the most widely used models of concurrency. However, they still lack, in our view, a satisfactory semantics: on the one hand the ‘token game’ is too intensional, even in its more abstract interpretations in terms of nonsequential processes and monoidal categories; on the other hand, Winskel's basic unfolding construction, which provides a coreflection between nets and finitary prime algebraic domains, works only for safe nets. In this paper we extend Winskel's result to PT nets. We start with a rather general category PTNets of PT nets, we introduce a category DecOcc of decorated (nondeterministic) occurrence nets and we define adjunctions between PTNets and DecOcc and between DecOcc and Occ, the category of occurrence nets. The role of DecOcc is to provide natural unfoldings for PT nets, i.e., acyclic safe nets where a notion of family is used to relate multiple instances of the same place. The unfolding functor from PTNets to Occ reduces to Winskel's when restricted to safe nets. Moreover, the standard coreflection between Occ and Dom, the category of finitary prime algebraic domains, when composed with the unfolding functor above, determines a chain of adjunctions between PTNets and Dom.

The multiset-based relational model of linear logic induces a semantics of the untyped λ-calculus, which corresponds with a non-idempotent intersection type system, System R. We prove that, in System R, the size of type derivations and the size of types are closely related to the execution time of λ-terms in a particular environment machine, Krivine's machine.

We show that, aside from the semiring equations, three equations and two equation schemes characterize the semiring of regular sets with the Kleene star operation.

The ambient logic is a modal logic that was proposed for the description of the structural and computational properties of distributed and mobile computation. The structural part of the ambient logic is, essentially, a logic of labelled trees, hence it turns out to be a good foundation for query languages for semistructured data, much in the same way as first-order logic is a fitting foundation for relational query languages. We define here a query language for semistructured data that is based on the ambient logic, and we outline an execution model for this language. The language turns out to be quite expressive. Its strong foundations and the equivalences that hold in the ambient logic are helpful in the definition of the language semantics and execution model.

We investigate, in a process algebraic setting, a new notion of correctness for service compositions, which we call strong service compliance: composed services are strong compliant if their composition is both deadlock and livelock free (this is the traditional notion of compliance), and whenever a message can be sent to invoke a service, it is guranteed to be ready to serve the invocation. We also define a new notion of refinement, called strong subcontract pre-order, suitable for strong compliance: given a composition of strong compliant services, we can replace any service with any other service in subcontract relation while preserving the overall strong compliance. Finally, we present a characterisation of the strong subcontract pre-order by resorting to the theory of a (should) testing pre-order.

Geometrical methods in concurrency theory (and in distributed systems theory) have appeared recently for modelling and analyzing the behaviour of systems and also for solving computability and complexity issues. We identify some of the main directions of research, and survey some of the major ideas on which all this is based (some of which are now more than thirty years old).

A constructive characterization is given of the isomorphisms which must hold in all models of the typed lambda calculus with surjective pairing. Using the close relation between closed Cartesian categories and models of these calculi, we also produce a characterization of those isomorphisms which hold in all CCC's. Using the correspondence between these calculi and proofs in intuitionistic positive propositional logic, we thus provide a characterization of equivalent formulae of this logic, where the definition of equivalence of terms depends on having “invertible” proofs between the two terms. Work of Rittri (1989), on types as search keys in program libraries, provides an interesting example of use of these characterizations.

This is a short overview of an experimental library of Mathematics formalized in the Coq proof assistant using the univalent interpretation of the underlying type theory of Coq. I started to work on this library in February 2010 in order to gain experience with formalization of Mathematics in a constructive type theory based on the intuition gained from the univalent models (see Kapulkin et al. 2012).

Full formal descriptions of algorithms making use of quantum principles must take into account both quantum and classical computing components, as well as communications between these components. Moreover, to model concurrent and distributed quantum computations and quantum communication protocols, communications over quantum channels that move qubits physically from one place to another must also be taken into account.

Inspired by classical process algebras, which provide a framework for modelling cooperating computations, a process algebraic notation is defined. This notation provides a homogeneous style for formal descriptions of concurrent and distributed computations comprising both quantum and classical parts. Based upon an operational semantics that makes sure that quantum objects, operations and communications operate according to the postulates of quantum mechanics, an equivalence is defined among process states considered as having the same behaviour. This equivalence is a probabilistic branching bisimulation. From this relation, an equivalence on processes is defined. However, it is not a congruence because it is not preserved by parallel composition.

The ambient calculus is a process calculus for describing mobile computation. We develop a theory of Morris-style contextual equivalence for proving properties of mobile ambients. We prove a context lemma that allows derivation of contextual equivalences by considering contexts of a particular limited form, rather than all arbitrary contexts. We give an activity lemma that characterises the possible interactions between a process and a context. We prove several examples of contextual equivalence. The proofs depend on characterising reductions in the ambient calculus in terms of a labelled transition system.

In this paper we investigate the expressive power of three alternative approaches to the definition of infinite behaviours in process calculi, namely, recursive definitions, replication and iteration. We prove several results discriminating between the calculi obtained from a core CCS by adding the three mechanisms mentioned above. These results are derived by considering the decidability of four basic properties: termination (that is, all computations are finite); convergence (that is, the existence of a finite computation); barb (that is, the ability to perform an action on a given channel) and weak bisimulation.

Our results, which are summarised in Table 1, show that the three calculi form a strict expressiveness hierarchy in that: all the properties mentioned are undecidable in CCS with recursion; only termination and barb are decidable in CCS with replication; all the properties are decidable in CCS with iteration.

As a corollary, we also obtain a strict expressiveness hierarchy with respect to weak bisimulation, since there exist weak bisimulation preserving encodings of iteration in replication and of replication in recursion, whereas there are no weak bisimulation preserving encodings in the other directions.

This paper uses concepts from sheaf theory to explain phenomena in concurrent systems, including object, inheritance, deadlock, and non-interference, as used in computer security. The approach is very; general, and applies not only to concurrent object oriented systems, but also to systems of differential equations, electrical circuits, hardware description languages, and much more. Time can be discrete or continuous, linear or branching, and distribution is allowed over space as well as time. Concepts from categpru theory help to achieve this generality: objects are modelled by sheaves; inheritance by sheaf morphisms; systems by diagrams; and interconnection by diagrams of diagrams. In addition, behaviour is given by limit, and the result of interconnection by colimit. The approach is illustrated with many examples, including a semantics for a simple concurrent object-based programming language.

Ciliates have developed a unique nuclear dualism, having two nuclei of different functionality: the germline micronucleus and the somatic macronucleus. The way that ciliates assemble the macronuclear genes after cell mating constitutes one of the most intricate DNA processings in living organisms. This processing is also very interesting from the computational point of view. In this paper, we investigate the operations of loop excision and hairpin excision/reinsertion used in the assembly process. In particular, we consider three levels of formalization of this process, culminating in graph reduction systems.

Using a categorical semantics that has been developed recently as a basis, we study composition and refinement as horizontal and vertical structuring techniques for typed graph transformation systems. Composition of graph transformation systems with respect to common subsystems is shown to be compatible with the semantics, i.e., the semantics of the composed system is obtained as the composition of the semantics of the component systems. Moreover, the structure of a composed graph transformation system is preserved during a refinement step in the sense that compatible refinements of the components induce a refinement of the composition. The concepts and results are illustrated by a sample development of a small information system using entity relationship modelling techniques.

Mixins are modules in which some components are deferred, that is, their definition has to be provided by another module. Moreover, in contrast to parameterized modules (like ML functors), mixin modules can be mutually dependent and their composition supports the redefinition of components (overriding). In this paper, we present a formal model of mixins and their basic composition operators. These operators can be viewed as a kernel language with clean semantics in which one can express more complex operators of existing modular languages, including variants of inheritance in object-oriented programming. Our formal model is given in an ‘institution independent’ way, that is, it is parameterized by the semantic framework modelling the underlying core language.

Provenance is information recording the source, derivation or history of some information. Provenance tracking has been studied in a variety of settings, particularly database management systems. However, although many candidate definitions of provenance have been proposed, the mathematical or semantic foundations of data provenance have received comparatively little attention. In this paper, we argue that dependency analysis techniques familiar from program analysis and program slicing provide a formal foundation for forms of provenance that are intended to show how (part of) the output of a query depends on (parts of) its input. We introduce a semantic characterisation of such dependency provenance for a core database query language, show that minimal dependency provenance is not computable, and provide dynamic and static approximation techniques. We also discuss preliminary implementation experience with using dependency provenance to compute data slices, or summaries of the parts of the input relevant to a given part of the output.

We construct an approximating chain of simple valuations on the upper space of a compact metric space whose lub is a given probability measure on the metric space. We show that whenever a separable metric space is homeomorphic to a Gδ subset of an ω-continuous dcpo equipped with its Scott topology, the space of probability measures of the metric space equipped with the weak topology is homeomorphic with a subset of the maximal elements of the probabilistic power domain of the ω-continuous dcpo. Given an effective approximation of a probability measure by an increasing chain of normalised valuations on the upper space of a compact metric space, we show that the expected value of any Hölder continuous function on the space can be obtained up to any given accuracy. We present a novel application in computing integrals in dynamical systems. We obtain an algorithm to compute the expected value of any Hölder continuous function with respect to the unique invariant measure of the Feigenbaum map in the periodic doubling route to chaos.