Structural Synthesis for GXW Specifications

We define the GXW fragment of linear temporal logic (LTL) as the basis for synthesizing embedded control software for safety-critical applications. Since GXW includes the use of a weak-until operator we are able to specify a number of diverse programmable logic control (PLC) problems, which we have compiled from industrial training sets. For GXW controller specifications, we develop a novel approach for synthesizing a set of synchronously communicating actor-based controllers. This synthesis algorithm proceeds by means of recursing over the structure of GXW specifications, and generates a set of dedicated and synchronously communicating sub-controllers according to the formula structure. In a subsequent step, 2QBF constraint solving identifies and tries to resolve potential conflicts between individual GXW specifications. This structural approach to GXW synthesis supports traceability between requirements and the generated control code as mandated by certification regimes for safety-critical software. Synthesis for GXW specifications is in PSPACE compared to 2EXPTIME-completeness of full-fledged LTL synthesis. Indeed our experimental results suggest that GXW synthesis scales well to industrial-sized control synthesis problems with 20 input and output ports and beyond.Comment: The long (including appendix) version being reviewed by CAV'16 program committee. Compared to the submitted version, one author (out of her wish) is moved to the Acknowledgement. (v2) Corrected typos. (v3) Add an additional remark over environment assumption and easy corner case

Monetary policy and stock market boom-bust cycles

We explore the dynamic effects of news about a future technology improvement which turns out ex post to be overoptimistic. We find that it is difficult to generate a boom-bust cycle (a period in which stock prices, consumption, investment and employment all rise and then crash) in response to such a news shock, in a standard real business cycle model. However, a monetized version of the model which stresses sticky wages and a Taylorrule based monetary policy naturally generates a welfare-reducing boom-bust cycle in response to a news shock. We explore the possibility that integrating credit growth into monetary policy may result in improved performance. We discuss the robustness of our analysis to alternative specifications of the labor market, in which wage-setting frictions do not distort on going firm/worker relations. JEL Classification: C11, C51, E5, E13, E32Asset price boom-busts, DSGE Models, monetary policy

Conversion from linear to circular polarization in FPGA in real time

ABSTRACT Radio astronomical receivers are now expanding their frequency range to cover large (octave) fractional bandwidths for sensitivity and spectral flexibility, which makes the design of good analogue circular polarizers challenging. Better polarization purity requires a flatter phase response over increasingly wide bandwidth, which is most easily achieved with digital techniques. They offer the ability to form circular polarization with perfect polarization purity over arbitrarily wide fractional bandwidths, due to the ease of introducing a perfect quadrature phase shift. In analogue systems the quadrature phase shift is not accurate in the regions away from the design point or frequency. In digital systems on the contrary, it is possible to introduce the exact quadrature phase shift vectorially to each frequency point in the band thus producing a perfect quadrature phase shift throughout the band. Further, the rapid improvements in field programmable gate arrays provide the high processing power, low cost, portability and reconfigurability needed to make practical the implementation of the formation of circular polarization digitally. It will be possible to carry out broadband polarization observations. Circular polarization is used in very long baseline interferometry (VLBI) due to geometrical and stability considerations. VLBI is often used to explore polarization of radio emission, which often occurs due to synchrotron mechanism, Zeeman effect in atoms and molecules, cyclotron radiation and plasma oscillations in the solar atmosphere. Also VLBI finds application in methods like rotation measure synthesis that can be used to find the magnetic field strength and whose multiwavelength observations determine the direction of magnetic field. So a digital circular polarizer would find a considerable application in VLBI systems. Here I explore the performance of a circular polarizer implemented with digital techniques. I designed a digital circular polarizer in which the intermediate frequency signals from a receiver with native linear polarizations were sampled and converted to circular polarization. The frequency-dependent instrumental phase difference and gain scaling factors were determined using an injected noise signal and applied to the two linear polarizations to equalize the transfer characteristics of the two polarization channels. This equalization was performed in 512 frequency channels over a 500 MHz bandwidth. Circular polarization was formed by quadrature phase shifting and summing the equalized linear polarization signals. I obtained polarization purity of -58 dB corresponding to a D-term of 0.0012 over the whole bandwidth. This value of D-term is an upper limit. This technique enables construction of broad-band radio astronomy receivers with native linear polarization to form circular polarization for VLBI

Computer Aided Verification

This open access two-volume set LNCS 13371 and 13372 constitutes the refereed proceedings of the 34rd International Conference on Computer Aided Verification, CAV 2022, which was held in Haifa, Israel, in August 2022. The 40 full papers presented together with 9 tool papers and 2 case studies were carefully reviewed and selected from 209 submissions. The papers were organized in the following topical sections: Part I: Invited papers; formal methods for probabilistic programs; formal methods for neural networks; software Verification and model checking; hyperproperties and security; formal methods for hardware, cyber-physical, and hybrid systems. Part II: Probabilistic techniques; automata and logic; deductive verification and decision procedures; machine learning; synthesis and concurrency. This is an open access book

Symbolic reactive synthesis

In this thesis, we develop symbolic algorithms for the synthesis of reactive systems. Synthesis, that is the task of deriving correct-by-construction implementations from formal specifications, has the potential to eliminate the need for the manual—and error-prone—programming task. The synthesis problem can be formulated as an infinite two-player game, where the system player has the objective to satisfy the specification against all possible actions of the environment player. The standard synthesis algorithms represent the underlying synthesis game explicitly and, thus, they scale poorly with respect to the size of the specification. We provide an algorithmic framework to solve the synthesis problem symbolically. In contrast to the standard approaches, we use a succinct representation of the synthesis game which leads to improved scalability in terms of the symbolically represented parameters. Our algorithm reduces the synthesis game to the satisfiability problem of quantified Boolean formulas (QBF) and dependency quantified Boolean formulas (DQBF). In the encodings, we use propositional quantification to succinctly represent different parts of the implementation, such as the state space and the transition function. We develop highly optimized satisfiability algorithms for QBF and DQBF. Based on a counterexample-guided abstraction refinement (CEGAR) loop, our algorithms avoid an exponential blow-up by using the structure of the underlying symbolic encodings. Further, we extend the solving algorithms to extract certificates in the form of Boolean functions, from which we construct implementations for the synthesis problem. Our empirical evaluation shows that our symbolic approach significantly outperforms previous explicit synthesis algorithms with respect to scalability and solution quality.In dieser Dissertation werden symbolische Algorithmen für die Synthese von reaktiven Systemen entwickelt. Synthese, d.h. die Aufgabe, aus formalen Spezifikationen korrekte Implementierungen abzuleiten, hat das Potenzial, die manuelle und fehleranfällige Programmierung überflüssig zu machen. Das Syntheseproblem kann als unendliches Zweispielerspiel verstanden werden, bei dem der Systemspieler das Ziel hat, die Spezifikation gegen alle möglichen Handlungen des Umgebungsspielers zu erfüllen. Die Standardsynthesealgorithmen stellen das zugrunde liegende Synthesespiel explizit dar und skalieren daher schlecht in Bezug auf die Größe der Spezifikation. Diese Arbeit präsentiert einen algorithmischen Ansatz, der das Syntheseproblem symbolisch löst. Im Gegensatz zu den Standardansätzen wird eine kompakte Darstellung des Synthesespiels verwendet, die zu einer verbesserten Skalierbarkeit der symbolisch dargestellten Parameter führt. Der Algorithmus reduziert das Synthesespiel auf das Erfüllbarkeitsproblem von quantifizierten booleschen Formeln (QBF) und abhängigkeitsquantifizierten booleschen Formeln (DQBF). In den Kodierungen verwenden wir propositionale Quantifizierung, um verschiedene Teile der Implementierung, wie den Zustandsraum und die Übergangsfunktion, kompakt darzustellen. Wir entwickeln hochoptimierte Erfüllbarkeitsalgorithmen für QBF und DQBF. Basierend auf einer gegenbeispielgeführten Abstraktionsverfeinerungsschleife (CEGAR) vermeiden diese Algorithmen ein exponentielles Blow-up, indem sie die Struktur der zugrunde liegenden symbolischen Kodierungen verwenden. Weiterhin werden die Lösungsalgorithmen um Zertifikate in Form von booleschen Funktionen erweitert, aus denen Implementierungen für das Syntheseproblem abgeleitet werden. Unsere empirische Auswertung zeigt, dass unser symbolischer Ansatz die bisherigen expliziten Synthesealgorithmen in Bezug auf Skalierbarkeit und Lösungsqualität deutlich übertrifft

Risk management in Norwegian avalanche rescue operations. Managing uncertainty, complexity, overcommitment and the long-term monitoring of accident risk

PhD thesis in Risk management and societal safetyIntroduction: Avalanche incidents commonly take place in adverse environmental conditions, and the expected survival time of avalanche victims is short. These situations require an immediate rescue response, which may pose a serious challenge to the safety of both rescuers and avalanche victims. Historically, the Norwegian rescue service has experienced few serious accidents, but undesirable incidents where rescuers are dangerously exposed in avalanche runout zones seem more frequent. Risk management in the avalanche rescue service is multifaceted, influenced by its multi-organizational structure. Individuals acting in this socio-technical rescue system are easily caught between two imperatives: saving lives and staying alive. The aim of risk management is to maintain equilibrium in rescue commitment. This project analysed whether the Norwegian avalanche rescue system is correctly balanced to withstand the extra load of common risk influencing factors in rescue operations. Aim: The fundamental aim of this thesis was to contribute to new knowledge on factors that are important for risk management and performance in the Norwegian avalanche rescue service. Methods: Mixed methods research was applied to answer the specific research questions. This implied multiple research activities in a combination of quantitative and qualitative methodologies. Study number 1 was a retrospective study to characterize Norwegian avalanche incidents and rescue response (Paper I). A comprehensive study comprising avalanche rescue statistics, cross-case analysis, factor analysis and risk modelling was conducted to gain insight into avalanche rescue performance (Paper II). In a phenomenological study to explore the concept of overcommitment, nine air ambulance crews from five bases took part in focus group interviews (Papers III and IV). Lastly, a systemic safety analysis was conducted in two separate seminars, supported by the insight of six experts in Norwegian avalanche rescue operations (Paper V). The thesis itself is a cross-paper synthesis of results. Results: The studies returned results which contribute to justified beliefs about patient and rescuer safety in Norwegian avalanche rescue operations. Conclusion: A synthesis of results from the various studies indicates that the Norwegian rescue service is vulnerable to common risk sources in rescue operations, affecting the safety of both rescuers and patients. The avalanche rescue system could benefit from a focus on the integrity of already established safety barriers. This implies an interorganizational effort to identify and reach common goals and system requirements. This thesis may serve as input to discussions on risk acceptance levels in the rescue service, the applicability and validity of control algorithms in rescue management and how to adjust the degree of commitment in various rescue missions

The metabolism and environmental fate of the cyanobacterial neurotoxin Beta-N-methylamino-L-alanine

The neurotoxic amino acid β-­‐N-­‐methylamino-L­alanine (BMAA)is present in environmentally ubiquitous cyanobacteria and bioaccumulates and biomagnifies within the environment. The implication of BMAA in the development of neurodegenerative disease has raised concerns over the potential risk of human exposure to this neurotoxin, and has focussed attention on identifying possible routes of exposure that include direct contact with cyanobacteria and the ingestion of BMAA-­‐containing plant and animal products

Protein Structure

Since the dawn of recorded history, and probably even before, men and women have been grasping at the mechanisms by which they themselves exist. Only relatively recently, did this grasp yield anything of substance, and only within the last several decades did the proteins play a pivotal role in this existence. In this expose on the topic of protein structure some of the current issues in this scientific field are discussed. The aim is that a non-expert can gain some appreciation for the intricacies involved, and in the current state of affairs. The expert meanwhile, we hope, can gain a deeper understanding of the topic