We generalise the standard construction of realizability models (specifically, of categories of assemblies) to a wide class of computability structures, which is broad enough to embrace models of computation such as labelled transition systems and process algebras. We consider a general notion of simulation between such computability structures, and show how these simulations correspond precisely to certain functors between the realizability models. Furthermore, we show that our class of computability structures has good closure properties – in particular, it is ‘cartesian closed’ in a slightly relaxed sense. Finally, we investigate some important subclasses of computability structures and of simulations between them. We suggest that our 2-category of computability structures and simulations may offer a useful framework for investigating questions of computational power, abstraction and simulability for a wide range of models.

Raussen (2010) gave the trace space, which corresponds to the executions of parallel non-looped, non-branching processes as a prod-simplicial complex derived from a poset. The connected components represent equivalent executions. For looped processes, the state space is a torus and the trace space is a disjoint union of trace spaces of deloopings. In the current paper, we develop the index poset for the trace space of the deloopings from the once delooped case. When just one process is looped, the index poset is generated as words in a regular language. We also construct a corresponding automaton.

In earlier work we explored the expressiveness and algebraic theory Psi-calculi, which form a parametric framework for extensions of the pi-calculus. In the current paper we consider higher-order psi-calculi through a technically surprisingly simple extension of the framework, and show how an arbitrary psi-calculus can be lifted to its higher-order counterpart in a canonical way. We illustrate this with examples and establish an algebraic theory of higher-order psi-calculi. The formal results are obtained by extending our proof repositories in Isabelle/Nominal.

In this paper we introduce Event Identifier Logic (EIL), which extends Hennessy–Milner logic by the addition of:

1. (1) reverse as well as forward modalities; and

2. (2) identifiers to keep track of events.

We consider structured specifications built from flat specifications using union, translation and hiding with their standard model-class semantics in the context of an arbitrary institution. We examine the alternative of sound property-oriented semantics for such specifications, and study their relationship to model-class semantics. An exact correspondence between the two (completeness) is not achievable in general. We show through general results on property-oriented semantics that the semantics arising from the standard proof system is the strongest sound and compositional property-oriented semantics in a wide class of such semantics. We also sharpen one of the conditions that does guarantee completeness and show that it is a necessary condition.

We give a constructive proof showing that every finitely generated polynomial ideal has a Gröbner basis, provided the ring of coefficients is Noetherian in the sense of Richman and Seidenberg. That is, we give a constructive termination proof for a variant of the well-known algorithm for computing the Gröbner basis. In combination with a purely order-theoretic result we have proved in a separate paper, this yields a unified constructive proof of the Hilbert basis theorem for all Noether classes: if a ring belongs to a Noether class, then so does the polynomial ring. Our proof can be seen as a constructive reworking of one of the classical proofs, in the spirit of the partial realisation of Hilbert's programme in algebra put forward by Coquand and Lombardi. The rings under consideration need not be commutative, but are assumed to be coherent and strongly discrete: that is, they admit a membership test for every finitely generated ideal. As a complement to the proof, we provide a prime decomposition for commutative rings possessing the finite-depth property.