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  • 19301.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Some experiences of teacher cooperation involving statistics, geochemistry, and a computer in the classroom1995Inngår i: Proceedings of the 50th Session of the International Statistical Institute, 1995, Vol. Book 2, s. 1218-1219Konferansepaper (Fagfellevurdert)
  • 19302.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Statistics in industry and implications for teaching1986Inngår i: Proceedings: the Second International Conference on Teaching Statistics / [ed] Roger Davidson ; Jim Swift, Victoria, BC: University of Victoria , 1986, s. 355-356Konferansepaper (Fagfellevurdert)
  • 19303.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Statistics teaching through teacher cooperation and computer in the classroom1997Inngår i: Teorija slucajnych processov, ISSN 0321-3900, Vol. 3(19), nr 1-2, s. 76-87Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    The author describes his experience in developing the course of statistical analysis applications in geochemistry. During this course the students study graphical statistical methods, exploratory data analysis, smoothing and outlier treatment, comparison of two and more samples and regression analysis including residual analysis and influence analysis. The geochemistry part of the course includes monitoring techniques for water quality data and normalising of hydrogeochemical data. The analysis was performed by Data Desk program. The specific feature of the course was that two teachers one from geochemistry and one from statistics were active simultaneously in the classroom during part of the course. They participated in discussions on the interpretation of results. Students were actively involved in the discussions and statistical analysis.

  • 19304.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    The circular safety region: a useful graphical tool in capability analysis2005Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 21, nr 5, s. 529-538Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    When measuring the capability of a manufacturing process some form of process capability index is often used. To assess the capability, using a random sample, confidence intervals or hypothesis tests on the index are frequently used. Here, an alternative approach is presented. A process is usually defined to be capable if the capability index exceeds a stated threshold value, e.g. Cpm > 4/3. This inequality can be expressed graphically as a region in the plane defined by the process parameters (µ, σ). This graphical region, or safety region, similar to a confidence region for ((µ, σ). can be plotted to test for capability. Under the assumption of normality a circular safety region for the capability index Cpm is constructed that can be used to draw conclusions about the capability at a given significance level. This simple graphical approach is helpful when trying to understand if it is the variability, the deviation from target or both that need to be reduced to improve the capability. Using circular regions several characteristics can be monitored in the same plot.

  • 19305.
    Vännman, Kerstin
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Two teachers, a computer, and statistics: experiences of teacher cooperation1995Inngår i: 8th SEFI European Seminar on Mathematics in Engineering Education: proceedinga, Prague, 1995, Vol. 1, s. 205-211Konferansepaper (Fagfellevurdert)
  • 19306.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability indices for one-sided specification intervals and skewed distributions2007Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 23, nr 6, s. 755-765Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    One-sided specification intervals are frequent in industry, but the process capability analysis is not well developed theoretically for this case. Most of the published articles about process capability focus on the case when the specification interval is two-sided. Furthermore, usually the assumption of normality is necessary. However, a common practical situation is process capability analysis when the studied characteristic has a skewed distribution with a long tail towards large values and an upper specification limit only exists. In such situations it is not uncommon that the smallest possible value of the characteristic is 0 and that this also is the best value to obtain. We propose a new class of indices for such a situation with an upper specification limit, a target value zero, and where the studied characteristic has a skewed, zero-bound distribution with a long tail towards large values. A confidence interval for an index in the proposed class, as well as a decision procedure for deeming a process as capable or not, is discussed. These results are based on large sample properties of the distribution of a suggested estimator of the index. A simulation study is performed, assuming the quality characteristic is Weibull distributed, to investigate the properties of the suggested decision procedure.

  • 19307.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability plots for one-sided specification limits2007Inngår i: Quality Technology & Quantitative Management, ISSN 1684-3703, E-ISSN 1811-4857, Vol. 4, nr 4, s. 569-590Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We extend the idea of process capability plots from the case of two-sided specification intervals to derive a graphical method useful when doing capability analysis having one-sided specification limits. The derived process capability plots are based on existing capability indices for one-sided specification limits. Both the cases with and without a target value are investigated. Under the assumption of normality we suggest estimated process capability plots to be used to assess process capability at a given significance level. Theoretical results are given for determining the significance level as well as power for the method. The presented graphical approach is helpful to determine if it is the variability, the deviation from target, or both that need to be reduced to improve the capability. Examples are presented.

  • 19308.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Albing, Malin
    Process capability plots for one-sided specification limits2005Rapport (Annet vitenskapelig)
  • 19309.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Castagliola, Philippe
    Monitoring Unstable But Capable Processes with the CP(u,v) System of Capability Indices2006Inngår i: 2006 proceedings: Twelfth ISSAT International Conference Reliability and Quality in Design : August 3 - 5, 2006, Chicago, Illinois, U.S.A. / [ed] Hoang Pham, New Brunswick, NJ: International Society of Science and Applied Technologies , 2006, s. 220-224Konferansepaper (Fagfellevurdert)
  • 19310.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Cederwall, Krister
    Luleå tekniska universitet.
    A statistical approach to composite beams1982Inngår i: Nordic Concrete Research, Publication No 1, The Nordic Concrete Federation , 1982Konferansepaper (Annet vitenskapelig)
  • 19311.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Dunkels, Andrejs
    Luleå tekniska universitet.
    Att vara detektiv bland siffror från verkligheten1985Inngår i: Nämnaren : tidskrift för matematikundervisning, ISSN 0348-2723, nr 4, s. 69-Artikkel i tidsskrift (Annet vitenskapelig)
  • 19312.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Dunkels, Andrejs
    Luleå tekniska universitet.
    Kreativ statistik med EDA1984Bok (Annet (populærvitenskap, debatt, mm))
  • 19313.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hubele, Norma
    The distribution of Cpm when using a pooled variance estimator with some results2001Inngår i: 2001 proceedings, American Statistical Association: papers presented at the annual meeting of the American Statistical Association: joint statistical meetings, Atlanta, Georgia, August 5 - 9, 2001, and other ASA-sponsored conferences, Acoustical Society of America (ASA), 2001Konferansepaper (Fagfellevurdert)
  • 19314.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hubele, Norma Faris
    Arizona State University.
    Distributional properties of estimated capability indices based on subsamples2003Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 19, nr 2, s. 111-128Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Under the assumption of normality, the distribution of estimators of a class of capability indices, containing the indices , , and , is derived when the process parameters are estimated from subsamples. The process mean is estimated using the grand average and the process variance is estimated using the pooled variance from subsamples collected over time for an in-control process. The derived theory is then applied to study the use of hypothesis testing to assess process capability. Numerical investigations are made to explore the effect of the size and number of subsamples on the efficiency of the hypothesis test for some indices in the studied class. The results for and indicate that, even when the total number of sampled observations remains constant, the power of the test decreases as the subsample size decreases. It is shown how the power of the test is dependent not only on the subsample size and the number of subsamples, but also on the relative location of the process mean from the target value. As part of this investigation, a simple form of the cumulative distribution function for the non-central -distribution is also provided

  • 19315.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Hubele, Norma Faris
    The effect of subsample sizes on Cpm when using pooled and un-pooled variance estimators2001Rapport (Annet vitenskapelig)
  • 19316.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Klefsjö, Bengt
    Luleå tekniska universitet, Institutionen för ekonomi, teknik och samhälle, Industriell Ekonomi.
    Ärligare undersökningar tack vare slumpen1983Inngår i: Elementa : tidskrift för matematik, fysik och kemi, ISSN 0013-5933, nr 1, s. 30-34Artikkel i tidsskrift (Annet (populærvitenskap, debatt, mm))
  • 19317.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kotz, Samuel
    A superstructure of capability indices: asymptotics and its implications1995Inngår i: International Journal of Reliability, Quality and Safety Engineering (IJRQSE), ISSN 0218-5393, Vol. 2, nr 4, s. 343-360Artikkel i tidsskrift (Fagfellevurdert)
  • 19318.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kotz, Samuel
    University of Maryland.
    A superstructure of capability indices: distributional properties and implications1995Inngår i: Scandinavian Journal of Statistics, ISSN 0303-6898, E-ISSN 1467-9469, Vol. 22, nr 4, s. 477-491Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A family of capability indices, containing the indices Cp, Cpk, Cpm, and Cpmk, has earlier been defined by {\it K. Vännman} [Stat. Sin. 5, No. 2, 805-820 (1995). for the case of two-sided specification intervals. By varying the parameters of the family various indices with suitable properties can be obtained. Under the assumption of normality an explicit form of the distribution of the family of the estimated indices is provided. Numerical investigations are made to explore the behavior of the estimators of the indices for different values of the parameters. A decision rule, based on the estimator, that can be used to determine whether the process can be considered capable or not is provided and suitable criteria for choosing an index from the family are suggested.

  • 19319.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kulachi, Murat
    Informatics and Mathematical Modelling, Technical University of Denmark.
    A model free approach to eliminate autocorrelation when testing for process capability2006Rapport (Annet vitenskapelig)
  • 19320.
    Vännman, Kerstin
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Kulahci, Murat
    Informatics and Mathematical Modelling, Technical University of Denmark.
    A model-free approach to eliminate autocorrelation when testing for process capability2008Inngår i: Quality and Reliability Engineering International, ISSN 0748-8017, E-ISSN 1099-1638, Vol. 24, nr 2, s. 213-228Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    There is an increasing use of on-line data acquisition systems in industry. This usually leads to autocorrelated data and implies that the assumption of independent observations has to be re-examined. Most decision procedures for capability analysis assume independent data. In this article we present a new way of performing capability analysis when data are autocorrelated. This method is based on what can be called the 'iterative skipping' strategy. In that, by skipping a pre-determined number of observations, e.g. considering every fifth observation, the data set is divided into subsamples for which the independence assumption may be valid. For each such subsample of the data we estimate a capability index. Then traditional tests, assuming independence, can be performed based on each estimated capability index from the subsamples. By combining the information from each test statistic based on the subsamples in a suitable way, a new and efficient decision procedure is obtained. We discuss different ways of combining the information from these individual tests. A main appeal of our proposed method is that no time-series model is needed.

  • 19321.
    Waara, Einar
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Matematiska institutionen, Analys och sannolikhetsteori.
    Gauss and Jacobi Sums and the Congruence Zeta Function2018Independent thesis Basic level (degree of Bachelor), 10 poäng / 15 hpOppgave
  • 19322.
    Wadbro, Eddie
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Analysis and design of acoustic transition sections for impedance matching and mode conversion2014Inngår i: Structural and multidisciplinary optimization (Print), ISSN 1615-147X, E-ISSN 1615-1488, Vol. 50, nr 3, s. 395-408Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    This work considers the problem of designing passive transition sections to provide impedance matching and mode conversion for acoustic wave propagation. The base configuration consists of two waveguides connected by a transition section. The objective is to find a placement of material inside this section to make it function as an impedance matcher or a mode converter with minimal losses. A finite element approximation of the Helmholtz equation in a truncated domain together with Dirichlet-to-Neumann type non-reflecting boundary conditions models the wave propagation. Material distribution techniques solve the resulting topology optimization problem and the resulting interfacial devices show good transmission properties.

  • 19323.
    Wadbro, Eddie
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Microwave tomographic imaging as a sequence of topology optimization problems2010Inngår i: Optimization and Engineering, ISSN 1389-4420, E-ISSN 1573-2924, Vol. 11, s. 597-610Artikkel i tidsskrift (Fagfellevurdert)
  • 19324.
    Wadbro, Eddie
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    On the far-field properties of an acoustic horn2006Rapport (Annet vitenskapelig)
  • 19325.
    Wadbro, Eddie
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Topology optimization for acoustic wave propagation problems2006Licentiatavhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    The aim of this study is to develop numerical techniques for the analysis and optimization of acoustic horns for time harmonic wave propagation. An acoustic horn may be viewed as an impedance transformer, designed to give an impedance matching between the feeding waveguide and the surrounding air. When modifying the shape of the horn, the quality of this impedance matching changes, as well as the angular distribution of the radiated wave in the far field (the directivity). The dimensions of the horns considered are in the order of the wavelength. In this wavelength region the wave physics is complicated, and it is hard to apply elementary physical reasoning to enhance the performance of the horn. Here, topology optimization is applied to improve the efficiency and to gain control over the directivity of the acoustic horn.

  • 19326.
    Wadbro, Eddie
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Topology Optimization for Wave Propagation Problems2009Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis considers topology optimization methods for wave propagation problems. These methods make no a priori assumptions on topological properties such as the number of bodies involved in the design. The performed studies address problems from two different areas, acoustic wave propagation and microwave tomography. The final study discusses implementation aspects concerning the efficient solution of large scale material distribution problems.

    Acoustic horns may be viewed as impedance transformers between the feeding waveguide and the surrounding air. Modifying the shape of an acoustic horn changes the quality of the impedance match as well as the angular distribution of the radiated waves in the far field (the directivity). This thesis presents strategies to optimize acoustic devices with respect to efficiency and directivity simultaneously. The resulting devices exhibit desired far field properties and high efficiency throughout wide frequency ranges.

    In microwave tomography, microwaves illuminate an object, and measurements of the scattered electrical field are used to depict the object's conductive and dielectric properties. Microwave tomography has unique features for medical applications. However, the reconstruction problem is difficult due to strongly diffracting waves in combination with large dielectric contrasts. This thesis demonstrates a new method to perform the reconstruction using techniques originally developed for topology optimization of linearly elastic structures. Numerical experiments illustrate the method and produce good estimates of dielectric properties corresponding to biological objects.

    Material distribution problems are typically cast as large (for high resolutions) nonlinear programming problems over coefficients in partial differential equations. Here, the computational power of a modern graphics processing unit (GPU) efficiently solves a pixel based material distribution problem with over 4 million unknowns using a gradient based optimality criteria method.

  • 19327.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    High contrast microwave tomography using topology optimization techniques2007Inngår i: Proc. 8th International Conference on Mathematical and Numerical Aspects of Waves: WAVES 2007, Reading, UK: Department of Mathematics, University of Reading , 2007, s. 448-450Konferansepaper (Annet vitenskapelig)
  • 19328.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    High contrast microwave tomography using topology optimization techniques2010Inngår i: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 234, s. 1773-1780Artikkel i tidsskrift (Fagfellevurdert)
  • 19329.
    Wadbro, Eddie
    et al.
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    Berggren, Martin
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    High contrast microwave tomography using topology optimization techniques2007Inngår i: Proceedings of Waves 2007: The 8th International Conference on Mathematical and Numerical Aspects of Waves / [ed] N. Biggs et al., Reading, U.K.: University of Reading , 2007, s. 448-450Konferansepaper (Fagfellevurdert)
  • 19330.
    Wadbro, Eddie
    et al.
    Uppsala University,Department of Information Technology, Division of Scientific Computing.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    High contrast microwave tomography using topology optimization techniques2010Inngår i: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 234, nr 6, s. 1773-1780Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Microwave tomography for medical applications leads to a difficult reconstruction problem for the dielectric properties of biological tissue due to strongly diffracting waves in combination with large dielectric contrasts. We apply the material distribution technique used for topology optimization of elastic structures in order to solve the nonlinear least-squares problem underlying the reconstruction problem. Using simulated numerical data with an approximate signal-to-noise ratio of 40 dB and geometrical a priori information on the unknown objects, we obtain good estimates of the dielectric properties corresponding to biological objects.

  • 19331.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    Megapixel topology optimization on a graphics processing unit2009Inngår i: SIAM Review, ISSN 0036-1445, E-ISSN 1095-7200, Vol. 51, s. 707-721Artikkel i tidsskrift (Fagfellevurdert)
  • 19332.
    Wadbro, Eddie
    et al.
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Megapixel topology optimization on a graphics processing unit2009Inngår i: SIAM Review, ISSN 0036-1445, E-ISSN 1095-7200, Vol. 51, nr 4, s. 707-721Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    We show how the computational power and programmability of modern graphics processing units (GPUs) can be used to efficiently solve large-scale pixel-based material distribution problems using a gradient-based optimality criterion method. To illustrate the principle, a so-called topology optimization problem that results in a constrained nonlinear programming problem with over 4 million decision variables is solved on a commodity GPU.

  • 19333.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    Microwave tomography using topology optimization techniques2008Inngår i: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 30, s. 1613-1633Artikkel i tidsskrift (Fagfellevurdert)
  • 19334.
    Wadbro, Eddie
    et al.
    Uppsala University, Department of Information Technology, Division of Scientific Computing.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Microwave tomography using topology optimization techniques2008Inngår i: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 30, nr 3, s. 1613-1633Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Microwave tomography is a technique in which microwaves illuminate a specimen, and measurements of the scattered electrical field are used to determine and depict the specimen's dielectric and conductive properties. This article presents a new method to perform the reconstruction. The reconstruction method is illustrated by assuming time harmonic scattering in two space dimensions in a setup tailored for medical applications. We prove that the resulting constrained nonlinear least-squares problem admits a solution. The governing Helmholtz equation is discretized by using the finite-element method, and the dielectric properties are allowed to attain different values at each element within a given region. The reconstruction algorithm uses methodologies borrowed from topology optimization of linearly elastic structures. Numerical examples illustrate the reconstruction method in a parameter range typical for human tissue and for the challenging case where the size of the object is in the same order as the wavelength. A reasonable estimate of the dielectric properties is obtained by using one observation per 20 unknowns when the permittivity is allowed to vary continuously within a given interval. Using a priori information that the permittivity attains only certain values results in a good estimate and a sharp image. As opposed to topology optimization for structures, there is no indication of mesh dependency and checkerboarding when forcing the permittivity to attain discrete values.

  • 19335.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Topology optimization of an acoustic horn2006Inngår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 196, s. 420-436Artikkel i tidsskrift (Fagfellevurdert)
  • 19336.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Topology optimization of wave transducers2006Inngår i: Topological Design Optimization of Structures, Machines and Materials: Status and Perspectives, Dordrecht, The Netherlands: Springer, 2006, s. 301-310Konferansepaper (Fagfellevurdert)
  • 19337.
    Wadbro, Eddie
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Engström, Christian
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för matematik och matematisk statistik.
    Topology and shape optimization of plasmonic nano-antennas2015Inngår i: Computer Methods in Applied Mechanics and Engineering, ISSN 0045-7825, E-ISSN 1879-2138, Vol. 293, s. 155-169Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Metallic nano-antennas are devices used to concentrate the energy in light into regions that are much smaller than the wavelength. These structures are currently used to develop new measurement and printing techniques, such as optical microscopy with sub-wavelength resolution, and high-resolution lithography. Here, we analyze and design a nano-antenna in a two-dimensional setting with the source being a planar TE-polarized wave. The design problem is to place silver and air in a pre-specified design region to maximize the electric energy in a small given target region. At optical frequencies silver exhibits extreme dielectric properties, having permittivity with a negative real part. We prove existence and uniqueness of solutions to the governing nonstandard Helmholtz equation with absorbing boundary conditions. To solve the design optimization problem, we develop a two-stage procedure. The first stage uses a material distribution parameterization and aims at finding a conceptual design without imposing any a priori information about the number of shapes of components comprising the nano-antenna. The second design stage uses a domain variation approach and aims at finding a precise shape. Both of the above design problems are formulated as non-linear mathematical programming problems that are solved using the method of moving asymptotes. The final designs perform very well and the electric energy in the target region is several orders of magnitude larger than when there is only air in the design region. The performance of the optimized designs is verified with a high order interior penalty method.

  • 19338.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Udawalpola, Rajitha
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Shape and topology optimization of an acoustic horn-lens combination2007Inngår i: Proc. 8th International Conference on Mathematical and Numerical Aspects of Waves: WAVES 2007, Reading, UK: Department of Mathematics, University of Reading , 2007, s. 451-453Konferansepaper (Annet vitenskapelig)
  • 19339.
    Wadbro, Eddie
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Udawalpola, Rajitha
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för teknisk databehandling. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    Shape and topology optimization of an acoustic horn-lens combination2010Inngår i: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 234, s. 1781-1787Artikkel i tidsskrift (Fagfellevurdert)
  • 19340. Wadbro, Eddie
    et al.
    Zahedi, Sara
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för beräkningsvetenskap. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Kreiss, Gunilla
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för beräkningsvetenskap. Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Numerisk analys.
    Berggren, Martin
    A uniformly well-conditioned, unfitted Nitsche method for interface problems2013Inngår i: BIT Numerical Mathematics, ISSN 0006-3835, E-ISSN 1572-9125, Vol. 53, s. 791-820Artikkel i tidsskrift (Fagfellevurdert)
  • 19341. Wadbro, Eddie
    et al.
    Zahedi, Sara
    KTH, Skolan för datavetenskap och kommunikation (CSC), Numerisk analys, NA (stängd 2012-06-30).
    Kreiss, Gunilla
    Berggren, Martin
    A uniformly well-conditioned, unfitted Nitsche method for interface problems2013Inngår i: BIT Numerical Mathematics, ISSN 0006-3835, E-ISSN 1572-9125, Vol. 53, s. 791-820Artikkel i tidsskrift (Fagfellevurdert)
  • 19342.
    Wadbro, Eddie
    et al.
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    Zahedi, Sara
    Department of Information Technology, Uppsala University, Uppsala.
    Kreiss, Gunilla
    Department of Information Technology, Uppsala University, Uppsala.
    Berggren, Martin
    Umeå universitet, Teknisk-naturvetenskapliga fakulteten, Institutionen för datavetenskap.
    A uniformly well-conditioned, unfitted Nitsche method for interface problems2013Inngår i: BIT Numerical Mathematics, ISSN 0006-3835, E-ISSN 1572-9125, Vol. 53, nr 3, s. 791-820Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    A finite element method for elliptic partial differential equations that allows for discontinuities along an interface not aligned with the mesh is presented. The solution on each side of the interface is separately expanded in standard continuous, piecewise-linear functions, and jump conditions at the interface are weakly enforced using a variant of Nitsche’s method. In our method, the solutions on each side of the interface are extended to the entire domain which results in a fixed number of unknowns independent of the location of the interface. A stabilization procedure is included to ensure well-defined extensions. We prove that the method provides optimal convergence order in the energy and the L2 norms and a condition number of the system matrix that is independent of the position of the interface relative to the mesh. Numerical experiments confirm the theoretical results and demonstrate optimal convergence order also for the pointwise errors.

  • 19343.
    Waern, Caroline
    et al.
    Växjö universitet, Fakulteten för humaniora och samhällsvetenskap, Institutionen för pedagogik.
    Johansson, Emma
    Växjö universitet, Fakulteten för humaniora och samhällsvetenskap, Institutionen för pedagogik.
    Barns möte med matematik: Hur synliggörs matematiken för förskolebarn?2007Independent thesis Basic level (degree of Bachelor), 10 poäng / 15 hpOppgave
    Abstract [sv]

    Denna studie handlar om synliggörandet av matematik i förskolan. Då läroplanen föreskriver att alla barn skall utmanas och utvecklas i sitt matematiska tänkande och lärande så är syftet med undersökningen att få mer kunskap om hur och vad det är för matematik som synliggörs i förskolan. Vi ville också ta reda på hur pedagogerna ser på sin roll och kompetens i arbetet med små barn och matematik. Genom vår litteraturstudie har vi fått många idéer och kunskap om hur man kan arbeta med matematik i förskolan och förskoleklass. Vi genomförde en kvalitativ studie där vi använde oss av enkäter med öppna frågor. Dessa enkäter riktade sig till hela arbetslaget, detta för att vi ville få igång en diskussion i arbetslaget om hur de arbetar kring matematiken. Sammanfattningsvis visar resultatet att pedagogerna har en medvetenhet om vad matematik är och hur den ska synliggöras för förskolebarnen. Genom vårt arbete har vi insett, vilken stor betydelse vi pedagoger har för att matematiken ska bli synlig för barnen i förskolan. Vi hoppas att detta arbete kan inspirera andra pedagoger att få in mer matematik i den dagliga verksamheten i förskolan.

  • 19344.
    Waern, Tom
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för beräkningsvetenskap.
    Mahmod, Shad
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Matematisk-datavetenskapliga sektionen, Institutionen för informationsteknologi, Avdelningen för beräkningsvetenskap.
    Trendanalys inom futurehandel2016Independent thesis Basic level (degree of Bachelor), 10 poäng / 15 hpOppgave
    Abstract [sv]

    Futurehandel är en investeringsform där trendanalys vanligen tillämpas. Målet med projektet är att prediktera marknadsförändringar genom att optimera modeller som använder sig av historiska marknadspriser. Resultaten från dessa metoder jämförs sedan med resultaten erhållna genom att endast ta en lång position. För att vidare jämföra resultaten hos de olika metoderna har deras sharpekvoter jämförts. Resultatet av studien visar att överlag har linjära anpassningar genererat högst sharpekvoter, främst ridge regression. Undantaget är vid handel av valutor då medelvärdesbaserade metoder är de mest tillförlitliga.

  • 19345.
    Waernbaum, Ingeborg
    Umeå universitet, Samhällsvetenskapliga fakulteten, Statistiska institutionen.
    Covariate selection and propensity score specification in causal inference2008Doktoravhandling, med artikler (Annet vitenskapelig)
    Abstract [en]

    This thesis makes contributions to the statistical research field of causal inference in observational studies. The results obtained are directly applicable in many scientific fields where effects of treatments are investigated and yet controlled experiments are difficult or impossible to implement.

    In the first paper we define a partially specified directed acyclic graph (DAG) describing the independence structure of the variables under study. Using the DAG we show that given that unconfoundedness holds we can use the observed data to select minimal sets of covariates to control for. General covariate selection algorithms are proposed to target the defined minimal subsets.

    The results of the first paper are generalized in Paper II to include the presence of unobserved covariates. Morevoer, the identification assumptions from the first paper are relaxed.

    To implement the covariate selection without parametric assumptions we propose in the third paper the use of a model-free variable selection method from the framework of sufficient dimension reduction. By simulation the performance of the proposed selection methods are investigated. Additionally, we study finite sample properties of treatment effect estimators based on the selected covariate sets.

    In paper IV we investigate misspecifications of parametric models of a scalar summary of the covariates, the propensity score. Motivated by common model specification strategies we describe misspecifications of parametric models for which unbiased estimators of the treatment effect are available. Consequences of the misspecification for the efficiency of treatment effect estimators are also studied.

  • 19346.
    Waernbaum, Ingeborg
    Umeå universitet, Samhällsvetenskapliga fakulteten, Statistiska institutionen.
    Model misspecification and robustness in causal inference: comparing matching with doubly robust estimation2012Inngår i: Statistics in Medicine, ISSN 0277-6715, E-ISSN 1097-0258, Vol. 31, nr 15, s. 1572-1581Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    In this paper we compare the robustness properties of a matching estimator with a doubly robust estimator. We describe the robustness properties of matching and subclassification estimators by showing how misspecification of the propensity score model cam result in consistent estimation of the average causal effect. The propensity scores are covariate scores, which are a class of functions that removes bias due to all observed covariates. When matching on a parametric model (e.g. a propensity or prognostic score), the matching estimator is robust to model misspecifications if the misspecified model belongs to the class of covariate scores. The implication is that there are multiple possibilities for the matching estimator in contrast to the doubly robust estimator in which the researcher has two chances to make reliable inference. In simulations, we compare the finite sample properties of the matching estimator with a simple inverse probability weighting estimator and a doubly robust estimator. For the misspecifications in our study the mean square error of the matching estimator is smaller than the mean square error of both the simple inverse probability weighting estimator and the doubly robust estimator-

  • 19347.
    Waernbaum, Ingeborg
    Umeå universitet, Samhällsvetenskapliga fakulteten, Statistiska institutionen.
    Propensity score model specification for estimation of average treatment effects2010Inngår i: Journal of Statistical Planning and Inference, ISSN 0378-3758, E-ISSN 1873-1171, Vol. 140, nr 7, s. 1948-1956Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Treatment effect estimators that utilize the propensity score as a balancing score, e.g., matching and blocking estimators are robust to misspecifications of the propensity score model when the misspecification is a balancing score. Such misspecifications arise from using the balancing property of the propensity score in the specification procedure. Here, we study misspecifications of a parametric propensity score model written as a linear predictor in a strictly monotonic function, e.g. a generalized linear model representation. Under mild assumptions we show that for misspecifications, such as not adding enough higher order terms or choosing the wrong link function, the true propensity score is a function of the misspecified model. Hence, the latter does not bring bias to the treatment effect estimator. It is also shown that a misspecification of the propensity score does not necessarily lead to less efficient estimation of the treatment effect. The results of the paper are highlighted in simulations where different misspecifications are studied.

  • 19348.
    Wagner, J
    et al.
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Newman, R C
    Imperial College of Science.
    Davidson, B R
    Imperial College of Science.
    Westwater, S P
    University of Liverpool.
    Bullough, T J
    University of Liverpool.
    Joyce, T B
    University of Liverpool.
    Latham, Chris
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Jones, R
    University of Exeter.
    Öberg, Sven
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Di-Carbon defects in annealed highly carbon doped GaAs1997Inngår i: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 78, nr 1, s. 74-77Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Formation of bonded dicarbon C-C centers is deduced from the observation of Raman lines at 1742, 1708, and 1674 cm -1 in GaAs codoped with 12C and 13C after annealing at 850 °C with concomitant loss of vibrational scattering from CAs. The frequencies agree with results of ab initio theory for a C-C split interstitial (deep donor) formed by the trapping of a mobile interstitial C (displaced CAs) atom by an undisplaced CAs acceptor. Other mechanisms of carrier loss are inferred since a weaker Raman triplet is detected at 1859, 1824, and 1788 cm -1 from a different C-C complex.

  • 19349.
    Wagner, Philipp
    et al.
    Université de Nantes.
    Ivanovskaya, Viktoria V
    Université de Nantes.
    Rayson, Mark
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Briddon, Patrick
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Matematiska vetenskaper.
    Ewels, Christopher P
    Université de Nantes.
    Mechanical properties of nanosheets and nanotubes investigated using a new geometry independent volume definition2013Inngår i: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, nr 15, s. 155302-Artikkel i tidsskrift (Fagfellevurdert)
    Abstract [en]

    Cross-sectional area and volume become difficult to define as material dimensions approach the atomic scale. This limits the transferability of macroscopic concepts such as Young's modulus. We propose a new volume definition where the enclosed nanosheet or nanotube average electron density matches that of the parent layered bulk material. We calculate the Young's moduli for various nanosheets (including graphene, BN and MoS2) and nanotubes. Further implications of this new volume definition such as a Fermi level dependent Young's modulus and out-of-plane Poisson's ratio are shown.

  • 19350.
    Wagner, Stefan
    Blekinge Tekniska Högskola, Fakulteten för teknikvetenskaper, Institutionen för matematik och naturvetenskap.
    Secondary Characteristic Classes of Lie Algebra ExtensionsManuskript (preprint) (Annet vitenskapelig)
    Abstract [en]

    We introduce a notion of secondary characteristic classes of Lie algebra extensions. As a spin-off of our construction we obtain a new proof of Lecomte’s generalization of the Chern–Weil homomorphism.

384385386387388389390 19301 - 19350 of 20385
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