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  • 201.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Demissie, Bizuneh Minda
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Addis Ababa University, Ethiopia.
    Sinnamon, Gord
    University of Western Ontario, Canada.
    End-point norm estimates for Cesaro and Copson operators2023In: Annali di Matematica Pura ed Applicata, ISSN 0373-3114, E-ISSN 1618-1891Article in journal (Refereed)
    Abstract [en]

    For a large class of operators acting between weighted l(infinity) spaces, exact formulas are given for their norms and the norms of their restrictions to the cones of nonnegative sequences; nonnegative, nonincreasing sequences; and nonnegative, nondecreasing sequences. The weights involved are arbitrary nonnegative sequences and may differ in the domain and codomain spaces. The results are applied to the Cesaro and Copson operators, giving their norms and their distances to the identity operator on the whole space and on the cones. Simplifications of these formulas are derived in the case of these operators acting on power-weighted l(infinity). As an application, best constants are given for inequalities relating the weighted l(infinity) norms of the Cesaro and Copson operators both for general weights and for power weights.

  • 202.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Mathematics.
    Lind, Martin
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    A new variational characterization of Sobolev spaces2015In: Journal of Geometric Analysis, ISSN 1050-6926, E-ISSN 1559-002X, Vol. 25, no 4, p. 2185-2195Article in journal (Refereed)
    Abstract [en]

    We obtain a new variational characterization of the Sobolev space $W_p^1(\Omega)$ (where $\Omega\subseteq\R^n$ and $p>n$). This is a generalization of a classical result of F. Riesz. We also consider some related results.

  • 203.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Marcoci, Anca N.
    Department of Mathematics and Computer Science, Technical University of Civil Engineering Bucharest, Romania.
    Marcoci, Liviu G.
    Department of Mathematics and Computer Science, Technical University of Civil Engineering Bucharest, Romania.
    Factorizations of Weighted Hardy Inequalities2018In: Bulletin of the Brazilian Mathematical Society, ISSN 1678-7544, E-ISSN 1678-7714, Vol. 49, no 4, p. 915-932Article in journal (Refereed)
    Abstract [en]

    We present factorizations of weighted Lebesgue, Cesàro and Copson spaces, for weights satisfying the conditions which assure the boundedness of the Hardy’s integral operator between weighted Lebesgue spaces. Our results enhance, among other, the best known forms of weighted Hardy inequalities.

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  • 204.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Mathematics. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Marcoci, Anca Nicoleta
    Technical University of Civil Engineering Bucharest, Department of Mathematics and Computer Science, Bucharest, Romania.
    Marcoci, Liviu Gabriel
    Technical University of Civil Engineering Bucharest, Department of Mathematics and Computer Science, Bucharest, Romania.
    Persson, Lars-Erik
    Luleå University of Technology, Department of Mathematics.
    Optimal estimates in Lorentz spaces of sequences with an increasing weight2013In: Proceedings of the Romanian Academy: Series A, Mathematics, Physics, Technical Sciences, Information Science, ISSN 1454-9069, Vol. 14, no 1, p. 20-27Article in journal (Refereed)
  • 205.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Nikolova, Ljudmila
    Sofia University, Bulgaria.
    Persson, Lars-Erik
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Yimer, Markos Fisseha
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Addis Ababa University, Ethiopia.
    Some Hardy-type inequalities in Banach function spaces2021In: Mathematical Inequalities & Applications, ISSN 1331-4343, E-ISSN 1848-9966, Vol. 24, no 4, p. 1001-1002Article in journal (Refereed)
    Abstract [en]

    Some new inequalities of Hardy-type in Banach function space settings are proved anddiscussed. In particular, these results generalize and unify several classical Hardy-type inequalities. Some results are new also in the classical situation.

  • 206.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Mathematics. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Nikolova, Ludmila
    Sofia University, BGR.
    Carleson- and Hardy type inequalities in some Banach function spaces2019In: Nonlinear Studies, ISSN 1359-8678, Vol. 26, no 4, p. 755-766Article in journal (Refereed)
    Abstract [en]

    Some new inequalities of Carleson- and Hardy- type in Banach function space settings are proved and discussed. In particular, these theorems both generalize and unify several classical inequalities e.g. those by Hardy, Polya-Knopp, Carleman, Hardy-Knopp and Carleson. As applications some new inequalities are pointed out.

  • 207.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Pecaric, Josep
    Persson, Lars-Erik
    Carlson type inequalities1998In: Journal of inequalities and applications, ISSN 1025-5834, E-ISSN 1029-242X, Vol. 2, no 2, p. 121-135Article in journal (Refereed)
  • 208.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Persson, Lars-Erik
    Luleå tekniska universitet.
    Some new sharp limit Hardy-type inequalities via convexity2014In: Journal of inequalities and applications, ISSN 1025-5834, E-ISSN 1029-242X, no 6, p. 1-10Article in journal (Refereed)
    Abstract [en]

    Some new limit cases of Hardy-type inequalities are proved, discussed and compared. In particular, some new refinements of Bennett’s inequalities are proved. Each of these refined inequalities contain two constants, and both constants are in fact sharp. The technique in the proofs is new and based on some convexity arguments of independent interest.

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  • 209.
    Barza, Sorina
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Silvestre, Pilar
    Department of Mathematics and Systems Analysis, Aalto University, Finland.
    Functions of bounded second p-variation2014In: Revista Matemática Complutense, ISSN 1139-1138, E-ISSN 1988-2807, Vol. 27, p. 69-91Article in journal (Refereed)
    Abstract [en]

    The generalized functionals of Merentes type generate a scale of spaces connecting the class of functions of bounded second p -variation with the Sobolev space of functions with p-integrable second derivative. We prove some limiting relations for these functionals as well as sharp estimates in terms of the fractional modulus of order 2−1/p . These results extend the results in Lind (Math Inequal Appl 16:2139, 2013) for functions of bounded variation but are not consequence of the last.

  • 210. Bauschert, Thomas
    et al.
    D’Andreagiovanni, Fabio
    Kassler, Andreas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Wang, Chenghao
    A matheuristic for green and robust 5G virtual network function placement2019In: Applications of Evolutionary Computation / [ed] Paul Kaufmann, Pedro A. Castillo, Cham: Springer, 2019, p. 430-438Conference paper (Refereed)
    Abstract [en]

    We investigate the problem of optimally placing virtual network functions in 5G-based virtualized infrastructures according to a green paradigm that pursues energy-efficiency. This optimization problem can be modelled as an articulated 0-1 Linear Program based on a flow model. Since the problem can prove hard to be solved by a state-of-the-art optimization software, even for instances of moderate size, we propose a new fast matheuristic for its solution. Preliminary computational tests on a set of realistic instances return encouraging results, showing that our algorithm can find better solutions in considerably less time than a state-of-the-art solver.

  • 211.
    Bawack, Ransome Epie
    et al.
    University Capitole, Toulouse, FRA.
    Ahmad, Muhammad Ovais
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Understanding business analytics continuance in agile information system development projects: an expectation-confirmation perspective2021In: Information Technology and People, ISSN 0959-3845, E-ISSN 1758-5813, Vol. 34, no 6, p. 1551-1569Article in journal (Refereed)
    Abstract [en]

    Purpose: This paper seeks to examine how expectations from business analytics (BA) by members of agile information systems development (ISD) teams affect their perceptions and continuous use of BA in ISD projects. Design/methodology/approach: Data was collected from 153 respondents working in agile ISD projects and analysed using partial least squares structural equation modelling techniques (PLS-SEM). Findings: Perceived usefulness and technological compatibility are the most salient factors that affect BA continuance intention in agile ISD projects. The proposed model explains 48.4% of the variance for BA continuance intention, 50.6% of the variance in satisfaction, 36.7% of the variance in perceived usefulness and 31.9% of the variance in technological compatibility. Research limitations/implications: First, this study advances understanding of the factors that affect the continuous use of BA in agile ISD projects; second, it contextualizes the expectation-confirmation model by integrating technological compatibility in the context of agile ISD projects. Originality/value: This is the first study to investigate BA continuance intention from an employee perspective in the context of agile ISD projects.

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  • 212.
    Bayram, Firas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Towards Robust and Adaptive Machine Learning: A Fresh Perspective on Evaluation and Adaptation Methodologies in Non-Stationary Environments2023Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Machine learning (ML) has become ubiquitous in various disciplines and applications, serving as a powerful tool for developing predictive models to analyze diverse variables of interest. With the advent of the digital era, the proliferation of data has presented numerous opportunities for growth and expansion across various domains. However, along with these opportunities, there is a unique set of challenges that arises due to the dynamic and ever-changing nature of data. These challenges include concept drift, which refers to shifting data distributions over time, and other data-related issues that can be framed as learning problems. Traditional static models are inadequate in handling these issues, underscoring the need for novel approaches to enhance the performance robustness and reliability of ML models to effectively navigate the inherent non-stationarity in the online world. The field of concept drift is characterized by several intricate aspects that challenge learning algorithms, including the analysis of model performance, which requires evaluating and understanding how the ML model's predictive capability is affected by different problem settings. Additionally, determining the magnitude of drift necessary for change detection is an indispensable task, as it involves identifying substantial shifts in data distributions. Moreover, the integration of adaptive methodologies is essential for updating ML models in response to data dynamics, enabling them to maintain their effectiveness and reliability in evolving environments. In light of the significance and complexity of the topic, this dissertation offers a fresh perspective on the performance robustness and adaptivity of ML models in non-stationary environments. The main contributions of this research include exploring and organizing the literature, analyzing the performance of ML models in the presence of different types of drift, and proposing innovative methodologies for drift detection and adaptation that solve real-world problems. By addressing these challenges, this research paves the way for the development of more robust and adaptive ML solutions capable of thriving in dynamic and evolving data landscapes.

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  • 213.
    Bayram, Firas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Ahmed, Bestoun S.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Czech Technical University in Prague, Czech Republic.
    A domain-region based evaluation of ML performance robustness to covariate shift2023In: Neural Computing & Applications, ISSN 0941-0643, E-ISSN 1433-3058, Vol. 35, no 24, p. 17555-17577Article in journal (Refereed)
    Abstract [en]

    Most machine learning methods assume that the input data distribution is the same in the training and testing phases.However, in practice, this stationarity is usually not met and the distribution of inputs differs, leading to unexpectedperformance of the learned model in deployment. The issue in which the training and test data inputs follow differentprobability distributions while the input–output relationship remains unchanged is referred to as covariate shift. In thispaper, the performance of conventional machine learning models was experimentally evaluated in the presence of covariateshift. Furthermore, a region-based evaluation was performed by decomposing the domain of probability density function ofthe input data to assess the classifier’s performance per domain region. Distributional changes were simulated in a twodimensional classification problem. Subsequently, a higher four-dimensional experiments were conducted. Based on theexperimental analysis, the Random Forests algorithm is the most robust classifier in the two-dimensional case, showing thelowest degradation rate for accuracy and F1-score metrics, with a range between 0.1% and 2.08%. Moreover, the resultsreveal that in higher-dimensional experiments, the performance of the models is predominantly influenced by the complexity of the classification function, leading to degradation rates exceeding 25% in most cases. It is also concluded that themodels exhibit high bias toward the region with high density in the input space domain of the training samples.

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  • 214.
    Bayram, Firas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Ahmed, Bestoun S.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Hallin, ERIK
    Uddeholms AB, Värmlands län.
    Engman, Anton
    Uddeholms AB, Värmlands län.
    A Drift Handling Approach for Self-Adaptive ML Software in Scalable Industrial Processes2022In: Proceedings of the 37th IEEE/ACM International Conference on Automated Software Engineering / [ed] Mario Aehnelt and Thomas Kirste, Association for Computing Machinery (ACM), 2022, p. 1-5, article id 129Conference paper (Refereed)
    Abstract [en]

    Most industrial processes in real-world manufacturing applications are characterized by the scalability property, which requires an automated strategy to self-adapt machine learning (ML) software systems to the new conditions. In this paper, we investigate an Electroslag Remelting (ESR) use case process from the Uddeholms AB steel company. The use case involves predicting the minimum pressure value for a vacuum pumping event. Taking into account the long time required to collect new records and efficiently integrate the new machines with the built ML software system. Additionally, to accommodate the changes and satisfy the non-functional requirement of the software system, namely adaptability, we propose an automated and adaptive approach based on a drift handling technique called importance weighting. The aim is to address the problem of adding a new furnace to production and enable the adaptability attribute of the ML software. The overall results demonstrate the improvements in ML software performance achieved by implementing the proposed approach over the classical non-adaptive approach. 

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  • 215.
    Bayram, Firas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Ahmed, Bestoun S.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Hallin, Erik
    Uddeholms AB, Sweden.
    Engman, Anton
    Uddeholms AB, Sweden.
    DQSOps: Data Quality Scoring Operations Framework for Data-Driven Applications2023In: EASE '23: Proceedings of the 27th International Conference on Evaluation and Assessment in Software Engineering, Association for Computing Machinery (ACM), 2023, p. 32-41Conference paper (Refereed)
    Abstract [en]

    Data quality assessment has become a prominent component in the successful execution of complex data-driven artificial intelligence (AI) software systems. In practice, real-world applications generate huge volumes of data at speeds. These data streams require analysis and preprocessing before being permanently stored or used in a learning task. Therefore, significant attention has been paid to the systematic management and construction of high-quality datasets. Nevertheless, managing voluminous and high-velocity data streams is usually performed manually (i.e. offline), making it an impractical strategy in production environments. To address this challenge, DataOps has emerged to achieve life-cycle automation of data processes using DevOps principles. However, determining the data quality based on a fitness scale constitutes a complex task within the framework of DataOps. This paper presents a novel Data Quality Scoring Operations (DQSOps) framework that yields a quality score for production data in DataOps workflows. The framework incorporates two scoring approaches, an ML prediction-based approach that predicts the data quality score and a standard-based approach that periodically produces the ground-truth scores based on assessing several data quality dimensions. We deploy the DQSOps framework in a real-world industrial use case. The results show that DQSOps achieves significant computational speedup rates compared to the conventional approach of data quality scoring while maintaining high prediction performance.

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  • 216.
    Bayram, Firas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Ahmed, Bestoun S.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Kassler, Andreas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    From concept drift to model degradation: An overview on performance-aware drift detectors2022In: Knowledge-Based Systems, ISSN 0950-7051, E-ISSN 1872-7409, Vol. 245, article id 108632Article, review/survey (Refereed)
    Abstract [en]

    The dynamicity of real-world systems poses a significant challenge to deployed predictive machine learning (ML) models. Changes in the system on which the ML model has been trained may lead to performance degradation during the system’s life cycle. Recent advances that study non-stationary environments have mainly focused on identifying and addressing such changes caused by a phenomenon called concept drift. Different terms have been used in the literature to refer to the same type of concept drift and the same term for various types. This lack of unified terminology is set out to create confusion on distinguishing between different concept drift variants. In this paper, we start by grouping concept drift types by their mathematical definitions and survey the different terms used in the literature to build a consolidated taxonomy of the field. We also review and classify performance-based concept drift detection methods proposed in the last decade. These methods utilize the predictive model’s performance degradation to signal substantial changes in the systems. The classification is outlined in a hierarchical diagram to provide an orderly navigation between the methods. We present a comprehensive analysis of the main attributes and strategies for tracking and evaluating the model’s performance in the predictive system. The paper concludes by discussing open research challenges and possible research directions.

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  • 217.
    Bayram, Firas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Aupke, Phil
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Ahmed, Bestoun S.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Kassler, Andreas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Theocharis, Andreas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics (from 2013).
    Forsman, Jonas
    CGI, Karlstad, Sweden.
    DA-LSTM: A dynamic drift-adaptive learning framework for interval load forecasting with LSTM networks2023In: Engineering applications of artificial intelligence, ISSN 0952-1976, E-ISSN 1873-6769, Vol. 123, article id 106480Article in journal (Refereed)
    Abstract [en]

    Load forecasting is a crucial topic in energy management systems (EMS) due to its vital role in optimizing energy scheduling and enabling more flexible and intelligent power grid systems. As a result, these systems allow power utility companies to respond promptly to demands in the electricity market. Deep learning (DL) models have been commonly employed in load forecasting problems supported by adaptation mechanisms to cope with the changing pattern of consumption by customers, known as concept drift. A drift magnitude threshold should be defined to design change detection methods to identify drifts. While the drift magnitude in load forecasting problems can vary significantly over time, existing literature often assumes a fixed drift magnitude threshold, which should be dynamically adjusted rather than fixed during system evolution. To address this gap, in this paper, we propose a dynamic drift-adaptive Long Short-Term Memory (DA-LSTM) framework that can improve the performance of load forecasting models without requiring a drift threshold setting. We integrate several strategies into the framework based on active and passive adaptation approaches. To evaluate DA-LSTM in real-life settings, we thoroughly analyze the proposed framework and deploy it in a real-world problem through a cloud-based environment. Efficiency is evaluated in terms of the prediction performance of each approach and computational cost. The experiments show performance improvements on multiple evaluation metrics achieved by our framework compared to baseline methods from the literature. Finally, we present a trade-off analysis between prediction performance and computational costs.

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  • 218.
    Beckerle, Matthias
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Magnusson, Jonathan
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Pulls, Tobias
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Splitting Hairs and Network Traces: Improved Attacks Against Traffic Splitting as a Website Fingerprinting Defense2022In: WPES 2022: Proceedings of the 21st Workshop on Privacy in the Electronic Society / [ed] Yuan Hong; Lingyu Wang, Association for Computing Machinery (ACM), 2022, p. 15-27Conference paper (Refereed)
    Abstract [en]

    The widespread use of encryption and anonymization technologies - -e.g., HTTPS, VPNs, Tor, and iCloud Private Relay - -makes network attackers likely to resort to traffic analysis to learn of client activity. For web traffic, such analysis of encrypted traffic is referred to as Website Fingerprinting (WF). WF attacks have improved greatly in large parts thanks to advancements in Deep Learning (DL). In 2019, a new category of defenses was proposed: traffic splitting, where traffic from the client is split over two or more network paths with the assumption that some paths are unobservable by the attacker. In this paper, we take a look at three recently proposed defenses based on traffic splitting: HyWF, CoMPS, and TrafficSliver BWR5. We analyze real-world and simulated datasets for all three defenses to better understand their splitting strategies and effectiveness as defenses. Using our improved DL attack Maturesc on real-world datasets, we improve the classification accuracy wrt. state-of-the-art from 49.2% to 66.7% for HyWF, the F1 score from 32.9% to 72.4% for CoMPS, and the accuracy from 8.07% to 53.8% for TrafficSliver BWR5. We find that a majority of wrongly classified traces contain less than a couple hundred of packets/cells: e.g., in every dataset 25% of traces contain less than 155 packets. What cannot be observed cannot be classified. Our results show that the proposed traffic splitting defenses on average provide less protection against WF attacks than simply randomly selecting one path and sending all traffic over that path.

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  • 219.
    Beckerle, Matthias
    et al.
    Technische Universität Darmstadt,, Germany.
    Martucci, Leonardo
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Formal Definitions for Usable Access Control Rule Sets: From Goals to Metrics2013In: Proceedings of the Ninth Symposium on Usable Privacy and Security, Association for Computing Machinery (ACM), 2013, p. 2:1-2:11Conference paper (Refereed)
    Abstract [en]

    Access control policies describe high level requirements for access control systems. Access control rule sets ideally trans-late these policies into a coherent and manageable collectionof Allow/Deny rules. Designing rule sets that reflect desired policies is a difficult and time-consuming task. The result is that rule sets are difficult to understand and manage. The goal of this paper is to provide means for obtaining usable access control rule sets, which we define as rule sets that (i) reflect the access control policy and (ii) are easy to understand and manage. In this paper, we formally define the challenges that users face when generating usable access control rule sets and provide formal tools to handle them more easily. We started our research with a pilot study in which specialists were interviewed. The objective was to list usability challenges regarding the management of access control rule sets and verify how those challenges were handled by specialists. The results of the pilot study were compared and combined with results from related work and refined into six novel, formally defined metrics that are used to measure the security and usability aspects of access control rule sets. We validated our findings with two user studies, which demonstrate that our metrics help users generate statistically significant better rule sets.

  • 220.
    Beckman, Claes
    et al.
    KTH, Center for Wireless Systems.
    Garcia, Johan
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Alfredsson, Stefan
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Brunström, Anna
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    On the Impact of Velocity on the Train-to-Earth MIMO Propagation Channel: Statistical Observations and Qualitative Analysis2017In: 2017 IEEE International Symposium on Antennas and Propagation & USNC/URSI National Radio Science Meeting, IEEE, 2017, p. 1865-1866Conference paper (Refereed)
    Abstract [en]

    We provide measured data collected from 97 trains completing over 7000 journeys in Sweden showing that the throughput over LTE is impacted by train velocity. In order to explain these observations we assume that the underlying causes can be found in the implementation of the MIMO system into LTE Rel. 8 and the diffuse scattering of signals from ground reflections.

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  • 221. Beentjes, Casper
    et al.
    Di Bucchianico, Alessandro
    Hamster, Christian
    Kadu, Ajinkya
    Man, Irene
    Myerscough, Keith
    Regis, Marta
    Richardson, Omar
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Equalizing the Cost of Health Insurance2017In: Proceedings of the 126th European Study GroupMathematics with Industry / [ed] Daan Crommelin, Stella Kapodistria, Guus Regts, Chris Stolk, Peter van de Ven, SWI , 2017, p. 29-49Conference paper (Other academic)
    Abstract [en]

    The Dutch government compensates health insurance companies when insuringindividuals who are estimated to have high health care costs. This is necessaryto avoid insurers not offering services to certain groups or not providing themwith a high quality of service. It is, however, unknown to what extent thedifferences in health care expenses by different groups of people are truly due toa poorer or better health status. We explore several statistical approaches thatfacilitate explaining the cause of these differences.

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  • 222.
    Behrenfors, Louise
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Norlén, Kevin
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Progressiv webbapplikation med bildigenkänning2020Independent thesis Basic level (professional degree), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    More and more people in Sweden have access to both the Internet and smartphones to a greater extent than ever before. There is an application for almost everything, such as training-, games- and news applications. Based on this and the increasing interest for the environment in general and recycling, the goal was set for this project. A progressive web application with the use of image recognition with the help of machine learning that will make it easier for people to sort and recycle their household trash.

    The final result is an application that can be run equally on most devices (mobile, PC, tablets, etc.), regardless of model or operating system. The application is used to identify household waste through an image recognition service. This report addresses the technologies used to build the application and implementation process of it. Finally we discuss the results of the development and some selected people got to test the app and answer some questions and voice their opinions.

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  • 223.
    Beilina, Larissa
    et al.
    Chalmers University, Gothenburg University.
    Shestopalov, YuriKarlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Inverse problems and large-scale computations2013Conference proceedings (editor) (Refereed)
  • 224.
    Berdén, Daniel
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Traxler, Johan
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Low-code-plattformar: En översikt: Undersökning via applikationsutveckling2021Independent thesis Basic level (university diploma), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Development of digital solutions that seamlessly integrate and display relevant informationhas become one of the most important tools for many companies. Low-codeplatforms have been created to simplify this work. The question is if these platformssucceed in this goal and to what extent. This paper has as its goal to process this questionand arrive at a guiding answer. Initially, the low-code concept was examined and thenthree different platforms were selected. These platforms were: OutSystems, MicrosoftPower Apps, and Mendix. To investigate and evaluate these platforms, a demo applicationwas designed and subsequently implemented in each of the platforms. The resultwas a description of the development work and an evaluation of the various platforms. Inthe evaluation of the development work, it was concluded that the investigated low-codeplatforms contribute to smoother development with a focus on user interfaces, but thatdespite their simpler systems, they still require a certain development habit for efficientuse. Proposals for future work in the area and with the work presented in the report aredescribed.

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  • 225.
    Bergenudd, Anton
    Karlstad University, Faculty of Economic Sciences, Communication and IT, Department of Computer Science. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Improving information gathering for IT experts.: Combining text summarization and individualized information recommendation.2022Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Information gathering and information overload is an ever growing topic of concernfor Information Technology (IT) experts. The amount of information dealt withon an everyday basis is large enough to take up valuable time having to scatterthrough it all to find the relevant information. As for the application area of IT,time is directly related to money as having to waste valuable production time ininformation gathering and allocation of human resources is a direct loss of profitsfor any given company. Two issues are mainly addressed through this thesis: textsare too lengthy and the difficulty of finding relevant information. Through the useof Natural Language Processes (NLP) methods such as topic modelling and textsummarization, a proposed solution is constructed in the form of a technical basiswhich can be implemented in most business areas. An experiment along with anevaluation session is setup in order to evaluate the performance of the technical basisand enforce the focus of this paper, namely ”How effective is text summarizationcombined with individualized information recommendation in improving informationgathering of IT experts?”. Furthermore, the solution includes a construction of userprofiles in an attempt to individualize content and theoretically present more relevantinformation. The results for this project are affected by the substandard quality andmagnitude of data points, however positive trends are discovered. It is stated thatthe use of user profiles further enhances the amount of relevant articles presentedby the model along with the increasing recall and precision values per iteration andaccuracy per number of updates made per user. Not enough time is spent as for theextent of the evaluation process to confidently state the validity of the results morethan them being inconsistent and insufficient in magnitude. However, the positivetrends discovered creates further speculations on if the project is given enough timeand resources to reach its full potential. Essentially, one can theoretically improveinformation gathering by summarizing texts combined with individualization.

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  • 226.
    Berglind, Raymond
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Larsson, Louise
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Mobile SIF-application: Cross-platform development of a Xamarin application utilizing data from a SharePoint intranet solution2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Xamarin, a cross-platform development framework, was used in this project for the development of a proof-of-concept mobile client application for an existing intranet solution. The intranet consists of Microsoft SharePoint components and is a pre-assembled product produced by the requestor of this project; Sogeti. To fetch the intranet’s data, a web API was implemented as an intermediary between the mobile application and the intranet. The Xamarin application targets three different mobile platforms including iOS, Android and Universal Windows Platform, and since Xamarin allows for shared logic between different operating systems, this approach was adopted to the greatest extent possible. However, during the development of the project several problems were encountered that prompted platform-specific implementations. For example, the user authentication process needed to be implemented in a platform-specific manner in the client application and some parts of the graphical user interface needed to be adapted to each individual platform as well. Even though the development of the applications was not completely finished, four out of seven of the original requirements set forth by Sogeti at the beginning of the project were achieved. These fulfilled requirements include, among other functionality, the implementation of displaying news articles and attention messages from a user’s specific intranet solution. Overall, the project was found to be successful, especially with regard to evaluating how well Xamarin and cross-platform development works with these kinds of mobile applications.

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  • 227.
    Berglund, Simon
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Digital skyltning på Androidenheter: Ett Android integrationsprojekt2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [sv]

    Digital skyltning är en speciell form av skyltning där information, bilder, videoklipp eller liknande media visas på digitala skärmar. Skyltningen sker vanligtvis på LCD eller LED skärmar och placeras vanligtvis på antingen offentliga platser eller på arbetsplatser.

    Projektet går ut på att expandera en färdig digital-skyltningsmjukvara där koden huvudsakligen är skriven i HTML5 och JavaScript genom att göra den exekverbar och anpassad för operativsystemet Android.

    Resultatet av projektet gjorde mjukvaran anpassad samt exekverbar på Android. Verktyget Cordova användes för att göra webb-koden exekverbar på Android. Anpassningen gäller huvudsakligen att ge mjukvaran tillgång till Androidenhetens funktioner och resurser som exempelvis filsystemet och skärmdumps-funktionen.

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  • 228.
    Bergman, Andreas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Pieskä, Marcus
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Westlinder, Simon
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Rust, Josefine
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Grinnemo, Karl-Johan
    Karlstad University, Faculty of Economic Sciences, Communication and IT, Department of Computer Science. Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Socket Intents Extended for SCTP: Extended Version of Socket Intents to Use the Transport Protocol SCTP2016Report (Other (popular science, discussion, etc.))
    Abstract [en]

    This report covers a project in the course Computer Engineering Project, DVAE08, at Karlstad University. The aim of the project was to modify an already existing solution for selecting the most fitting path for known traffic online, with a proactive approach instead of a reactive, called Socket Intents. The purpose of the modified version is to make the previous solution compatible with the transport protocol SCTP. This solution consists of three new implemented components; a header parser, a sniffer, and a query manager. The header parser and sniffer receive packets from the traffic and send them to one another. The query manager handles queries from the policies to the sniffer, as well as the response. Together, these components will gather information about the state of the network, and select the most fitting path that fulfill application needs. The results achieved from the modification are good work for the SCTP one-to-one type. 

  • 229.
    Bergqvist, Tomas
    et al.
    Umeå universitet.
    Liljekvist, Yvonne
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    van Bommel, Jorryt
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Österholm, Magnus
    Umeå universitet.
    Evaluation of a large scale professional development program2017In: Proceedings of the 41st Conference of the International Group for the Psychology of Mathematics Education: Volume 2 / [ed] B. Kaur, W.K. Ho, T.L Toh, B.H. Choy, Singapore: PME , 2017, p. 153-160Conference paper (Refereed)
    Abstract [en]

    This paper reports on an evaluation of the professional development program Boost for Mathematics in Sweden. 200 mathematics lessons were visited, and the teachers were interviewed after each lesson.The analysis used an analytical framework based on Lithner et al. (2010).The findings indicate that the PD-program has had a significant impact on the teachers’ knowledge about the mathematical competencies as they are presented in the national curriculum documents, and that the teaching practice has improved and give the students better possibilities todevelop the competencies.The results also show that these improvements are still present one year after the program had ended.

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  • 230. Bergqvist, Tomas
    et al.
    Österholm, Magnus
    van Bommel, Jorryt
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Karlstad University, Faculty of Health, Science and Technology (starting 2013), Science, Mathematics and Engineering Education Research.
    Liljekvist, Yvonne
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Karlstad University, Faculty of Arts and Social Sciences (starting 2013), Department of Educational Studies (from 2013). Karlstad University, Faculty of Health, Science and Technology (starting 2013), Science, Mathematics and Engineering Education Research.
    Resultat av utvärderingen av Matematiklyftet angående undervisning och fortbildning2018Conference paper (Other (popular science, discussion, etc.))
  • 231.
    Bergwall, Andreas
    et al.
    Örebro University, Sweden.
    Mellroth, Elisabet
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Karlstad University, Faculty of Health, Science and Technology (starting 2013), Science, Mathematics and Engineering Education Research.
    Teachers’ choice of a challenging task through collaborative learning2021In: Teachers’ choice of a challenging task through collaborative learning, Svensk förening för MatematikDidaktisk Forskning - SMDF, 2021Conference paper (Refereed)
    Abstract [en]

    Challenging mathematical tasks are important for all students’ learning processes, and the demanding job of finding and developing such tasks is preferably done through teacher collaboration. Using cultural-historical activity theory, we analyze three upper secondary teachers’ collaborative learning process in choosing and rejecting tasks for a collection of challenging tasks they have agreed to develop. They collaboratively chose one task to fulfil the criteria of challenging tasks; one task was rejected as it did not fulfil the criteria, and another was temporarily rejected as not all the teachers knew how to solve it themselves. The analysis revealed a positive and open atmosphere among the teachers, with content-focused discussion highlighting mathematical content and teaching related to the tasks discussed. While their discussion showed several signs of collaborative learning, whether their work will result in changes to teaching practice remains to be explored.

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    Teachers' choice of a challenging task through collaborative learning.
  • 232.
    Bergwall, Andreas
    et al.
    Örebro universitet.
    Mellroth, Elisabet
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Karlstad University, Faculty of Health, Science and Technology (starting 2013), Science, Mathematics and Engineering Education Research. Örebro universitet.
    Jansson, Torbjörn
    Tullängsgymnasiet, Örebro kommun.
    Nordin, Johan
    Tullängsgymnasiet, Örebro kommun.
    Teachers’ characterizations of challenging introductory and enrichment tasks2022In: Proceedings of the Twelfth Congress of  European Research Society in Mathematics Education (CERME12) / [ed] J. Hodgen; E. Geraniou; G. Bolondi; F. Ferretti, European Society for Research in Mathematics Education, 2022Conference paper (Refereed)
    Abstract [en]

    Developing tasks for use in mixed-ability classrooms presents teachers with several dilemmas. By making one such dilemma an explicit object of inquiry, this study aims to capture characteristics for challenging tasks suitable for introduction or enrichment. It is based on eight teachers’ collaborative and retrospective analysis of challenging tasks developed in a combined research and school development project. Among the results are the observation that introductory tasks should have an easy entry level and not require pre-knowledge of the upcoming concept, while an enrichment task should require relatively deep conceptual pre-knowledge. It is suggested that attention to seemingly contradictory features of introductory and enrichment tasks can fuel collaborative learning processes so that they include several important aspects of tasks aimed at challenging all students. Teachers’ verbalization of task characteristics is one outcome of such a process.

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    Teachers’ characterizations of challenging introductory and enrichment tasks
  • 233. Bernhard, David
    et al.
    Kulyk, Oksana
    Volkamer, Melanie
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Security Proofs for Participation Privacy and Stronger Verifiability for Helios2016Report (Other academic)
  • 234.
    Bernhard, David
    et al.
    University of Bristol, UK.
    Oksana, Kulyk
    Technische Universität Darmstadt, Germany.
    Volkamer, Melanie
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013). Technische Universität Darmstadt, Germany.
    Security proofs for Participation privacy, receipt-freeness and ballot privacy for the helios voting scheme2017In: ARES '17 Proceedings of the 12th International Conference on Availability, Reliability and Security, New York: Association for Computing Machinery (ACM), 2017, article id UNSP 1Conference paper (Refereed)
    Abstract [en]

    The Helios voting scheme is well studied including formal proofs for verifiability and ballot privacy. However, depending on its version, the scheme provides either participation privacy (hiding who participated in the election) or verifiability against malicious bulletin board (preventing election manipulation by ballot stuffing), but not both at the same time. It also does not provide receipt-freeness, thus enabling vote buying by letting the voters construct receipts proving how they voted. Recently, an extension to Helios, further referred to as KTV-Helios, has been proposed that claims to provide these additional security properties. However, the authors of KTV-Helios did not prove their claims. Our contribution is to provide formal definitions for participation privacy and receipt-freeness that we applied to KTV-Helios. In order to evaluate the fulfillment of participation privacy and receipt-freeness, we furthermore applied the existing definition of ballot privacy, which was also used for evaluating the security of Helios, in order to show that ballot privacy also holds for KTV-Helios

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  • 235.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Boundary Layers and Shock Profiles for the Broadwell Model2016In: International Journal of Differential Equations, ISSN 1687-9643, E-ISSN 1687-9651, Vol. 2016, p. 1-8, article id 5801728Article in journal (Refereed)
    Abstract [en]

    We consider the existence of nonlinear boundary layers and the typically nonlinear problem of existence of shock profiles for the Broadwell model, which is a simplified discrete velocity model for the Boltzmann equation. We find explicit expressions for the nonlinear boundary layers and the shock profiles. In spite of the few velocities used for the Broadwell model, the solutions are (at least partly) in qualitatively good agreement with the results for the discrete Boltzmann equation, that is the general discrete velocity model, and the full Boltzmann equation.

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  • 236.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Boundary layers for discrete kinetic models: Multicomponent mixtures, polyatomic molecules, bimolecular reactions, and quantum kinetic equations2017In: Kinetic and Related Models, ISSN 1937-5093, E-ISSN 1937-5077, Vol. 10, no 4, p. 925-955Article in journal (Refereed)
    Abstract [en]

    We consider some extensions of the classical discrete Boltzmann equation to the cases of multicomponent mixtures, polyatomic molecules (with a finite number of different internal energies), and chemical reactions, but also general discrete quantum kinetic Boltzmann-like equations; discrete versions of the Nordheim-Boltzmann (or Uehling-Uhlenbeck) equation for bosons and fermions and a kinetic equation for excitations in a Bose gas interacting witha Bose-Einstein condensate. In each case we have an H-theorem and so for the planar stationary half-space problem, we have convergence to an equilibrium distribution at infinity (or at least a manifold of equilibrium distributions). In particular, we consider the nonlinear half-space problem of condensation and evaporation for these discrete Boltzmann-like equations. We assume that the flow tends to a stationary point at infinity and that the outgoing flow is known at the wall, maybe also partly linearly depending on the incoming flow. We find that the systems we obtain are of similar structures as for the classical discrete Boltzmann equation (for single species), and that previously obtained results for the discrete Boltzmann equation can be applied after being generalized. Then the number of conditions on the assigned data at the wall needed for existence of a unique solution is found. The number of parameters to be specified in the boundary conditions depends on if we have subsonic or supersonic condensation or evaporation. All our results are valid for any finite number of velocities.

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  • 237.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Compactness Property of the Linearized Boltzmann Collision Operator for a Mixture of Monatomic and Polyatomic Species2024In: Journal of statistical physics, ISSN 0022-4715, E-ISSN 1572-9613, Vol. 191, no 3, article id 32Article in journal (Refereed)
    Abstract [en]

    The linearized Boltzmann collision operator has a central role in many important applications of the Boltzmann equation. Recently some important classical properties of the linearized collision operator for monatomic single species were extended to multicomponent monatomic gases and polyatomic single species. For multicomponent polyatomic gases, the case where the polyatomicity is modelled by a discrete internal energy variable was considered lately. Here we consider the corresponding case for a continuous internal energy variable. Compactness results, stating that the linearized operator can be decomposed into a sum of a positive multiplication operator, the collision frequency, and a compact operator, bringing e.g., self-adjointness, is extended from the classical result for monatomic single species, under reasonable assumptions on the collision kernel. With a probabilistic formulation of the collision operator as a starting point, the compactness property is shown by a decomposition, such that the terms are, or at least are uniform limits of, Hilbert-Schmidt integral operators and therefore are compact operators. Moreover, bounds on-including coercivity of-the collision frequency are obtained for hard sphere like, as well as hard potentials with cutoff like, models, from which Fredholmness of the linearized collision operator follows, as well as its domain.

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  • 238.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Discrete quantum Boltzmann equation2019In: AIP Conference Proceedings, American Institute of Physics (AIP), 2019, Vol. 2132, p. 1-9, article id 130011Conference paper (Refereed)
    Abstract [en]

    In this work, we consider a Boltzmann equation for anyons. In particular, we study a general discrete velocity model of the equation, where the velocity variable is assumed to only take values from a given finite-such that the (finite) number of velocities is arbitrary-set of velocities. Included, as two limiting cases, is the discrete quantum Boltzmann equation (Nordheim-Boltzmann/Uehling-Uhlenbeck equation) for bosons and fermions. Mass, momentum, and energy are assumed to be conserved during collisions, and considering suitable discrete velocity models, they will also be the only collision invariants. The equilibrium distributions will be given by a transcendental equation, and only in some few cases-including the two limiting cases where they are Planckians-they will be explicitly expressed. However, there is an H-theorem, and therefore one can prove that for the spatially homogeneous equation, as time tends to infinity, as well as, for the steady equation in a half-space with slab-symmetry, as the space variable tends to infinity, the distribution function converges to an equilibrium distribution. Linearizing around an equilibrium distribution in a suitable way, we find that the obtained linearized operator has similar properties as the corresponding linearized operator for the discrete Boltzmann equation: E.g. it is symmetric and positive semi-definite. Hence, previously obtained results for the spatially homogeneous Cauchy problem and the steady half-space problem in a slab symmetry for the discrete Boltzmann equation, can be applied also in the considered quantum case.

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  • 239.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Discrete Quantum Kinetic Equation2023In: La Matematica, E-ISSN 2730-9657, Vol. 2, no 4, p. 836-860Article in journal (Refereed)
    Abstract [en]

    A semi-classical approach to the study of the evolution of bosonic or fermionic excitations is through the Nordheim—Boltzmann- or, Uehling—Uhlenbeck—equation, also known as the quantum Boltzmann equation. In some low ranges of temperatures—e.g., in the presence of a Bose condensate—also other types of collision operators may render in essential contributions. Therefore, extended— or, even other—collision operators are to be considered as well. This work concerns a discretized version—a system of partial differential equations—of such a quantum equation with an extended collision operator. Trend to equilibrium is studied for a planar stationary system, as well as the spatially homogeneous system. Some essential properties of the linearized operator are proven, implying that results for general half-space problems for the discrete Boltzmann equation can be applied. A more general collision operator is also introduced, and similar results are obtained also for this general equation.

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  • 240.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Discrete velocity models for multicomponent mixtures and polyatomic molecules without nonphysical collision invariants and shock profiles2016In: 30th International Symposium on Rarefied Gas Dynamics: RGD 30 / [ed] Andrew Ketsdever, Henning Struchtrup, American Institute of Physics (AIP), 2016, p. 040005-1-040005-8, article id 040005Conference paper (Refereed)
    Abstract [en]

    An important aspect of constructing discrete velocity models (DVMs) for the Boltzmann equation is to obtain the right number of collision invariants. It is a well-known fact that, in difference to in the continuous case, DVMs can have extra collision invariants, so called spurious collision invariants, in plus to the physical ones. A DVM with only physical collision invariants, and so without spurious ones, is called normal. The construction of such normal DVMs has been studied a lot in the literature for single species as well as for binary mixtures. For binary mixtures also the concept of supernormal DVMs has been introduced by Bobylevand Vinerean. Supernormal DVMs are defined as normal DVMs such that both restrictions to the different species are normal as DVMs for single species.

    In this presentation we extend the concept of supernormal DVMs to the case of multicomponent mixtures and introduce it for polyatomic molecules. By polyatomic molecules we mean here that each molecule has one of a finite number of different internal energies, which can change, or not, during a collision. We will present some general algorithms for constructing such models, but also give some concrete examples of such constructions.

    The two different approaches above can be combined to obtain multicomponent mixtures with a finite number of different internal energies, and then be extended in a natural way to chemical reactions.

    The DVMs are constructed in such a way that we for the shock-wave problem obtain similar structures as for the classical discrete Boltzmann equation (DBE) for one species, and therefore will be able to apply previously obtained results for the DBE. In fact the DBE becomes a system of ordinary dierential equations (dynamical system) and the shock profiles can be seen as heteroclinic orbits connecting two singular points (Maxwellians). The previous results for the DBE then give us the existence of shock profiles for shock speeds close to a typical speed, corresponding to the sound speed in the continuous case. For binary mixtures this extension has already been addressed before by the author.

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  • 241.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Discrete Velocity Models for Polyatomic Molecules Without Nonphysical Collision Invariants2018In: Journal of statistical physics, ISSN 0022-4715, E-ISSN 1572-9613, Vol. 172, no 3, p. 742-761Article in journal (Refereed)
    Abstract [en]

    An important aspect of constructing discrete velocity models (DVMs) for the Boltzmann equation is to obtain the right number of collision invariants. Unlike for the Boltzmann equation, for DVMs there can appear extra collision invariants, so called spurious collision invariants, in plus to the physical ones. A DVM with only physical collision invariants, and hence, without spurious ones, is called normal. The construction of such normal DVMs has been studied a lot in the literature for single species, but also for binary mixtures and recently extensively for multicomponent mixtures. In this paper, we address ways of constructing normal DVMs for polyatomic molecules (here represented by that each molecule has an internal energy, to account for non-translational energies, which can change during collisions), under the assumption that the set of allowed internal energies are finite. We present general algorithms for constructing such models, but we also give concrete examples of such constructions. This approach can also be combined with similar constructions of multicomponent mixtures to obtain multicomponent mixtures with polyatomic molecules, which is also briefly outlined. Then also, chemical reactions can be added.

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  • 242.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Half-Space Problems for a Linearized Discrete Quantum Kinetic Equation2015In: Journal of statistical physics, ISSN 0022-4715, E-ISSN 1572-9613, Vol. 159, no 2, p. 358-379Article in journal (Refereed)
    Abstract [en]

    We study typical half-space problems of rarefied gas dynamics, including the problems of Milne and Kramer, for a general discrete model of a quantum kinetic equation for excitations in a Bose gas. In the discrete case the plane stationary quantum kinetic equation reduces to a system of ordinary differential equations. These systems are studied close to equilibrium and are proved to have the same structure as corresponding systems for the discrete Boltzmann equation. Then a classification of well-posed half-space problems for the homogeneous, as well as the inhomogeneous, linearized discrete kinetic equation can be made. The number of additional conditions that need to be imposed for well-posedness is given by some characteristic numbers. These characteristic numbers are calculated for discrete models axially symmetric with respect to the x-axis. When the characteristic numbers change is found in the discrete as well as the continuous case. As an illustration explicit solutions are found for a small-sized model.

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  • 243.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Half-Space Problems for the Boltzmann Equation of Multicomponent Mixtures2023In: From Kinetic Theory to Turbulence Modeling: The Legacy of Carlo Cercignani / [ed] Paolo Barbante, Francesco D. Belgiorno, Silvia Lorenzani, Lorenzo Valdettaro, Springer, 2023, Vol. 51, p. 45-57Conference paper (Refereed)
    Abstract [en]

    Half-space problems in the kinetic theory of gases are of great importance in the study of the asymptotic behavior of solutions of boundary value problems for the Boltzmann equation for small Knudsen numbers. They provide the boundary conditions for the fluid-dynamic-type equations and Knudsen-layer corrections to the solution of the fluid-dynamic-type equations in a neighborhood of the boundary. These problems are well studied for single species, including some important contributions by Carlo Cercignani, and it is well-known that the number of additional conditions needed to be imposed depends on different regimes for the Mach number (corresponding to subsonic/supersonic evaporation/condensation). However, the case of mixtures is not as well studied in the literature. We will address some extensions of the results for half-space problems for single species to the case of multicomponent mixtures. 

  • 244.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Linear half-space problems in kinetic theory: Abstract formulation and regime transitions2023In: International Journal of Mathematics, ISSN 0129-167X, E-ISSN 1793-6519, article id 2350091Article in journal (Refereed)
    Abstract [en]

    In this work, a general formulation, which is based on steady boundary layer problems for the Boltzmann equation, of a half-space problem is considered. The number of conditions on the indata at the interface needed to obtain well-posedness is investigated. The solutions will converge exponentially fast "far away" from the interface. For linearized kinetic half-space problems similar to the one of evaporation and condensation in kinetic theory, slowly varying modes might occur near regime transitions where the number of conditions needed to obtain well-posedness changes (corresponding to transition between evaporation and condensation, or subsonic and supersonic evaporation/condensation), preventing uniform exponential speed of convergence. However, those modes might be eliminated by imposing extra conditions on the indata at the interface. Flow velocities at the far end for which regime transitions occur are presented for Boltzmann equations: for monatomic and polyatomic single species and mixtures; as well as bosons and fermions.

  • 245.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Linearized Boltzmann Collision Operator: I. Polyatomic Molecules Modeled by a Discrete Internal Energy Variable and Multicomponent Mixtures2023In: Acta Applicandae Mathematicae - An International Survey Journal on Applying Mathematics and Mathematical Applications, ISSN 0167-8019, E-ISSN 1572-9036, Vol. 183, no 1, p. 1-45, article id 3Article in journal (Refereed)
    Abstract [en]

    The linearized Boltzmann collision operator appears in many important applications of the Boltzmann equation. Therefore, knowing its main properties is of great interest. This work extends some classical results for the linearized Boltzmann collision operator for monatomic single species to the case of polyatomic single species, while also reviewing corresponding results for multicomponent mixtures of monatomic species. The polyatomicity is modeled by a discrete internal energy variable, that can take a finite number of (given) different values. Results concerning the linearized Boltzmann collision operator being a nonnegative symmetric operator with a finite-dimensional kernel are reviewed. A compactness result, saying that the linearized operator can be decomposed into a sum of a positive multiplication operator, the collision frequency, and a compact operator, bringing e.g., self-adjointness, is extended from the classical result for monatomic single species, under reasonable assumptions on the collision kernel. With a probabilistic formulation of the collision operator as a starting point, the compactness property is shown by a splitting, such that the terms can be shown to be, or be the uniform limit of, Hilbert-Schmidt integral operators and as such being compact operators. Moreover, bounds on - including coercivity of - the collision frequency are obtained for a hard sphere like model, from which Fredholmness of the linearized collision operator follows, as well as its domain. 

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  • 246.
    Bernhoff, Niclas
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Linearized Boltzmann collision operator: II. Polyatomic molecules modeled by a continuous internal energy variable2023In: Kinetic and Related Models, ISSN 1937-5093, E-ISSN 1937-5077, Vol. 16, no 6, p. 828-849Article in journal (Refereed)
    Abstract [en]

    The linearized collision operator of the Boltzmann equation can in a natural way be written as a sum of a positive multiplication operator, the collision frequency, and an integral operator. Compactness of the integral operator for monatomic single species is a classical result, while corresponding results for mixtures and polyatomic single species where the polyatomicity is modeled by a discrete internal energy variable, are more recently obtained. In this work the compactness of the integral operator for polyatomic single species, for which the number of internal degrees of freedom is greater or equal to two and the polyatomicity is modeled by a continuous internal energy variable, is studied. Compactness of the integral operator is obtained by proving that its terms are, or, at least, can be approximated by, Hilbert-Schmidt integral operators, under some assumptions on the collision kernel. Self-adjointness of the linearized collision operator follows. Moreover, bounds on -including coercivity of -the collision frequency, are obtained for some particular collision kernels -corresponding to hard sphere like models, but also hard potential with cut-off like models. Then it follows that the linearized collision operator is a Fredholm operator.

  • 247.
    Bernhoff, Niclas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Golse, Francois
    Ecole Polytech, CMLS, FRA.
    On the Boundary Layer Equations with Phase Transition in the Kinetic Theory of Gases2021In: Archive for Rational Mechanics and Analysis, ISSN 0003-9527, E-ISSN 1432-0673, Vol. 240, no 1, p. 51-98Article in journal (Refereed)
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  • 248.
    Bernhoff, Niclas
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Vinerean, Mirela
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science (from 2013).
    Discrete Velocity Models for Mixtures Without Nonphysical Collision Invariants2016In: Journal of statistical physics, ISSN 0022-4715, E-ISSN 1572-9613, Vol. 165, no 2, p. 434-453Article in journal (Refereed)
    Abstract [en]

    An important aspect of constructing discrete velocity models (DVMs) for the Boltzmann equation is to obtain the right number of collision invariants. It is a well-known fact that DVMs can also have extra collision invariants, so called spurious collision invariants, in plus to the physical ones. A DVM with only physical collision invariants, and so without spurious ones, is called normal. For binary mixtures also the concept of supernormal DVMs was introduced, meaning that in addition to the DVM being normal, the restriction of the DVM to any single species also is normal. Here we introduce generalizations of this concept to DVMs for multicomponent mixtures. We also present some general algorithms for constructing such models and give some concrete examples of such constructions. One of our main results is that for any given number of species, and any given rational mass ratios we can construct a supernormal DVM. The DVMs are constructed in such a way that for half-space problems, as the Milne and Kramers problems, but also nonlinear ones, we obtain similar structures as for the classical discrete Boltzmann equation for one species, and therefore we can apply obtained results for the classical Boltzmann equation.

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  • 249.
    Berthold, Stefan
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    Inter-temporal Privacy Metrics2014Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Informational privacy of individuals has significantly gained importance after information technology has become widely deployed. Data, once digitalised, can be copied, distributed, and long-term stored at negligible costs. This has dramatic consequences for individuals that leave traces in the form of personal data whenever they interact with information technology, for instance, computers and phones; or even when information technology is recording the personal data of aware or unaware individuals. The right of individuals for informational privacy, in particular to control the flow and use of their personal data, is easily undermined by those controlling the information technology.

    The objective of this thesis is to study the measurement of informational privacy with a particular focus on scenarios where an individual discloses personal data to a second party which uses this data for re-identifying the individual within a set of other individuals. We contribute with privacy metrics for several instances of this scenario in the publications included in this thesis, most notably one which adds a time dimension to the scenario for modelling the effects of the time passed between data disclosure and usage. The result is a new framework for inter-temporal privacy metrics.

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  • 250.
    Berthold, Stefan
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Mathematics and Computer Science.
    The Privacy Option Language: Specification & Implementation2013Report (Other academic)
    Abstract [en]

    The data protection laws in Europe require that data controllers provide privacy policies to inform individuals about the prospective processing of their personal data. The ever growing expressiveness of privacy policy languages allows to specify policies in a growing number of details. This and new options for policy negotiations transformed rather general privacy policies into specific privacy contracts between the data controller and the individual.

    In this report, we specify a privacy contract language and call it the Privacy Option Language. It is modelled after the analogy between financial option contracts and data disclosures which has been presented in previous work and led to the Privacy Option notion. The language specification provides privacy by design through its data minimisation provisions, i.e., all contracts are automatically reduced to their canonical form so that individual differences in the contract formulation are inherently normalised. The language specification is extensible in two ways. First, hooks are specified in the core language and can be used to connect sublanguages. The freedom to choose any suitable sublanguage allows to specify language details independent of the core language. Second, the Privacy Option Language itself can be used as a sublanguage within a more general-domain language. We give examples for both types of extensions. Additionally, we provide tools for evaluating semantics such as human-readable presentations of Privacy Options and contract management. The definitions of the semantics are kept simple and serve as templates for more practical ones.

    All functionality can be checked by interactive tests in a standard multi-purpose programming language interpreter, since the Privacy Option Language is specified as an embedded domain-specific language within Haskell. Hands-on examples are provided along with the language specification.

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    The Privacy Option Language
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