A Survey on Continuous Time Computations

We provide an overview of theories of continuous time computation. These theories allow us to understand both the hardness of questions related to continuous time dynamical systems and the computational power of continuous time analog models. We survey the existing models, summarizing results, and point to relevant references in the literature

Verification of Timed Automata Using Rewrite Rules and Strategies

ELAN is a powerful language and environment for specifying and prototyping deduction systems in a language based on rewrite rules controlled by strategies. Timed automata is a class of continuous real-time models of reactive systems for which efficient model-checking algorithms have been devised. In this paper, we show that these algorithms can very easily be prototyped in the ELAN system. This paper argues through this example that rewriting based systems relying on rules and strategies are a good framework to prototype, study and test rather efficiently symbolic model-checking algorithms, i.e. algorithms which involve combination of graph exploration rules, deduction rules, constraint solving techniques and decision procedures

Bitcoin: the wrong implementation of the right idea at the right time

This paper is a study into some of the regulatory implications of cryptocurrencies using the CAMPO research framework (Context, Actors, Methods, Methods, Practice, Outcomes). We explain in CAMPO format why virtual currencies are of interest, how self-regulation has failed, and what useful lessons can be learned. We are hopeful that the full paper will produce useful and semi-permanent findings into the usefulness of virtual currencies in general, block chains as a means of mining currency, and the profundity of current ‘media darling’ currency Bitcoin as compared with the development of block chain generator Ethereum. While virtual currencies can play a role in creating better trading conditions in virtual communities, despite the risks of non-sovereign issuance and therefore only regulation by code (Brown/Marsden 2013), the methodology used poses significant challenges to researching this ‘community’, if BitCoin can even be said to have created a single community, as opposed to enabling an alternate method of exchange for potentially all virtual community transactions. First, BitCoin users have transparency of ownership but anonymity in many transactions, necessary for libertarians or outright criminals in such illicit markets as #SilkRoad. Studying community dynamics is therefore made much more difficult than even such pseudonymous or avatar based communities as Habbo Hotel, World of Warcraft or SecondLife. The ethical implications of studying such communities raise similar problems as those of Tor, Anonymous, Lulzsec and other anonymous hacker communities. Second, the journalistic accounts of BitCoin markets are subject to sensationalism, hype and inaccuracy, even more so than in the earlier hype cycle for SecondLife, exacerbated by the first issue of anonymity. Third, the virtual currency area is subject to slowly emerging regulation by financial authorities and police forces, which appears to be driving much of the early adopter community ‘underground’. Thus, the community in 2016 may not bear much resemblance to that in 2012. Fourth, there has been relatively little academic empirical study of the community, or indeed of virtual currencies in general, until relatively recently. Fifth, the dynamism of the virtual currency environment in the face of the deepening mistrust of the financial system after the 2008 crisis is such that any research conclusions must by their nature be provisional and transient. All these challenges, particularly the final three, also raise the motivation for research – an alternative financial system which is separated from the real-world sovereign and which can use code regulation with limited enforcement from offline policing, both returns the study to the libertarian self-regulated environment of early 1990s MUDs, and offers a tantalising prospect of a tool to evade the perils of ‘private profit, socialized risk’ which existing large financial institutions created in the 2008-12 disaster. The need for further research into virtual currencies based on blockchain mining, and for their usage by virtual communities, is thus pressing and should motivate researchers to solve the many problems in methodology for exploring such an environment

On computing fixpoints in well-structured regular model checking, with applications to lossy channel systems

We prove a general finite convergence theorem for "upward-guarded" fixpoint expressions over a well-quasi-ordered set. This has immediate applications in regular model checking of well-structured systems, where a main issue is the eventual convergence of fixpoint computations. In particular, we are able to directly obtain several new decidability results on lossy channel systems.Comment: 16 page

Exploiting the Hierarchical Structure of Rule-Based Specifications for Decision Planning

Rule-based specifications have been very successful as a declarative approach in many domains, due to the handy yet solid foundations offered by rule-based machineries like term and graph rewriting. Realistic problems, however, call for suitable techniques to guarantee scalability. For instance, many domains exhibit a hierarchical structure that can be exploited conveniently. This is particularly evident for composition associations of models. We propose an explicit representation of such structured models and a methodology that exploits it for the description and analysis of model- and rule-based systems. The approach is presented in the framework of rewriting logic and its efficient implementation in the rewrite engine Maude and is illustrated with a case study.