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  • 1751.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Börstler, Jürgen
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    An Analysis of Success Factors for a Team-Project Software Development Course2006In: Proceedings of the 19th Conference on Software Engineering Education and Training, 2006, p. 95-102Conference paper (Refereed)
  • 1752.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Computing Science.
    Eriksson, Staffan
    Football animations for mobile phones2002In: Proceedings of the second Nordic conference on Human-computer interaction, 2002, p. 255-258Chapter in book (Other academic)
  • 1753.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Computing Science.
    Eriksson, Staffan
    Östberg, Frida
    Designing a football experience for mobile phonesManuscript (Other academic)
  • 1754.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Nilsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Untruncated eliminations in the EY-NPMA MAC protocol: performance and optimality2007In: IEEE Communications Letters, ISSN 1089-7798, E-ISSN 1558-2558, Vol. 11, no 2, p. 213-215Article in journal (Refereed)
  • 1755.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Nilsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Dougherty, Mark S
    Prioritized repeated eliminations multiple access: a novel protocol for wireless networksManuscript (preprint) (Other academic)
  • 1756.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Nilsson, Thomas
    Umeå University, Faculty of Science and Technology, Department of Computing Science. Umeå University, Faculty of Science and Technology, Department of Computing Science.
    S. Dougherty, Mark
    Prioritized repeated eliminations multiple access: A novel protocol for wireless networks2008In: The 27th Conference on Computer Communications (IEEE INFOCOM 2008), 2008, p. 1561-1569Chapter in book (Other academic)
    Abstract [en]

    A new and simple MAC protocol is proposed. Each node transmits a burst with length sampled from a geometric distribution with parameter q followed by a carrier sense slot. A node repeats the previous operations until it senses a busy slot. A node will access the channel and transmit its payload when it has sensed h idle slots. The parameter q can be adjusted for each node to achieve desired levels of relative priority with little impact on channel utilization and without a need for explicit knowledge about other traffic. Comparison to existing tree-splitting, carrier sensing and bursting protocols through analysis and simulations shows that our algorithm scales very well to the number of nodes. It has very high success probability, channel utilization and fairness. Extensions are provided for hidden terminal scenarios and are shown in simulations to provide adequate performance.

  • 1757.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Computing Science.
    Schedin, Lennart
    Elg, Fredrik
    Effects of delay and delay visulaization on Pong players2006In: UMINF, ISSN 0348-0542, no 06.42Article in journal (Refereed)
  • 1758.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Computing Science.
    Sun, Jiong
    Determining utility functions for streaming low bit rate football video2004In: UMINF, ISSN 0348-0542, no 04.14Article in journal (Refereed)
  • 1759.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Computing Science.
    Sun, Jiong
    Users complicate utility-based scheduling in mobile and wireless networksManuscript (Other academic)
  • 1760.
    Wikstrand, Greger
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Söderström, Ulrik
    Umeå University, Faculty of Science and Technology, Applied Physics and Electronics.
    Internet card play with video conferencing2006In: Proceedings SSBA 2006: Symposium on Image Analysis, Umeå, March 16-17, 2006 / [ed] Fredrik Georgsson, Niclas Börlin, Umeå: Umeå universitet , 2006, p. 93-96Chapter in book (Other academic)
    Abstract [en]

    In an experiment, groups of four participants played “bluffstopp” — a card game based on deception — over the Internet while communicating through multicast video and audio. Higher frame rates lead to lower video quality ratings. The result is explained as an effect of increasing visual workload.

  • 1761.
    Wikström, Daniel
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Knowledge Transfer between User Interfaces: Exploring mental model influence in learning new UI - A case study at Skatteverket.2017Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The phenomenon of mental models has long been studied in both cognitive science and HCI, yet their impact in knowledge transfer between user interfaces are not as commonly studied. Rarely are practical examples of this phenomenon studied from an academic perspective. Using a real world example, Skatteverket’s (Swedish tax authorities) information system Basregister, this case study seeks to explore the role of mental models in knowledge transfer between user interfaces.

    Method of investigation entailed utilizing two design iterations, three interview phases, and five participants employed at Skatteverket. This study investigated if, and how much design elements from well-known external- and the original- system impact the acquisition of new mental models for the newly produced user interface designs. Main results conclude that both external and internal design elements may be to tremendous benefit when the goal is to design with maximum knowledge transfer, as to reduce cognitive load on the user. However, that such implementations should be carefully instigated when designing new UI, and always from a user-centric approach.

  • 1762.
    Wikström, Gunilla
    Umeå University, Faculty of Science and Technology, Computing Science.
    Computation of Parameters in Separable Nonlinear Dynamic Models by use of Short-Cut Methods2002In: Technical Report UMINF, Vol. 02.14Article in journal (Refereed)
  • 1763.
    Wikström, Gunilla
    Umeå University, Faculty of Science and Technology, Computing Science.
    Computation of Parameters in some Mathematical Models2002Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    In computational science it is common to describe dynamic systems by mathematical models in forms of differential or integral equations. These models may contain parameters that have to be computed for the model to be complete. For the special type of ordinary differential equations studied in this thesis, the resulting parameter estimation problem is a separable nonlinear least squares problem with equality constraints. This problem can be solved by iteration, but due to complicated computations of derivatives and the existence of several local minima, so called short-cut methods may be an alternative. These methods are based on simplified versions of the original problem. An algorithm, called the modified Kaufman algorithm, is proposed and it takes the separability into account. Moreover, different kinds of discretizations and formulations of the optimization problem are discussed as well as the effect of ill-conditioning.

    Computation of parameters often includes as a part solution of linear system of equations Ax = b. The corresponding pseudoinverse solution depends on the properties of the matrix A and vector b. The singular value decomposition of A can then be used to construct error propagation matrices and by use of these it is possible to investigate how changes in the input data affect the solution x. Theoretical error bounds based on condition numbers indicate the worst case but the use of experimental error analysis makes it possible to also have information about the effect of a more limited amount of perturbations and in that sense be more realistic. It is shown how the effect of perturbations can be analyzed by a semi-experimental analysis. The analysis combines the theory of the error propagation matrices with an experimental error analysis based on randomly generated perturbations that takes the structure of A into account

  • 1764.
    Wikström, Gunilla
    et al.
    Umeå University, Faculty of Science and Technology, Computing Science.
    Wedin, P.-Å.
    Interpretation and Practical Use of Error Propagation Matrices2002In: Technical Report UMINF, Vol. 02.15Article in journal (Refereed)
  • 1765.
    Wikström, Hannes
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Sandström, Jonas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Incorporating Video Support in an Innovative Communications Solution2010Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Ericsson is a large actor in the telecom industry, providing infrastructure and communication solutions connecting billions of people around the world. The Multimedia branch of Ericsson focuses on the development of communications solutions, and one of these new innovative solutions is called Cloud Communication. The Cloud Communication system provides an anonymous peer-to-peer communication service through multiple channels. This master thesis is a step to further expand the channels that the system supports as well as rebuild and redesign the applications that make the communication possible. Methodologies that helped achieving this was mainly the design process and to help plan and execute the project Scrum was used. The results of the thesis were added support for video conversations and two entirely new clients supporting the new feature together with the old ones. These new clients can be used to demonstrate the capabilities of the Cloud Communication system and the features it supports. Video conversations in an anonymous context is however questioned and the conclusion is that the video-use case mainly should be applied in a collaboration/business context in which the clients currently are implemented.

  • 1766.
    Wilson, Holly
    et al.
    University of Bath.
    Theodorou, Andreas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Slam the Brakes: Perceptions of Moral Decisions in Driving Dilemmas2019Conference paper (Other academic)
  • 1767.
    Winter, Stephan
    et al.
    Department of Infrastructure Engineering, University of Melbourne.
    Tomko, Martin
    Department of Infrastructure Engineering, University of Melbourne.
    Vasardani, Maria
    Department of Infrastructure Engineering, University of Melbourne.
    Richter, Kai-Florian
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Koshelham, Kourosh
    Department of Infrastructure Engineering, University of Melbourne.
    Indoor localization and navigation independent of sensor based technologies2017Report (Other academic)
    Abstract [en]

    In this short article we present concepts of indoor localization and navigation that are independent of sensors embedded in the environment, and thus, standing against the tide of technology-based indoor localization. The motivation for doing so is clear: We seek solutions that are independent of particular environments, and thus globally applicable.

  • 1768.
    Wiseen Åberg, Erik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Implementing a PPP-based SSL VPN Client for Clavister Security Gateways2016Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    In this thesis, we describe how an SSL VPN client utilizing the UNIX program called pppd was developed. pppd enables the creation of point to point links through the Point-To-Protoco l(PPP) which is capable of carrying protocols such as IP, allowing the VPN to connect to the SSL VPN service of Clavisters security gateways. We then perform ananalysis of the degradation of network communication through put caused by this approach. Our findings suggest that the usage of pppd may be the cause of lower than acceptable throughput, making this approach non-viable as the basis for a VPN solution

  • 1769.
    Woldemariam, Yonas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Predicting User Competence from Text2017In: The 21st world multi-conference on systemics, cybernetics and informatics: proceedings : volume 1 / [ed] Nagib Callaos, Belkis Sánches, Michael Savoie, Andrés Tremante, International Institute of Informatics and Systemics, 2017, p. 147-152Conference paper (Refereed)
    Abstract [en]

    We explore the possibility of learning user competence from a text by using natural language processing and machine learning (ML) methods. In our context, competence is defined as the ability to identify the wildlife appearing in images and classifying into species correctly. We evaluate and compare the performance (regarding accuracy and F-measure) of the three ML methods, Naive Bayes (NB), Decision Trees (DT) and K-nearest neighbors (KNN), applied to the text corpus obtained from the Snapshot Senrengeti discussion forum posts. The baseline results show, that regarding accuracy, DT outperforms NB and KNN by 16.00%, and 15.00% respectively. Regarding F-measure, K-NN outperforms NB and DT by 12.08% and 1.17%, respectively. We also propose a hybrid model that combines the three models (DT, NB and KNN). We improve the baseline results with the calibration technique and additional features. Adding a bi-gram feature has shown a dramatic increase (from 48.38% to 64.40%) of accuracy for NB model. We achieved to push the accuracy limit in the baseline models from 93.39% to 94.09%

  • 1770.
    Woldemariam, Yonas
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Sentiment Analysis in A Cross-Media Analysis Framework2016In: PROCEEDINGS OF 2016 IEEE INTERNATIONAL CONFERENCE ON BIG DATA ANALYSIS (ICBDA), 2016, p. 27-31Conference paper (Refereed)
    Abstract [en]

    This paper introduces the implementation and integration of a sentiment analysis pipeline into the ongoing open source cross-media analysis framework. The pipeline includes the following components; chat room cleaner, NLP and sentiment analyzer. Before the integration, we also compare two broad categories of sentiment analysis methods, namely lexicon-based and machine learning approaches. We mainly focus on finding out which method is appropriate to detect sentiments from forum discussion posts. In order to conduct our experiments, we use the apache-hadoop framework with its lexicon-based sentiment prediction algorithm and Stanford coreNLP library with the Recursive Neural Tensor Network (RNTN) model. The lexicon-based uses sentiment dictionary containing words annotated with sentiment labels and other basic lexical features, and the later one is trained on Sentiment Treebank with 215,154 phrases, labeled using Amazon Turk. Our overall performance evaluation shows that RNTN outperforms the lexicon-based by 9.88% accuracy on variable length positive, negative, and neutral comments. How-ever, the lexicon-based shows better performance on classifying positive comments. We also found out that the F1-score values of the Lexicon-based is greater by 0.16 from the RNTN.

  • 1771.
    Woldemariam, Yonas Demeke
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Bensch, Suna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Björklund, Henrik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Predicting User Competence from Linguistic Data2017In: Proceedings of the 14th International Conference on Natural Language Processing (ICON-2017) / [ed] Sivaji Bandyopadhyay, Jadavpur University , 2017, p. 476-484Conference paper (Refereed)
    Abstract [en]

    We investigate the problem of predicting the competence of users of the crowd-sourcing platform Zooniverse by analyzing their chat texts. Zooniverse is an online platform where objects of different types are displayed to volunteer users to classify. Our research focuses on the Zoonivers Galaxy Zoo project, where users classify the images of galaxies and discuss their classifications in text. We apply natural language processing methods to extract linguistic features including syntactic categories, bag-of-words, and punctuation marks. We trained three supervised machine-learning classifiers on the resulting dataset: k-nearest neighbors, decision trees (with gradient boosting) and naive Bayes. They are evaluated (regarding accuracy and F-measure) with two different but related domain datasets. The performance of the classifiers varies across the feature set configurations designed during the training phase. A challenging part of this research is to compute the competence of the users without ground truth data available. We implemented a tool that estimates the proficiency of users and annotates their text with computed competence. Our evaluation results show that the trained classifier models give results that are significantly better than chance and can be deployed for other crowd-sourcing projects as well. 

  • 1772.
    Woldemariam, Yonas Demeke
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Björklund, Henrik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Bensch, Suna
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Predicting User Competence from Linguistic Data2017In: Proceedings of the 14th International Conference on Natural Language Processing (ICON-2017), NLP Association of India , 2017, p. 476-484Conference paper (Refereed)
    Abstract [en]

    We investigate the problem of predicting the competence of users of the crowdsourcing platform Zooniverse by analyzing their chat texts. Zooniverse is an online platform where objects of different types are displayed to volunteer users to classify. Our research focuses on the Zoonivers Galaxy Zoo project, where users classify the images of galaxies and discuss their classifications in text. We apply natural language processing methods to extract linguistic features including syntactic categories, bag-of-words, and punctuation marks. We trained three supervised machine-learning classifiers on the resulting dataset: k-nearest neighbors, decision trees (with gradient boosting) and naive Bayes. They are evaluated (regarding accuracy and F-measure) with two different but related domain datasets. The performance of the classifiers varies across the feature set configurations designed during the training phase. A challenging part of this research is to compute the competence of the users without ground truth data available. We implemented a tool that estimates the proficiency of users and annotates their text with computed competence. Our evaluation results show that the trained classifier models give results that are significantly better than chance and can be deployed for other crowd-sourcing projects as well.

  • 1773.
    Wu, Naomi
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A long-distance relationship: Reconnecting hotels with their guests via intuitive design2018Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    Currently for travel planning, guests will research via hotel websites while still preferring to book through third-party sites, which leads to a disconnect between hotels and their guests. A chat widget artifact that is added onto the hotel’s website and linked through messaging applications was created by a start-up company, Bookboost, to bridge this gap. The current intuitiveness of the artifact and future improvements that may increase intuitiveness was investigated through a case study of user and expert analysis. 10 participants – 5 hotel staff users and 5 guest users – were sampled at hotel lobbies via systematic sampling and non-random sampling. Participants ranged in age from 18 to 65 years old, with 30% being millennials. Task analysis, an interview, and a questionnaire were used for user analysis. The researcher acted as an evaluator and examined the artifact for flaws and possible improvements using activity theory’s human-artifact model (HAM). Analyses suggest that current intuitiveness is fairly high, but there is room for improvement. There seems to be a difference between millennials and non-millennials, especially regarding the amount of time taken and preference for the artifact (versus more familiar methods for communicating with others). Interest and comfort in technology usage was a factor in intuitiveness. Generally, those more comfortable with technology had higher zone of proximal development (ZPD) scores. Improvements have been suggested that may increase artifact intuitiveness, although this was not tested due to the scope of the study. Future research can continue to examine if the suggested improvements have indeed increased intuitiveness in the artifact for users of all ages.

  • 1774. Wu, Yu-Shu
    et al.
    Zhang, Keni
    Ding, Chris
    Pruess, K.
    Elmroth, Erik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Bodvarsson, G. S.
    An efficient parallel-computing method for modeling nonisothermal multiphase flow and multicomponent transport in porous and fractured media2002In: Advances in Water Resources, ISSN 0309-1708, E-ISSN 1872-9657, Vol. 25, p. 243-261Article in journal (Refereed)
  • 1775.
    Wärmedal, Björn
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Explaining kernel space with a real world example2012Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The basic premis of this report is to explain the relation of kernel space and user space. It will show how the processor and computer memory work together with the operating system to protect operating system routines from misuse by user programs. After reading through this the reader should have an understanding of what kernel space is, how it is protected and how user programs gain access to its contents in a controlled manner. Fuse is used as an example in the conclusion, explaining how that particular application can be used to implement a simple dependency tracking overlay in a filesystem.

  • 1776.
    Wärmedal, Linnea
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Computational Thinking - A New Approach for Teaching Computer Science to College Freshmen2014Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Felix, qui potuit rerum cognoscere causas

    The traditional way of introducing computer science to college freshmen is through a programming course. Such a course often account for programming, problem solving, efficiency, debugging, memory allocation and complexity. The student is presented to all of this within the first course in computer science. To be introduced to all these concepts during the first course could be compared to learning fundamental arithmetic alongside the mean value theorem. What if the student, instead of learning the traditional, where to learn the basics of computer science? What if programming didn’t come first?

    If the student where to get proficient in the basics of computer science they would be able to focus on the specifics of the different field of computer science.

    In this thesis I present a proposition of a syllabus for a college freshmen course to be taught to computer science minors. The course will be built on the concepts of computational thinking.

    The concept of computational thinking and how it is used in education will also be account for.

  • 1777.
    Xu, Lei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Elmroth, Erik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A time interval-based credit reservation approach for prepaid composite services in cloud environments2011In: Web Services (ECOWS), 2011 Ninth IEEE European Conference on, 2011, p. 158-165Conference paper (Refereed)
    Abstract [en]

    A typical business model in cloud environments for service providers is the provision of composite services built from resources and services offered by other providers. Customers of these composite services can make an advance payment before consumption. In this scenario, accounting systems will supervise each service comprising given composite services individually. As the number of constituent services and requests submitted by customers increase dramatically, scalability issues arise. To tackle this problem, we present an approach that estimates the highest cost of a given composite service composed of multiple pre-paid services, and then calculates the duration of a time interval based on which accounting systems access and update target customers' account. It is intended to reduce overhead imposed by supervising services individually in real-time. Commercial agreements between involved business partners and potential economic compensations due to the violation of Service Level Agreement (SLA) are mirrored in the cost estimation.

  • 1778.
    Xu, Lei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Jennings, Brendan
    Automating the generation, deployment and application of charging schemes for composed communications services2012In: 2012 IEEE Network operations and management symposium (NOMS), 2012, p. 853-859Conference paper (Refereed)
    Abstract [en]

    Creating composed services dynamically to meet the demand of every individual customer can be an inexpensive way for a service provider to differentiate itself from its competitors. Charging and billing systems are one of the key components supporting this business model. However, many existing charging and billing systems suffer from their time-consuming scheme 1 configuration process. This limits the flexibility of service providers to offer composed services. This paper proposes a framework named Accounting Logic Generator (ALG) that automates the generation and deployment of charging schemes for composed services. It processes a two-part charging scheme which contains rules that can be used to govern how the usage of a service is charged for when it is executed standalone, used in the context of a composed service, invoked within specific time intervals of interest or completed successfully in the context of composed service invocations where one or more other services fail. Furthermore, an approach that estimates service cost for the selection process of service composition is presented.

  • 1779.
    Xu, Lei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Lakew, Ewnetu Bayuh
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Hernandez-Rodriguez, Francisco
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Elmroth, Erik
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A Scalable Accounting Solution for Prepaid Services in Cloud Systems2012In: Proceedings of the 2012 IEEE Ninth International Conference on Services Computing, 2012, p. 81-89Conference paper (Refereed)
    Abstract [en]

    Prepaid charging, an essential option for the accounting of cloud services, provides effective financial control for both service providers and customers. However, it has to be supported by real-time credit checking and cost calculation. These real-time actions consume resources of the providers' network and impose high overhead. To tackle this issue, we present a scalable accounting solution in which an accounting component is hosted in every cluster constituting a cloud system. Each of our accounting component supervises service consumptions based on a calculated interval of a service bundle that is composed of all services hosted in a cluster and consumed by one customer simultaneously. Credit will be re-allocated when a customer's credit in one cluster is not enough to compensate further usage, and the allocation is performed based on service consumptions. This work is intended to reduce the cost of prepaid services and to ensure service provision is not hampered by the charging part. Additionally, we perform theoretical and experimental analyses that indicate this work can provide an inexpensive accounting solution for the long-lived services in storage clouds.

  • 1780.
    Xu, Lei
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Shi, Lei
    Wang, Runxin
    Jennings, Brendan
    A multiple criteria service composition selection algorithm supporting time-sensitive rules2011In: Proceedings of the12th IFIP/IEEE International Symposium on Integrated Network Management (IM 2011): Dublin, Ireland, 23—27 May 2011 / [ed] Nazim Agoulmine, Claudio Bartolini, Tom Pfeifer, Declan O'Sullivan, Institute of Electrical and Electronics Engineers (IEEE) , 2011, p. 718-721Conference paper (Refereed)
    Abstract [en]

    Constructing composite services by using of services offered by third parties is an attractive and inexpensive way for service brokers and aggregators to enhance differentiation from their competitors. When multiple services provide the same or similar functionalities, selecting those that satisfy users' non-functional requirements is crucial. In many cases, non-functional properties of services are heavily dependent on the activity of the network delivering those services whilst the network activity follows certain time-sensitive rules. We present a service selection algorithm that takes into account time-sensitive variations of non-functional propensities of services to identify a service combination offering the highest quality within a specified time interval.

  • 1781.
    Yan, Chunli
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Developing digital support for learning and diagnostic reasoning in clinical practice2018Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The two main purposes of clinical decision-support systems (CDSSs) are to provide healthcare professionals decision-making support based on evidence-based medical knowledge, and a continuing medical education. This thesis focuses on both purposes and shows how fundamental theory in the field of artificial intelligence can be developed, adapted and implemented in a CDSS for supporting learning and diagnostic reasoning in clinical practice. The main research problems addressed in this thesis are how to represent and manage uncertain, incomplete, inconsistent and distributed knowledge in automated reasoning and decision-making with the clinicians in the loop, how to facilitate the knowledge engineering and maintenance process, and how to detect and support learning and skill development in CDSS users.

    Research contributions include theories, methods, and algorithms based on possibilistic logic and formal argumentation for representing and managing uncertain, incomplete, inconsistent and distributed medical knowledge, and for supporting reasoning and decision-making when using a CDSS. The clinician is provided potentially conflicting arguments and their strength based on different diagnostic criteria and the available patient information in order to make an informed decision. The theoretical results were implemented in the Dementia Diagnosis and Management Support System - Web version (DMSS-W), in a multi-agent hypothesis-driven inquiry dialogue system, and in an inference engine serving as a module of ACKTUS.

    CDSS maintenance is challenging since new knowledge about diseases and treatments are continuously developed. Typically, knowledge and software engineers are needed to bridge medical experts and CDSSs, leading to time-consuming system development. ACKTUS (Activity-Centered Knowledge and Interaction Tailored to Users) was, as part of this research, further developed as a generic web-based platform for knowledge management and end-user development of CDSSs. It includes the inference engine and a content management system that the medical expert can use to manage knowledge, design and evaluate CDSSs. A graphical user interface generator synchronizes the interface to the ontology serving as the knowledge base. ACKTUS was used for developing DMSS-W, and facilitated the system development and maintenance.

    To offer person-tailored support for the clinician's learning, reasoning and decision-making, the CDSS design was based on theories of how novices and experts reason and make decisions. Pilot case studies involving physicians with different levels of expertise who applied DMSS-W in patient cases were conducted in clinical practice to explore methods for detecting skill levels and whether learning is taking place. The results indicated that the skill levels can be detected using the method. The novice was seen to develop reasoning strategies similar to an expert's, indicating that learning was taking place. In future work, tailored educational support will be developed, and evaluated using the methods.

  • 1782.
    Yan, Chunli
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Lindgren, Helena
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A Generic Approach for Data Management and End-User Development of Clinical Decision Support Systems2018Report (Other academic)
    Abstract [en]

    The main purposes of clinical decision-support systems (CDSS) are disseminating evidence-based medical knowledge (EBM), supporting a continued medical education, and improving clinical decision making and care. These purposes are traditionally achieved by using solutions that are relatively transparent and explainable to the end user. However, the development and maintenance of such solutions is resource demanding. Currently, there are four challenges existing in CDSSs when adapting to new circumstances. That is, when facing new knowledge, new diseases, different organizations and users with different skills, usually one needs to update the existing CDSS or develop a new CDSS, which requires lots of time and efforts. Hence, this paper aims for reusing an existing CDSS code by virtue of inputs from authorized medical domain expert users, and with minimal requirement of knowledge and software engineers. To facilitate knowledge elicitation and end-user development, an ACKTUS-based architecture for CDSS development and management is presented that contains: I) A knowledge base and a content management system built on Semantic Web technology to achieve modularity, reusability, customisation, and the possibility to allow medical experts to model the medical knowledge and to structure the information that builds up the design of the user interface; II) A user interface and an graphical user interface generator that automatically generates the user interface whenever the user logs in, so that the interface is synchronised with updates of the knowledge base; III) An inference engine that utilizes patient-specific data and applies various rules in the knowledge base to conduct the reasoning and decision making. These modules can be reused when adapting to new situations. A CDSS for dementia diagnosis is developed and used as an example in the presentation of the generic architecture. A pilot study of the CDSS is presented involving four medical professionals with different levels of expertise. The results show how the generic approach allows for easy knowledge representation and management of EBM, supports a continued medical education and may improve clinical decision making and care provision.

  • 1783.
    Yan, Chunli
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Lindgren, Helena
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Diagnostic Reasoning Guided by a Decision-Support System: a Case Study2017In: Proceedings of the European Conference on Cognitive Ergonomics 2017: Transforming the everyday, New York, NY, USA: ACM Digital Library, 2017, p. 25-30Conference paper (Refereed)
    Abstract [en]

    A clinical decision-support system for dementia investigation was used in clinical practice. User information was collected based on interactions with the application. The aim of this study is to identify features in logged data that can be used for detecting learning and reasoning patterns in the user. A case of a physician who is novice to both the application and the dementia domain was studied and compared to the case of an expert physician using the system. Diferences between them were found, and a clear pattern that indicates that learning takes place, both how to use the system and about dementia, was observed in the novice user. Further studies need to be conducted, focusing on whether patterns become stable over time, and with complementary methods that can detect reasons for observed behaviors. Software for automatic detection will be developed based on the results of this study.

  • 1784.
    Yan, Chunli
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Lindgren, Helena
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Hypothesis-Driven Agent Dialogues for Dementia Assessment2013In: VIII Workshop on Agents Applied in Health Care (A2HC), 2013, p. 13-24Conference paper (Refereed)
  • 1785.
    Yan, Chunli
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Lindgren, Helena
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Nieves, Juan Carlos
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A Dialogue-Based Approach for Dealing with Uncertain and Conflicting Information in Medical Diagnosis2018In: Autonomous Agents and Multi-Agent Systems, ISSN 1387-2532, E-ISSN 1573-7454, Vol. 32, no 6, p. 861-885Article in journal (Refereed)
    Abstract [en]

    In this paper, we propose a multi-agent framework to deal with situations involving uncertain or inconsistent information located in a distributed environment which cannot be combined into a single knowledge base. To this end, we introduce an inquiry dialogue approach based on a combination of possibilistic logic and a formal argumentation-based theory, where possibilistic logic is used to capture uncertain information, and the argumentation-based approach is used to deal with inconsistent knowledge in a distributed environment. We also modify the framework of earlier work, so that the system is not only easier to implement but also more suitable for educational purposes. The suggested approach is implemented in a clinical decision-support system in the domain of dementia diagnosis. The approach allows the physician to suggest a hypothetical diagnosis in a patient case, which is verified through the dialogue if sufficient patient information is present. If not, the user is informed about the missing information and potential inconsistencies in the information as a way to provide support for continuing medical education. The approach is presented, discussed, and applied to one scenario. The results contribute to the theory and application of inquiry dialogues in situations where the data are uncertain and inconsistent.

  • 1786.
    Yan, Chunli
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Nieves, Juan Carlos
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Lindgren, Helena
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A Multi-agent System for Nested Inquiry Dialogues2014In: Advances in Practical Applications of Heterogeneous Multi-Agent Systems: The PAAMS Collection : 12th International Conference, PAAMS 2014, Salamanca, Spain, June 4-6, 2014. Proceedings / [ed] Demazeau, Yves; Zambonelli, Franco; Corchado, JuanM.; Bajo, Javier, Springer, 2014, Vol. 8473, p. 303-314Chapter in book (Refereed)
    Abstract [en]

    Generating and evaluating arguments are two important aspects in argumentation-based dialogue systems. In current research, however, generating and evaluating arguments are normally treated separately. Also, there are rarely implementations of the approaches in real applications. In this paper, we generate inquiry dialogues and evaluate arguments during the dialogue procedure simultaneously. Furthermore, we have implemented this approach in a real medical domain and demonstrated a practical example extracted from this application.

  • 1787.
    Yedeg, Esubalewe Lakie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Analysis, Control, and Design Optimization of Engineering Mechanics Systems2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis considers applications of gradient-based optimization algorithms to the design and control of some mechanics systems. The material distribution approach to topology optimization is applied to design two different acoustic devices, a reactive muffler and an acoustic horn, and optimization is used to control a ball pitching robot.

    Reactive mufflers are widely used to attenuate the exhaust noise of internal combustion engines by reflecting the acoustic energy back to the source. A material distribution optimization method is developed to design the layout of sound-hard material inside the expansion chamber of a reactive muffler. The objective is to minimize the acoustic energy at the muffler outlet. The presence or absence of material is represented by design variables that are mapped to varying coefficients in the governing equation. An anisotropic design filter is used to control the minimum thickness of materials separately in different directions. Numerical results demonstrate that the approach can produce mufflers with high transmission loss for a broad range of frequencies.

    For acoustic devices, it is possible to improve their performance, without adding extended volumes of materials, by an appropriate placement of thin structures with suitable material properties. We apply layout optimization of thin sound-hard material in the interior of an acoustic horn to improve its far-field directivity properties. Absence or presence of thin sound-hard material is modeled by a surface transmission impedance, and the optimization determines the distribution of materials along a “ground structure” in the form of a grid inside the horn. Horns provided with the optimized scatterers show a much improved angular coverage, compared to the initial configuration.

    The surface impedance is handled by a new finite element method developed for Helmholtz equation in the situation where an interface is embedded in the computational domain. A Nitschetype method, different from the standard one, weakly enforces the impedance conditions for transmission through the interface. As opposed to a standard finite-element discretization of the problem, our method seamlessly handles both vanishing and non-vanishing interface conditions. We show the stability of the method for a quite general class of surface impedance functions, provided that possible surface waves are sufficiently resolved by the mesh.

    The thesis also presents a method for optimal control of a two-link ball pitching robot with the aim of throwing a ball as far as possible. The pitching robot is connected to a motor via a non-linear torsional spring at the shoulder joint. Constraints on the motor torque, power, and angular velocity of the motor shaft are included in the model. The control problem is solved by an interior point method to determine the optimal motor torque profile and release position. Numerical experiments show the effectiveness of the method and the effect of the constraints on the performance.

  • 1788.
    Yedeg, Esubalewe Lakie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Control and design of engineering mechanics systems2013Licentiate thesis, comprehensive summary (Other academic)
  • 1789.
    Yedeg, Esubalewe Lakie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    On the use of thin structures to control the far-field properties of an acoustic device2016Report (Other academic)
    Abstract [en]

    This work presents a sensitivity analysis for topology optimization of thin structures inside an acoustic horn in order to improve its far-field directivity properties. The objective function gradient is computed using an adjoint-based method, a well-known approach in the design optimization community due to its ability to compute the design sensitivities when there is a large number of design variables. The accuracy of the computed gradient is verified numerically against a first order finite difference approximation. The report also covers the matrix representations of the far-field pattern evaluation and the variational problem with an interface condition to model the thin structures. 

  • 1790.
    Yedeg, Esubalewe Lakie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Optimal ball pitchingArticle in journal (Other academic)
  • 1791.
    Yedeg, Esubalewe Lakie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Optimal control of a ball pitching robot2012In: 2012 17TH INTERNATIONAL CONFERENCE ON METHODS AND MODELS IN AUTOMATION AND ROBOTICS (MMAR), 2012, p. 456-456Conference paper (Refereed)
    Abstract [en]

    We study a two-link pitching robot, with an active gripping mechanism and aim to make it throw a ball as far as possible. The two links are connected at the elbow joint by a linear torsional spring. The gripping mechanism is able to hold a ball and release it at any specified time. The two-link pitching robot is connected to a motor shaft at the shoulder joint by a non-linear torsional spring. The shoulder joint is held fixed at the origin of the coordinate system. The configuration of the arm and the motor shaft is illustrated in Figure 1 and described by the angels q1 and qm, measured with respect to the horizontal axis, respectively, whereas q2 denote the angle change between the arm and forearm at the elbow joint.

  • 1792.
    Yedeg, Esubalewe Lakie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    State constrained optimal control of a ball pitching robot2013In: Mechanism and machine theory, ISSN 0094-114X, E-ISSN 1873-3999, Vol. 69, p. 337-349Article in journal (Refereed)
    Abstract [en]

    We present a method for offline optimal control of a two-link ball pitching robot with the aim of throwing a ball as far as possible. The pitching robot is connected to a motor via a non-linear torsional spring at the shoulder joint. The elbow joint is passive and loaded with a linear torsional spring. We model the system based on an Euler–Lagrange formulation. Constraints on the motor torque and power as well as the angular velocity of the motor shaft are included in the model. By using an interior point method with gradients supplied by a discrete adjoint method, we numerically solve the resulting constrained control problem of finding the optimal piecewise constant motor torque profile and release position. Numerical experiments illustrate the effectiveness of our strategy as well as the effect of the constraints on the objective. In our experiments, the optimal motor torque gives rise to motions comprising an initial backswing; a transition, where the elbow spring accumulates potential energy; and finally a fast acceleration phase leading up the ball release.

  • 1793.
    Yedeg, Esubalewe Lakie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Anisotropic Topology Optimization of a Reactive Muffler with a Perforated PipeArticle in journal (Other academic)
  • 1794.
    Yedeg, Esubalewe Lakie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Interior layout topology optimization of a reactive muffler2016In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 53, no 4, p. 645-656Article in journal (Refereed)
    Abstract [en]

    This article presents a material distribution method that is tailored to the interior design of reactive mufflers. Such devices are typically acoustically small, except in the length direction, and their interior consists of a combination of pipes, expansions, contractions, and Helmholtz resonators. In order to design the interior layout using material distribution optimization, it is necessary to be able to resolve thin sound-hard materials as well as thin sheets with a given acoustic impedance, such as perforated plates, and manage small channels to Helmholtz resonators. We develop a method that uses an anisotropic design filter in combination with a fine mesh in order to control the minimum thickness separately in different directions. A two-stage post processing procedure is used to control openings to resonators, and embedded thin impedance surfaces are modeled by a mortar-element method. Numerical results demonstrate that the approach can produce mufflers with high transmission loss for a broad range of frequencies. The optimized mufflers include components that resemble combinations of expansion chambers, cylindrical pipes, and Helmholtz resonators.

  • 1795.
    Yedeg, Esubalewe Lakie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Layout optimization of thin sound-hard material to improve the far-field directivity properties of an acoustic horn2017In: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 55, no 3, p. 795-808Article in journal (Refereed)
    Abstract [en]

    To improve the far-field directivity properties of a given mid-range acoustic horn, previously designed by shape optimization to exhibit almost ideal transmission properties in the frequency range 1.6–9.05 kHz, we apply layout optimization of thin sound-hard material in the interior of the horn. The purpose of the optimization is to place scattering material to prevent the sound intensity to increasingly be concentrated, with increasing frequency, along the horn axis. Absence or presence of thin sound-hard material is modeled by an equivalent surface transmission impedance, and the optimization algorithm determines the distribution of air or sound-hard material along a “ground structure” in the form of a grid inside the horn. The surface impedance is numerically handled using a newly developed finite-element formulation that allows exact enforcement of a vanishing impedance, corresponding to air, which would not be possible using a standard formulation. Horns provided with the optimized scatterers show a much improved angular coverage, compared to the initial configuration, with beam widths that exceed 60 ∘ uniformly over the operational frequency range, without destroying the good transmission properties of the initial horn.

  • 1796.
    Yedeg, Esubalewe Lakie
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Wadbro, Eddie
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Hansbo, Peter
    Larson, Mats G.
    Umeå University, Faculty of Science and Technology, Department of Mathematics and Mathematical Statistics.
    Berggren, Martin
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    A Nitsche-type Method for Helmholtz Equation with an Embedded Acoustically Permeable Interface2016In: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 304, p. 479-500Article in journal (Refereed)
    Abstract [en]

    We propose a new finite element method for Helmholtz equation in the situation where an acoustically permeable interface is embedded in the computational domain. A variant of Nitsche's method, different from the standard one, weakly enforces the impedance conditions for transmission through the interface. As opposed to a standard finite-element discretization of the problem, our method seamlessly handles a complex-valued impedance function Z that is allowed to vanish. In the case of a vanishing impedance, the proposed method reduces to the classic Nitsche method to weakly enforce continuity over the interface. We show stability of the method, in terms of a discrete Gårding inequality, for a quite general class of surface impedance functions, provided that possible surface waves are sufficiently resolved by the mesh. Moreover, we prove an a priori error estimate under the assumption that the absolute value of the impedance is bounded away from zero almost everywhere. Numerical experiments illustrate the performance of the method for a number of test cases in 2D and 3D with different interface conditions. 

  • 1797.
    Yekeh, Farahnaz
    et al.
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Pordel, Mostafa
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    Almeida, Luís
    University of Porto.
    Behnam, Moris
    Mälardalen University.
    Portugal, Paulo José
    University of Porto.
    Exploring alternatives to scale FTT-SE to large networks2011In: 6th IEEE International Symposium on Industrial Embedded Systems, 2011Conference paper (Refereed)
    Abstract [en]

    Nowadays, most complex embedded systems follow a distributed approach in which a network interconnects potentially large numbers of nodes. One technology that is being increasingly used is switched Ethernet, but real-time variants of this protocol typically limit scalability. In this paper, we focus on the scalability of the Flexible Time Triggered communication over Switched Ethernet (FTT-SE), which has been proposed to support hard real-time applications in a flexible and predictable manner. Moreover, time-triggered and event-triggered communication methods are supported in this protocol. FTT-SE has already been explored and investigated for small scale networked applications. In this paper we address the protocol scalability and suggest three different solutions with a qualitative assessment. © 2011 IEEE.

  • 1798.
    Yifter Lindgren, Emanuel
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    En studie om styrkor och svagheter hos sudokulösande algoritmer2014Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Sudoku is a popular game of logic in the form of a puzzle. Sudoku puzzles are printed daily in Swedish newspapers. The challenge of solving these puzzles have inspired many programmers to develop Sudokusolving algorithms. These algorithms use different approaches in order to quickly and efficiently solve the puzzles. One such approach is the use of logical strategies initially developed for human players. The purpose of this project is to benchmark and compare different approaches and algorithms. The results of the benchmarks is then used to evaluate the strength and weaknesses of each algorithm. The benchmark is performed with 12000 Sudoku puzzles of varying difficulty. The study shows that Dancing Links is the best performing algorithm in general. Dancing Links excels in each comparison criteria, one of which is solving speed. An algorithm that does not impress is Backtracking. The algorithm consistently takes the most time to solve a puzzle. The solving speed of the algorithm also varies greatly depending on the number of clues in the puzzle. This makes the algorithm less suitable for applications were solving speed is of concern. Only the strategy-based algorithm fails to solve all the puzzles. However the algorithm did manage to solve the easier puzzles and is consistently faster than Backtracking. A trend was observed where the algorithm’s speed was negatively affected by an increasing difficulty.

  • 1799. Yousefnezhad, Narges
    et al.
    Madhikermi, Manik
    Främling, Kary
    Umeå University, Faculty of Science and Technology, Department of Computing Science.
    MeDI: Measurement-based Device Identification Framework for Internet of Things2018In: 2018 IEEE 16TH INTERNATIONAL CONFERENCE ON INDUSTRIAL INFORMATICS (INDIN), IEEE , 2018, p. 95-100Conference paper (Refereed)
    Abstract [en]

    IoT systems may provide information from different sensors that may reveal potentially confidential data, such as a person's presence or not. The primary question to address is how we can identify the sensors and other devices in a reliable way before receiving data from them and using or sharing it. In other words, we need to verify the identity of sensors and devices. A malicious device could claim that it is the legitimate sensor and trigger security problems. For instance, it might send false data about the environment, harmfully affecting the outputs and behavior of the system. For this purpose, using only primary identity values such as IP address, MAC address, and even the public-key cryptography key pair is not enough since IPs can be dynamic, MACs can be spoofed, and cryptography key pairs can be stolen. Therefore, the server requires supplementary security considerations such as contextual features to verify the device identity. This paper presents a measurement-based method to detect and alert false data reports during the reception process by means of sensor behavior. As a proof of concept, we develop a classification-based methodology for device identification, which can be implemented in a real IoT scenario.

  • 1800. Zahrai, S.
    et al.
    Wikstrom, G.
    Umeå University, Faculty of Science and Technology, Departement of Computing Science.
    Mathematical modeling of flow and kinetics in a reactor for dilute-acid hydrolysis of cellulose particles - A mixture flow approach2007In: Applied Biochemistry and Biotechnology, Vol. 136, no 2, p. 141-152Article in journal (Refereed)
    Abstract [en]

    A mathematical model to simulate the dilute-acid hydrolysis process of cellulose particles is presented. In this model, the mass is treated as a mixture of different components. A test case is considered for which transport equations for components are developed and solved together with the momentum equation for the fluid flow. To solve the model equations, a commercially available flow solver was used. All input data were taken from previously published works. For the small static mixer considered as test geometry, the result, in terms of the conversion of the cellulose particles, was reasonable. With input parameters that are relevant to a plant-size reactor, the model can be used to predict the conversion of both cellulose and hemicellulose particles.

3334353637 1751 - 1800 of 1844
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