A Logic with Reverse Modalities for History-preserving Bisimulations

We introduce event identifier logic (EIL) which extends Hennessy-Milner logic by the addition of (1) reverse as well as forward modalities, and (2) identifiers to keep track of events. We show that this logic corresponds to hereditary history-preserving (HH) bisimulation equivalence within a particular true-concurrency model, namely stable configuration structures. We furthermore show how natural sublogics of EIL correspond to coarser equivalences. In particular we provide logical characterisations of weak history-preserving (WH) and history-preserving (H) bisimulation. Logics corresponding to HH and H bisimulation have been given previously, but not to WH bisimulation (when autoconcurrency is allowed), as far as we are aware. We also present characteristic formulas which characterise individual structures with respect to history-preserving equivalences.Comment: In Proceedings EXPRESS 2011, arXiv:1108.407

Expressiveness modulo Bisimilarity of Regular Expressions with Parallel Composition (Extended Abstract)

The languages accepted by finite automata are precisely the languages denoted by regular expressions. In contrast, finite automata may exhibit behaviours that cannot be described by regular expressions up to bisimilarity. In this paper, we consider extensions of the theory of regular expressions with various forms of parallel composition and study the effect on expressiveness. First we prove that adding pure interleaving to the theory of regular expressions strictly increases its expressiveness up to bisimilarity. Then, we prove that replacing the operation for pure interleaving by ACP-style parallel composition gives a further increase in expressiveness. Finally, we prove that the theory of regular expressions with ACP-style parallel composition and encapsulation is expressive enough to express all finite automata up to bisimilarity. Our results extend the expressiveness results obtained by Bergstra, Bethke and Ponse for process algebras with (the binary variant of) Kleene's star operation.Comment: In Proceedings EXPRESS'10, arXiv:1011.601

A criterion for separating process calculi

We introduce a new criterion, replacement freeness, to discern the relative expressiveness of process calculi. Intuitively, a calculus is strongly replacement free if replacing, within an enclosing context, a process that cannot perform any visible action by an arbitrary process never inhibits the capability of the resulting process to perform a visible action. We prove that there exists no compositional and interaction sensitive encoding of a not strongly replacement free calculus into any strongly replacement free one. We then define a weaker version of replacement freeness, by only considering replacement of closed processes, and prove that, if we additionally require the encoding to preserve name independence, it is not even possible to encode a non replacement free calculus into a weakly replacement free one. As a consequence of our encodability results, we get that many calculi equipped with priority are not replacement free and hence are not encodable into mainstream calculi like CCS and pi-calculus, that instead are strongly replacement free. We also prove that variants of pi-calculus with match among names, pattern matching or polyadic synchronization are only weakly replacement free, hence they are separated both from process calculi with priority and from mainstream calculi.Comment: In Proceedings EXPRESS'10, arXiv:1011.601

Magnetic Filter Field Dependence of the Performance of the RF Driven IPP Prototype Source for Negative Hydrogen Ions

The ITER neutral beam system requires a negative hydrogen ion beam of 48 A with an energy of 0.87 MeV and a negative deuterium beam of 40 A with an energy of 1 MeV. The beam is extracted from a large RF driven ion source with the dimension of 1.9 × 0.9 m2. An important role for the transport of the negative hydrogen ions to the extractor and the suppression of the co-extracted electrons is the magnetic filter field in front of the extractor. For the large ITER source the filter field will be generated by a current of up to 4 kA flowing through the first grid of the extractor. The extrapolation of the results obtained with the small IPP RF prototype source, where the filter field has a different 3D structure as it is generated by permanent magnets, is not straightforward. Furthermore, the filter field is by far not optimized due to the technical constraints of the RF source. Therefore, a frame that surrounds the ion sources and hosts permanent magnets was constructed for a fast and flexible change of the filter field. First results in hydrogen show that a minimum field of 3 mT in front of the extractor is needed for a sufficiently large number of extracted negative hydrogen ions, whereas sufficient co-extracted electron suppression is achieved by a source integrated magnetic field of more than 1.0 mTm.</jats:p

Logics and bisimulation games for concurrency, causality and conflict

Based on a simple axiomatization of concurrent behaviour we define two ways of observing parallel computations and show that in each case they are dual to conflict and causality, respectively. We give a logical characterization to those dualities and show that natural fixpoint modal logics can be extracted from such a characterization. We also study the equivalences induced by such logics and prove that they are decidable and can be related with well-known bisimulations for interleaving and noninterleaving concurrency. Moreover, by giving a game-theoretical characterization to the equivalence induced by the main logic, which is called Separation Fixpoint Logic (SFL), we show that the equivalence SFL induces is strictly stronger than a history-preserving bisimulation (hpb) and strictly weaker than a hereditary history-preserving bisimulation (hhpb). Our study considers branching-time models of concurrency based on transition systems and petri net structures