A Faster-Than Relation for Semi-Markov Decision Processes

When modeling concurrent or cyber-physical systems, non-functional requirements such as time are important to consider. In order to improve the timing aspects of a model, it is necessary to have some notion of what it means for a process to be faster than another, which can guide the stepwise refinement of the model. To this end we study a faster-than relation for semi-Markov decision processes and compare it to standard notions for relating systems. We consider the compositional aspects of this relation, and show that the faster-than relation is not a precongruence with respect to parallel composition, hence giving rise to so-called parallel timing anomalies. We take the first steps toward understanding this problem by identifying decidable conditions sufficient to avoid parallel timing anomalies in the absence of non-determinism.Comment: In Proceedings QAPL 2019, arXiv:2001.0616

Behavioural Preorders on Stochastic Systems - Logical, Topological, and Computational Aspects

Computer systems can be found everywhere: in space, in our homes, in our cars, in our pockets, and sometimes even in our own bodies. For concerns of safety, economy, and convenience, it is important that such systems work correctly. However, it is a notoriously difficult task to ensure that the software running on computers behaves correctly. One approach to ease this task is that of model checking, where a model of the system is made using some mathematical formalism. Requirements expressed in a formal language can then be verified against the model in order to give guarantees that the model satisfies the requirements. For many computer systems, time is an important factor. As such, we need our formalisms and requirement languages to be able to incorporate real time. We therefore develop formalisms and algorithms that allow us to compare and express properties about real-time systems. We first introduce a logical formalism for reasoning about upper and lower bounds on time, and study the properties of this formalism, including axiomatisation and algorithms for checking when a formula is satisfied. We then consider the question of when a system is faster than another system. We show that this is a difficult question which can not be answered in general, but we identify special cases where this question can be answered. We also show that under this notion of faster-than, a local increase in speed may lead to a global decrease in speed, and we take step towards avoiding this. Finally, we consider how to compare the real-time behaviour of systems not just qualitatively, but also quantitatively. Thus, we are interested in knowing how much one system is faster or slower than another system. This is done by introducing a distance between systems. We show how to compute this distance and that it behaves well with respect to certain properties.Comment: PhD dissertation from Aalborg Universit

Timed Comparisons of Semi-Markov Processes

Semi-Markov processes are Markovian processes in which the firing time of the transitions is modelled by probabilistic distributions over positive reals interpreted as the probability of firing a transition at a certain moment in time. In this paper we consider the trace-based semantics of semi-Markov processes, and investigate the question of how to compare two semi-Markov processes with respect to their time-dependent behaviour. To this end, we introduce the relation of being "faster than" between processes and study its algorithmic complexity. Through a connection to probabilistic automata we obtain hardness results showing in particular that this relation is undecidable. However, we present an additive approximation algorithm for a time-bounded variant of the faster-than problem over semi-Markov processes with slow residence-time functions, and a coNP algorithm for the exact faster-than problem over unambiguous semi-Markov processes

«Spilling blant unge i et avvikssosiologisk perspektiv» : Et kvalitativt forskningsprosjekt om unges erfaringer med dataspill i sammenheng med stigmatisering og støtte i spillkulturen

Denne masteroppgaven er resultatet av et forskningsprosjekt på unge studenter med interesse for og lang erfaring med video- og dataspill. Formålet med prosjektet var å kartlegge spillernes opplevelser av spill, med et todelt fokus på en eventuell sammenheng mellom spilling og stigmatisering, og spillkulturen som et støtteapparat. Fremgangsmåten for undersøkelsen har vært kvalitative intervjuer av fem studenter tilknyttet en spillgruppe ved en høgskole i Norge. Respondentene har ytret sine meninger om spill, deres spillgruppe og om spillere generelt. Disse meningene er basert på deres erfaring fra barne- og ungdomsalder, og deres liv som unge voksne spillere. Deres refleksjoner knyttes til et avvikssosiologisk perspektiv, hvor også et utvalg av tidligere forskning på feltet og annen relevant teori er en god bakgrunn for diskusjonen av resultatene. Det avvikssosiologiske perspektivet har fungert som bakteppet for besvarelsen av oppgavens problemstilling. Funnene i forskningen hadde flere elementer av avvikssosiologisk verdi. Det ble funnet sammenhenger mellom spilling og stigmatisering, noe som allerede var forsterket gjennom utvalget av tidligere forskning og teorien brukt i oppgaven. Det viste seg også at respondentene som spillere selv, tidvis stigmatiserte andre spillere. Det ble også funnet at spillkulturen kan fungere som et støtteapparat for spillere, og at ulike faktorer både kan svekke og styrke denne støtten. For eksempel kunne sosial tilhørighet og dannelse av vennskap styrke støtten i spillkulturen. Det viste seg også at gradene av sosial støtte fra disse vennskapene kan være varierende. De svake sosiale bånd som ofte dannes, fortrinnsvis i nettbaserte spill, kunne være et svekkende element i spillkulturens støtte. I tillegg kom det fram at spillernes gjensidige stigmatisering av hverandre kan være en motsetning til det som regnes som et støtteapparat. Derfor er spillkulturen som støtteapparat et kontekstavhengig begrep, hvor ulike faktorer vil være bestemmende for støtten som oppleves. Det kan også variere fra person til person. Disse konklusjonene ble trukket utfra det respondentene hevdet om sin spillgruppe, hvor spillgruppen kan tenkes å være en «modell» for resten av spillkulturen. Spillkulturen ellers, altså utenfor den aktuelle spillgruppen, ble også brukt i drøftingen av spillkulturen som støtteapparat

Oxygen supply to biocatalytic oxidations

Oxygen-dependent enzymes are becoming increasingly relevant in the synthesis of fine chemicals, flavours and fragrances as well as pharmaceutical intermediates. Oxidases are a notable subclass of oxidizing enzymes, which use molecular oxygen either as an oxidant or as an electron acceptor. This property makes them highly attractive for industrial manufacturing processes, avoiding the use of harmful metal oxidants However, supplying molecular oxygen with high transfer rates is still a major challenge when high reaction productivities need to be achieved to develop an economic feasible process. Commonly, bioreactors supply oxygen by sparging air into the reaction medium and the resultant mass transfer of oxygen from the gas to the liquid phase has proved to be a limiting factor due to the poor solubility of oxygen in water and since high oxygen demand is needed to achieve adequate reaction productivities (1). Furthermore, enzyme stability might become an issue since oxidases may deactivate at a gas-liquid interface (2). Therefore, in order to develop robust processes using oxygen-dependent enzymes, there is a necessity to quantify the oxygen affinity of the enzyme, explore the enzyme stability and quantify how fast the oxygen needs to be supplied to achieve high productivities. This can be done by characterizing the enzyme under relevant conditions for an industrial process. This contribution is focused on the characterization of enzyme and reaction kinetics of oxygen-dependent biocatalysts, with emphasis on the oxygen requirements, in order to provide guidance for the design and development of oxidative biocatalytic processes. The influence of KMO (kinetic Michaelis constant for oxygen) on enzyme efficiency will be discussed (3) as well as process limitations of oxygen transfer. Van Hecke W, Ludwig R, Dewulf J, Auly M, Messiaen T, Haltrich D, et al. Bubble-free oxygenation of a bi-enzymatic system: Effect on biocatalyst stability. Biotechnol Bioeng. 2009;102(1):122–31. Bommarius AS, Karau A. Deactivation of Formate Dehydrogenase (FDH) in solution and at gas-liquid interfaces. Biotechnol Prog. 2005;21(6):1663–72. Toftgaard Pedersen A, Birmingham WR, Rehn G, Charnock SJ, Turner NJ, Woodley JM. Process requirements of galactose oxidase catalyzed oxidation of alcohols. Org Process Res Dev. 2015;19:1580–9