UTP, Circus, and Isabelle

We dedicate this paper with great respect and friendship to He Jifeng on the occasion of his 80th birthday. Our research group owes much to him. The authors have over 150 publications on unifying theories of programming (UTP), a research topic Jifeng created with Tony Hoare. Our objective is to recount the history of Circus (a combination of Z, CSP, Dijkstra’s guarded command language, and Morgan’s refinement calculus) and the development of Isabelle/UTP. Our paper is in two parts. (1) We first discuss the activities needed to model systems: we need to formalise data models and their behaviours. We survey our work on these two aspects in the context of Circus. (2) Secondly, we describe our practical implementation of UTP in Isabelle/HOL. Mechanising UTP theories is the basis of novel verification tools. We also discuss ongoing and future work related to (1) and (2). Many colleagues have contributed to these works, and we acknowledge their support

An Analysis Tool for Models of Virtualized Systems

This paper gives an example-driven introduction to modelling and analyzing virtualized systems in, e.g., cloud computing, using virtually timed ambients, a process algebra developed to study timing aspects of resource management for (nested) virtual machines. The calculus supports nested virtualization and virtual machines compete with other processes for the resources of their host environment. Resource provisioning in virtually timed ambients extends the capabilities of mobile ambients to model the dynamic creation, migration, and destruction of virtual machines. Quality of service properties for virtually timed ambients can be formally expressed using modal contracts describing aspects of resource provisioning and verified using a model checker for virtually timed ambients, implemented in the rewriting system Maude

On language classes accepted by stateless 5′ → 3′ Watson-Crick finite automata

Watson-Crick automata are belonging to the natural computing paradigm as these finite automata are working on strings representing DNA molecules. Watson-Crick automata have two reading heads, and in the 5 ′ → 3 ′ models these two heads start from the two extremes of the input. This is well motivated by the fact that DNA strands have 5 ′ and 3 ′ ends based on the fact which carbon atoms of the sugar group is used in the covalent bonds to continue the strand. However, in the two stranded DNA, the directions of the strands are opposite, so that, if an enzyme would read the strand it may read each strand in its 5 ′ to 3 ′ direction, which means physically opposite directions starting from the two extremes of the molecule. On the other hand, enzymes may not have inner states, thus those Watson-Crick automata which are stateless (i.e. have exactly one state) are more realistic from this point of view. In this paper these stateless 5 ′ → 3 ′ Watson-Crick automata are studied and some properties of the language classes accepted by their variants are proven. We show hierarchy results, and also a “pumping”, i.e., iteration result for these languages that can be used to prove that some languages may not be in the class accepted by the class of stateless 5 ′ → 3 ′ Watson-Crick automata

On language classes accepted by stateless 5′ → 3′ Watson-Crick finite automata

Watson-Crick automata are belonging to the natural computing paradigm as these finite automata are working on strings representing DNA molecules. Watson-Crick automata have two reading heads, and in the 5 ′ → 3 ′ models these two heads start from the two extremes of the input. This is well motivated by the fact that DNA strands have 5 ′ and 3 ′ ends based on the fact which carbon atoms of the sugar group is used in the covalent bonds to continue the strand. However, in the two stranded DNA, the directions of the strands are opposite, so that, if an enzyme would read the strand it may read each strand in its 5 ′ to 3 ′ direction, which means physically opposite directions starting from the two extremes of the molecule. On the other hand, enzymes may not have inner states, thus those Watson-Crick automata which are stateless (i.e. have exactly one state) are more realistic from this point of view. In this paper these stateless 5 ′ → 3 ′ Watson-Crick automata are studied and some properties of the language classes accepted by their variants are proven. We show hierarchy results, and also a “pumping”, i.e., iteration result for these languages that can be used to prove that some languages may not be in the class accepted by the class of stateless 5 ′ → 3 ′ Watson-Crick automata

State-deterministic Finite Automata with Translucent Letters and Finite Automata with Nondeterministically Translucent Letters

Deterministic and nondeterministic finite automata with translucent letters were introduced by Nagy and Otto more than a decade ago as Cooperative Distributed systems of a kind of stateless restarting automata with window size one. These finite state machines have a surprisingly large expressive power: all commutative semi-linear languages and all rational trace languages can be accepted by them including various not context-free languages. While the nondeterministic variant defines a language class with nice closure properties, the deterministic variant is weaker, however it contains all regular languages, some non-regular context-free languages, as the Dyck language, and also some languages that are not even context-free. In all those models for each state, the letters of the alphabet could be in one of the following categories: the automaton cannot see the letter (it is translucent), there is a transition defined on the letter (maybe more than one transitions in nondeterministic case) or none of the above categories (the automaton gets stuck by seeing this letter at the given state and this computation is not accepting). State-deterministic automata are recent models, where the next state of the computation determined by the structure of the automata and it is independent of the processed letters. In this paper our aim is twofold, on the one hand, we investigate state-deterministic finite automata with translucent letters. These automata are specially restricted deterministic finite automata with translucent letters. In the other novel model we present, it is allowed that for a state the set of translucent letters and the set of letters for which transition is defined are not disjoint. One can interpret this fact that the automaton has a nondeterministic choice for each occurrence of such letters to see them (and then erase and make the transition) or not to see that occurrence at that time. Based on these semi-translucent letters, the expressive power of the automata increases, i.e., in this way a proper generalization of the previous models is obtained.Comment: In Proceedings AFL 2023, arXiv:2309.0112

Revisiting logical semantics for processes and their distances

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Informática, Departamento de Sistemas Informáticos y Computación, leída el 2-02-2016Esta tesis se enmarca en el amplio campo de la teoría de la concurrencia. Más específicamente, nos centramos en el estudio de las relaciones de similitud entre procesos concurrentes. Comenzamos estudiando la bisimulación, considerada la más importante de estas relaciones, y vemos después cómo podemos extender nuestros resultados al resto de las semánticas de procesos estudiadas durante las últimas décadas. En particular, nuestra contribución a la comunidad científica, se centra en dos puntos principales: – El desarrollo de una caracterización lógica uniforme de las semánticas de procesos: proponemos un esquema lógico común (enmarcado en la conocida lógica modal de Hennessy-Milner) e incluimos las diferentes semánticas en este esquema, enfatizando las diferencias y similitudes entre ellas, que se presentan del modo más claro posible. – La presentación de una nueva noción de distancia, tanto entre procesos finitos como infinitos: la misma se diferencia de las anteriormente propuestas en su carácter global, que acumula las diferencias que aportan los distintos cómputos, en lugar de quedarnos con la máxima de ellas...This thesis can be included in the broad field of concurrency theory. More specifically, we focus on the study of the similarities between concurrent processes. We start from bisimulation, the main of these relations, and then we see how we can extend the obtained results to the rest of the semantics developed along the last years. In particular, our main contributions can be roughly described by the following two items: – The development of a unified logical characterization of process semantics: we propose a common logical scheme (within the framework of the well known Hennessy-Milner Logic) and we set the different semantics in this scheme by emphasizing, in the clearest possible way, the (dis)similarities between them. – We present a new notion of distance for both finite and infinite processes. This novel notion differs from the previously available ones in its global character: instead of taking the maximum disagreement between the two compared processes, it adds all the differences provided by their whole sets of computations...Depto. de Sistemas Informáticos y ComputaciónFac. de InformáticaTRUEunpu

Zur Komplexität der Synthese von Petri-Netzen

Petri-Netz-Synthese fragt für ein reguläres Verhalten, das als Transitionssystem A gegeben ist, ob es ein Petri-Netz (einer bestimmten Klasse) gibt, das A implementiert. Diese Arbeit untersucht Varianten dieses Entscheidungsproblems für verschiedene Implementierungen und eine Vielzahl von Petri-Netz-Klassen aus Sicht der klassischen und der parametrisierten Komplexität. Ebenso wird die Komplexität der Modifikations-Techniken Neubeschriftung sowie Kanten-, Ereignis- und Zustandslöschung untersucht, die darauf abzielen, nicht-implememtierbare Transitionssysteme implementierbar zu machen.Petri net synthesis asks, for a regular behavior given as a transition system A, whether there exists a Petri net (of a particular class) that implements A. This work investigates variants of this decision problem for different implementations and a variety of Petri net classes from the point of view of classical and parameterized complexity. Also investigated is the complexity of the modification techniques of relabeling and edge, event, and state deletion, which aim to make non-implementable transition systems implementable