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  • 1.
    Abdul Salam, Parveena Shamim
    et al.
    Tu Kaiserslautern, Germany;Iit Madras, India.
    Bock, Wolfgang
    Tu Kaiserslautern, Germany.
    Klar, Axel
    Tu Kaiserslautern, Germany.
    Tiwari, Sudarshan
    Tu Kaiserslautern, Germany.
    Disease contagion models coupled to crowd motion and mesh-free simulation2021In: Mathematical Models and Methods in Applied Sciences, ISSN 0218-2025, Vol. 31, no 6, p. 1277-1295Article in journal (Refereed)
    Abstract [en]

    Modeling and simulation of disease spreading in pedestrian crowds have recently become a topic of increasing relevance. In this paper, we consider the influence of the crowd motion in a complex dynamical environment on the course of infection of the pedestrians. To model the pedestrian dynamics, we consider a kinetic equation for multi-group pedestrian flow based on a social force model coupled with an Eikonal equation. This model is coupled with a non-local SEIS contagion model for disease spread, where besides the description of local contacts, the influence of contact times has also been modeled. Hydrodynamic approximations of the coupled system are derived. Finally, simulations of the hydrodynamic model are carried out using a mesh-free particle method. Different numerical test cases are investigated, including uni- and bi-directional flow in a passage with and without obstacles.

  • 2.
    Agram, Nacira
    et al.
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Djehiche, Boualem
    KTH Royal instute of technology, Sweden.
    On a class of reflected backward stochastic Volterra integral equations and related time-inconsistent optimal stopping problems2021In: Systems & control letters (Print), ISSN 0167-6911, E-ISSN 1872-7956, Vol. 155, article id 104989Article in journal (Refereed)
    Abstract [en]

    We introduce a class of one-dimensional continuous reflected backward stochastic Volterra integral equations driven by Brownian motion, where the reflection keeps the solution above a given stochastic process (lower obstacle). We prove existence and uniqueness by a fixed point argument and derive a comparison result. Moreover, we show how the solution of our problem is related to a time-inconsistent optimal stopping problem and derive an optimal strategy. 

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  • 3.
    Agram, Nacira
    et al.
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Haadem, Sven
    University of Oslo, Norway.
    Øksendal, Bernt
    University of Oslo, Norway.
    Proske, Frank
    University of Oslo, Norway.
    Optimal stopping, randomized stopping and singular control with partial information flowManuscript (preprint) (Other academic)
    Abstract [en]

    The purpose of this paper is two-fold: We extend the well-known relation between optimal stopping and randomized stopping of a given stochastic process to a situation where the available information flow is a sub-filtration of the filtration of the process. We call these problems optimal stopping and randomized stopping with partial information. Following an idea of Krylov [K] we introduce a special singular stochastic control problem with partial information and show that this is also equivalent to the partial information optimal stopping and randomized stopping problems. Then we show that the solution of this singular control problem can be expressed in terms of (partial information) variational inequalities, which in turn can be rewritten as a reflected backward stochastic differential equation (RBSDE) with partial information.

  • 4.
    Agram, Nacira
    et al.
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Hilbert, Astrid
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Øksendal, Bernt
    University of Oslo, Norway.
    Singular control of SPDEs with space-mean dynamics2020In: Mathematical Control & Related Fields, ISSN 2156-8472, Vol. 10, no 2, p. 425-441Article in journal (Refereed)
    Abstract [en]

    We consider the problem of optimal singular control of a stochastic partial differential equation (SPDE) with space-mean dependence. Such systems are proposed as models for population growth in a random environment. We obtain sufficient and necessary maximum principles for such control problems. The corresponding adjoint equation is a reflected backward stochastic partial differential equation (BSPDE) with space-mean dependence. We prove existence and uniqueness results for such equations. As an application we study optimal harvesting from a population modelled as an SPDE with space-mean dependence.

  • 5.
    Al-Talibi, Haidar
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Nelson-type Limits for α-Stable Lévy Processes2010Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Brownian motion has met growing interest in mathematics, physics and particularly in finance since it was introduced in the beginning of the twentieth century. Stochastic processes generalizing Brownian motion have influenced many research fields theoretically and practically. Moreover, along with more refined techniques in measure theory and functional analysis more stochastic processes were constructed and studied. Lévy processes, with Brownian motionas a special case, have been of major interest in the recent decades. In addition, Lévy processes include a number of other important processes as special cases like Poisson processes and subordinators. They are also related to stable processes.

    In this thesis we generalize a result by S. Chandrasekhar [2] and Edward Nelson who gave a detailed proof of this result in his book in 1967 [12]. In Nelson’s first result standard Ornstein-Uhlenbeck processes are studied. Physically this describes free particles performing a random and irregular movement in water caused by collisions with the water molecules. In a further step he introduces a nonlinear drift in the position variable, i.e. he studies the case when these particles are exposed to an external field of force in physical terms.

    In this report, we aim to generalize the result of Edward Nelson to the case of α-stable Lévy processes. In other words we replace the driving noise of a standard Ornstein-Uhlenbeck process by an α-stable Lévy noise and introduce a scaling parameter uniformly in front of all vector fields in the cotangent space, even in front of the noise. This corresponds to time being sent to infinity. With Chandrasekhar’s and Nelson’s choice of the diffusion constant the stationary state of the velocity process (which is approached as time tends to infinity) is the Boltzmann distribution of statistical mechanics.The scaling limits we obtain in the absence and presence of a nonlinear drift term by using the scaling property of the characteristic functions and time change, can be extended to other types of processes rather than α-stable Lévy processes.

    In future, we will consider to generalize this one dimensional result to Euclidean space of arbitrary finite dimension. A challenging task is to consider the geodesic flow on the cotangent bundle of a Riemannian manifold with scaled drift and scaled Lévy noise. Geometrically the Ornstein-Uhlenbeck process is defined on the tangent bundle of the real line and the driving Lévy noise is defined on the cotangent space.

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  • 6.
    Aminbaghai, Mehdi
    et al.
    Vienna University of Technology, Austria.
    Dorn, Michael
    Linnaeus University, Faculty of Technology, Department of Building and Energy Technology. Vienna University of Technology, Austria.
    Eberhardsteiner, Josef
    Vienna University of Technology, Austria.
    Pichler, Bernhard
    Vienna University of Technology, Austria.
    A Matrix-Vector Operation-Based Numerical Solution Method for Linear m-th Order Ordinary Differential Equations: Application to Engineering Problems2013In: Advances in Applied Mathematics and Mechanics, ISSN 2070-0733, E-ISSN 2075-1354, Vol. 5, no 3, p. 269-308Article in journal (Refereed)
    Abstract [en]

    Many problems in engineering sciences can be described by linear, inhomogeneous, m-th order ordinary differential equations (ODEs) with variable coefficients. For this wide class of problems, we here present a new, simple, flexible, and robust solution method, based on piecewise exact integration of local approximation polynomials as well as on averaging local integrals. The method is designed for modern mathematical software providing efficient environments for numerical matrix-vector operation-based calculus. Based on cubic approximation polynomials, the presented method can be expected to perform (i) similar to the Runge-Kutta method, when applied to stiff initial value problems, and (ii) significantly better than the finite difference method, when applied to boundary value problems. Therefore, we use the presented method for the analysis of engineering problems including the oscillation of a modulated torsional spring pendulum, steady-state heat transfer through a cooling web, and the structural analysis of a slender tower based on second-order beam theory. Related convergence studies provide insight into the satisfying characteristics of the proposed solution scheme.

  • 7.
    Andersson, Anders
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering. Matematik.
    A modified Schwarz–Christoffel mapping for regions with piecewise smooth boundaries2008In: Journal of Computational and Applied Mathematics, ISSN 0377-0427, Vol. 213, no 1, p. 56-70Article in journal (Refereed)
    Abstract [en]

    A method where polygon corners in Schwarz-Christoffel mappings are rounded, is used to construct mappings from the upper half-plane to regions bounded by arbitrary piecewise smooth curves. From a given curve, a polygon is constructed by taking tangents to the curve in a number of carefully chosen so called tangent points. The Schwarz-Christoffel mapping for that polygon is then constructed and modified to round the corners.

    Since such a modification causes effects on the polygon outside the rounded corners, the parameters in the mapping have to be re-determined. This is done by comparing side-lengths in tangent polygons to the given curve and the curve produced by the modified Schwarz-Christoffel mapping. The set of equations that this comparison gives, can normally be solved using a quasi--Newton method.

    The resulting function maps the upper half--plane on a region bounded by a curve that apart from possible vertices is smooth, i.e., one time continuously differentiable, that passes through the tangent points on the given curve, has the same direction as the given curve in these points and changes direction monotonically between them. Furthermore, where the original curve has a vertex, the constructed curve has a vertex with the same inner angle.

    The method is especially useful for unbounded regions with smooth boundary curves that pass infinity as straight lines, such as channels with parallel walls at the ends. These properties are kept in the region produced by the constructed mapping.

  • 8.
    Andersson, Anders
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Modified Schwarz–Christoffel mappings using approximate curve factors2009In: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 233, no 4, p. 1117-1127Article in journal (Refereed)
    Abstract [en]

    The Schwarz–Christoffel mapping from the upper half-plane to a polygonal region in the complex plane is an integral of a product with several factors, where each factor corresponds to a certain vertex in the polygon. Different modifications of the Schwarz–Christoffel mapping in which factors are replaced with the so-called curve factors to achieve polygons with rounded corners are known since long times. Among other requisites, the arguments of a curve factor and its correspondent scl factor must be equal outside some closed interval on the real axis.

    In this paper, the term approximate curve factor is defined such that many of the already known curve factors are included as special cases. Additionally, by alleviating the requisite on the argument from exact to asymptotic equality, new types of curve factors are introduced. While traditional curve factors have a C1 regularity, C regular approximate curve factors can be constructed, resulting in smooth boundary curves when used in conformal mappings.

    Applications include modelling of wave scattering in waveguides. When using approximate curve factors in modified Schwarz–Christoffel mappings, numerical conformal mappings can be constructed that preserve two important properties in the waveguides. First, the direction of the boundary curve can be well controlled, especially towards infinity, where the application requires two straight parallel walls. Second, a smooth (C) boundary curve can be achieved.

  • 9.
    Andersson, Anders
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Numerical Conformal mappings for regions Bounded by Smooth Curves2006Licentiate thesis, monograph (Other academic)
    Abstract [en]

    In many applications, conformal mappings are used to transform two-dimensional regions into simpler ones. One such region for which conformal mappings are needed is a channel bounded by continuously differentiable curves. In the applications that have motivated this work, it is important that the region an approximate conformal mapping produces, has this property, but also that the direction of the curve can be controlled, especially in the ends of the channel.

    This thesis treats three different methods for numerically constructing conformal mappings between the upper half-plane or unit circle and a region bounded by a continuously differentiable curve, where the direction of the curve in a number of control points is controlled, exact or approximately.

    The first method is built on an idea by Peter Henrici, where a modified Schwarz-Christoffel mapping maps the upper half-plane conformally on a polygon with rounded corners. His idea is used in an algorithm by which mappings for arbitrary regions, bounded by smooth curves are constructed.

    The second method uses the fact that a Schwarz-Christoffel mapping from the upper half-plane or unit circle to a polygon maps a region Q inside the half-plane or circle, for example a circle with radius less than 1 or a sector in the half--plane, on a region Omega inside the polygon bounded by a smooth curve. Given such a region Omega, we develop methods to find a suitable outer polygon and corresponding Schwarz-Christoffel mapping that gives a mapping from Q to Omega.

    Both these methods use the concept of tangent polygons to numerically determine the coefficients in the mappings.

    Finally, we use one of Don Marshall's zipper algorithms to construct conformal mappings from the upper half--plane to channels bounded by arbitrary smooth curves, with the additional property that they are parallel straight lines when approaching infinity.

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  • 10.
    Andersson, Anders
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Numerical Conformal Mappings for Waveguides2009Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Acoustic or electro-magnetic scattering in a waveguide with varying direction and cross-section can be re-formulated as a two-dimensional scattering problem, provided that the variations take place in only one dimension at a time. By using the so-called Building Block Method, it is possible to construct the scattering properties of a combination of scatterers when the properties of each scatterer are known. Hence, variations in the waveguide geometry or in the boundary conditions can be treated one at a time. Using the Building Block Method, the problem takes the form of the Helmholtz equation for stationary waves in a waveguide of infinite length and with smoothly varying geometry and boundary conditions. A conformal mapping is used to transform the problem into a corresponding problem in a straight horizontal waveguide, and by expanding the field in Fourier trigonometric series, the problem can be reformulated as an infinite-dimensional ordinary differential equation. From this, numerically solvable differential equations for the reflection and transmission operators are derived. To be applicable in the Building Block Method, the numerical conformal mapping must be constructed such that the direction of the boundary curve can be controlled. At the channel ends ,it is an indispensable requirement, that the two boundary curves are (at least) asymptotically parallel and straight. Furthermore, to achieve bounded operators in the differential equations, the boundary curves must satisfy different regularity conditions, depending on the boundary conditions. In this work, several methods to accomplish such conformal mappings are presented. The Schwarz–Christoffel mapping, which is a natural starting point and for which also efficient numerical software exists, can be modified in different ways in order to achieve polygons with rounded corners. We present algorithms by which the parameters in the mappings can be determined after such modifications. We show also how the unmodified Schwarz–Christoffel mapping can be used for regions with a smooth boundary. This is done by constructing an appropriate outer polygon to the considered region.Finally, we introduce one method that is not Schwarz–Christoffel-related, by showing how one of the so-called zipper algorithms can be used for waveguides. Keywords: waveguides, building block method, numerical conformalmappings, Schwarz–Christoffel mapping, rounded corners method, approximate curve factors, outer polygon method, boundary curvature, zipper method, geodesic algorithm, acoustic wave scattering, electro-magnetic wave scattering

  • 11.
    Andersson, Anders
    Linnaeus University, Faculty of Science and Engineering, School of Computer Science, Physics and Mathematics.
    Numerical Conformal Mappings for Waveguides2010In: Computational Mathematics: Theory, Methods and Applications / [ed] Peter G. Chareton, Hauppauge, NY, USA: Nova Science Publishers, Inc., 2010Chapter in book (Other academic)
    Abstract [en]

    A number of newly developed numerical conformal mapping techniques are described. Their purpose is to achieve conformal mappings with good accuracy for regions bounded by smooth or piecewise smooth curves in which the boudary curve direction can be controllod, especially towards infinity in unbounded regions as for example waveguides.

    Most of the mappings are variants of the Schwarz-Christoffel mappings.

  • 12.
    Andersson, Anders
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    On the curvature of an inner curve in a Schwarz-Christoffel mapping2007Report (Other academic)
    Abstract [en]

    In the so called outer polygon method, an approximative conformal mapping for a given simply connected region \Omega is constructed using a Schwarz-­Christoffel mapping for an outer polygon, a polygonal region of which \Omega is a subset. The resulting region is then bounded by a C^\infty -curve, which among other things means that its curvature is bounded.

    In this work, we study the curvature of an inner curve in a polygon, i.e., the image under the Schwarz-­Christoffel mapping from R, the unit disk or upper half­plane, to a polygonal region P of a curve inside R. From the Schwarz-­Christoffel formula, explicit expressions for the curvature are derived, and for boundary curves, appearing in the outer polygon method, estimations of boundaries for the curvature are given.

  • 13.
    Andersson, Anders
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Schwarz–Christoffel Mappings for Nonpolygonal Regions2008In: SIAM Journal on Scientific Computing, ISSN 1064-8275, E-ISSN 1095-7197, Vol. 31, no 1, p. 94-111Article in journal (Refereed)
    Abstract [en]

    An approximate conformal mapping for an arbitrary region $\varOmega$ bounded by a smooth curve $\varGamma$ is constructed using the Schwarz–Christoffel mapping for a polygonal region in which $\varOmega$ is embedded. An algorithm for finding this so-called outer polygon is presented. The resulting function is a conformal mapping from the upper half-plane or the unit disk to a region $R$, approximately equal to $\varOmega$. $R$ is bounded by a $C^\infty$ curve, and since the mapping function originates from the Schwarz–Christoffel mapping and tangent polygons are used to determine it, important properties of $\Gamma$ such as direction, linear asymptotes, and inflexion points are preserved in the boundary of $R$. The method makes extensive use of existing Schwarz–Christoffel software in both the determination of outer polygons and the calculation of function values. By the use suggested here, the capabilities of such well-written software are extended.

  • 14.
    Andersson, Anders
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering. Matematik.
    Using a Zipper Algorithm to find a Conformal Map for a Channel with Smooth Boundary2006In: AIP Conference Proceedings: Second Conference on Mathematical Modeling of Wave Phenomena, American Institute of Physics, New York , 2006, p. 378-Conference paper (Refereed)
    Abstract [en]

    The so called geodesic algorithm, which is one of the zipper algorithms for conformal mappings, is combined with a Schwarz–Christoffel mapping, in its original or in a modified form, to produce a conformal mapping function between the upper half-plane and an arbitrary channel with smooth boundary and parallel walls at the end.

  • 15.
    Andersson, Anders
    et al.
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Nilsson, Börje
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Acoustic Transmission in Ducts of Various Shapes with an Impedance Condition2008In: International Conference on Numerical Analysis and Applied Mathematics 2008, AIP, Melville, USA , 2008, p. 33-36Conference paper (Refereed)
    Abstract [en]

    Propagation of acoustic waves in a two-dimensional duct with an impedance condition at the boundary, is studied. The duct is assumed to have two ends at infinity being asymptotically straight, but otherwise to be arbitrarily shaped.The so called Building Block Method allows us to synthesize propagation properties for ducts with complicated geometries from results for simpler ducts. Conformal mappings can be used to transform these simple ducts to straight ducts with constant cross-sections.By using recently developed techniques for numerical conformal mappings, it is possible to construct a transformation between an infinite strip and an arbitrarily shaped duct with smooth or piecewise smooth boundary, keeping both smoothness and the well controlled boundary direction towards infinity that the above mentioned method requires.To accomplish a stable formulation of the problem, we express it in terms of scattering operators. The resulting differential equation is solved using wave splitting and invariant embedding techniques. We expand the involved functions in Fourier series, and hence, it is possible to give the operators a matrix representation. Numerical results are produced using truncated matrices.

  • 16.
    Andersson, Anders
    et al.
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Nilsson, Börje
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Electro-Magnetic Scattering in Variously Shaped Waveguides with an Impedance Condition2009In: Mathematical modelling of wave phenomena: 3rd Conference on Mathematical Modelling of Wave Phenomena, Växjö, Sweden, 9 – 13 June 2008, Melville, New York: American Institute of Physics , 2009, p. 36-45Conference paper (Refereed)
    Abstract [en]

     

    Electro-magnetic scattering is studied in a waveguide with varying shape and crosssection. Furthermore, an impedance or admittance condition is applied to two of the waveguide walls. Under the condition that variations in geometry or impedance take place in only one plane at the time, the problem can be solved as a two-dimensional wave-scattering problems. By using newly developed numerical conformal mapping techniques, the problem is transformed into a wave-scattering problem in a straight two-dimensional channel. A numerically stable formulation is reached in terms of transmission and reflection operators. Numerical results are given for a slowly varying waveguide with a bend and for one more complex geometry.

     

  • 17.
    Andersson, Jerker
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Gymnasiematematik på distans: Varför så många avbryter sina distansstudier i matematik2006Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [sv]

    Syftet med studien är att undersöka varför det är så många distansstudenter som avbryter sina distansstudier i matematik. Distansutbildning (DU) är en studieform i stark frammarsch. Nyckelorden för en lyckad DU är bland annat flexibilitet och individanpassning. Medan flexibiliteten framförallt ökar tillgängligheten står individanpassningen som garant för en god lärsituation. Faktorer som i hög grad påverkar DU och genomströmningen är studiemotiv, artefakter och hur det sociala sammanhanget upplevs. Jag har i min undersökning samlat in data med hjälp av kvalitativa intervjuer och dessutom granskat distansupplägget som eleverna på den aktuella skolan haft. Urvalsgruppen består av fem stycken elever i olika åldersgrupper som redan klarat av halva kursen. Även analysen har skett med en kvalitativ ansats. I den aktuella studien kan man se att många av de riskfaktorer som tros ligga bakom många avhopp även föreligger här med det aktuella studieupplägget.

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  • 18.
    Araujo Cabarcas, Juan Carlos
    et al.
    Umeå University, Sweden.
    Compos, Carmen
    Polytechnic University of Valencia, Spain.
    Engström, Christian
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Roman, Jose
    Polytechnic University of Valencia, Spain.
    Computation of scattering resonances in absorptive and dispersive mediawith applications to metal-dielectric nano-structures2019Manuscript (preprint) (Other academic)
    Abstract [en]

    In this paper we consider scattering resonance computations in optics when the resonators consist of frequency dependent and lossy materials, such as metals at optical frequencies. The proposed computational approach combines a novel hp-FEM strategy, based on dispersion analysis for complex frequencies, with a fast implementation of the nonlinear eigenvalue solver NLEIGS. Numerical computations illustrate that the pre-asymptotic phase is significantly reduced compared to standard uniform h and p strategies. Moreover, the efficiency grows with the refractive index contrast, which makes the new strategy highly attractive for metal-dielectric structures. The hp-refinement strategy together with the efficient parallel code result in highly accurate approximations and short runtimes on multi processor platforms.

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  • 19.
    Araujo Caracas, Juan Carlos
    et al.
    Umeå University, Sweden.
    Engström, Christian
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Jarlebring, Elias
    KTH Royal Institute of Technology, Sweden.
    Efficient resonance computations for Helmholtz problems based on a Dirichlet-to-Neumann map2016Manuscript (preprint) (Other academic)
    Abstract [en]

    We present an efficient procedure for computing resonances and resonant modes of Helmholtz problems posed in exterior domains. The problem is formulated as a nonlinear eigenvalue problem (NEP), where the nonlinearity arises from the use of a Dirichlet-to-Neumann map, which accounts for modeling unbounded domains. We consider a variational formulation and show that the spectrum consists of isolated eigenvalues of finite multiplicity that only can accumulate at infinity. The proposed method is based on a high order finite element discretization combined with a specialization of the Tensor Infinite Arnoldi method. Using Toeplitz matrices, we show how to specialize this method to our specific structure. In particular we introduce a pole cancellation technique in order to increase the radius of convergence for computation of eigenvalues that lie close to the poles of the matrix-valued function. The solution scheme can be applied to multiple resonators with a varying refractive index that is not necessarily piecewise constant. We present two test cases to show stability, performance and numerical accuracy of the method. In particular the use of a high order finite element discretization together with TIAR results in an efficient and reliable method to compute resonances.

    Download full text (pdf)
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  • 20.
    Araujo, Juan
    et al.
    Umeå University, Sweden.
    Campos, Carmen
    Universitat Politècnica de València, Spain.
    Engström, Christian
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Roman, Jose
    Universitat Politècnica de València, Spain.
    Computation of scattering resonances in absorptive and dispersive media with applications to metal-dielectric nano-structures2020In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 407, p. 1-24, article id 109220Article in journal (Refereed)
    Abstract [en]

    In this paper we consider scattering resonance computations in optics when the resonators consist of frequency dependent and lossy materials, such as metals at optical frequencies. The proposed computational approach combines a novel hp-FEM strategy, based on dispersion analysis for complex frequencies, with a fast implementation of the nonlinear eigenvalue solver NLEIGS. Numerical computations illustrate that the pre-asymptotic phase is significantly reduced compared to standard uniform h and p strategies. Moreover, the efficiency grows with the refractive index contrast, which makes the new strategy highly attractive for metal-dielectric structures. The hp-refinement strategy together with the efficient parallel code result in highly accurate approximations and short runtimes on multi processor platforms.

  • 21.
    Araujo-Cabarcas, Juan Carlos
    et al.
    Umeå university, Sweden.
    Engström, Christian
    Umeå university, Sweden.
    Jarlebring, Elias
    KTH Royal Instute of Technology, Sweden.
    Efficient resonance computations for Helmholtz problems based on a Dirichlet-to-Neumann map2018In: Journal of Computational and Applied Mathematics, ISSN 0377-0427, E-ISSN 1879-1778, Vol. 330, p. 177-192Article in journal (Refereed)
    Abstract [en]

    We present an efficient procedure for computing resonances and resonant modes of Helmholtz problems posed in exterior domains. The problem is formulated as a nonlinear eigenvalue problem (NEP), where the nonlinearity arises from the use of a Dirichlet-to-Neumann map, which accounts for modeling unbounded domains. We consider a variational formulation and show that the spectrum consists of isolated eigenvalues of finite multiplicity that only can accumulate at infinity. The proposed method is based on a high order finite element discretization combined with a specialization of the Tensor Infinite Arnoldi method (TIAR). Using Toeplitz matrices, we show how to specialize this method to our specific structure. In particular we introduce a pole cancellation technique in order to increase the radius of convergence for computation of eigenvalues that lie close to the poles of the matrix-valued function. The solution scheme can be applied to multiple resonators with a varying refractive index that is not necessarily piecewise constant. We present two test cases to show stability, performance and numerical accuracy of the method. In particular the use of a high order finite element discretization together with TIAR results in an efficient and reliable method to compute resonances.

  • 22.
    Araújo C, Juan Carlos
    et al.
    Umeå University, Sweden.
    Engström, Christian
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    On spurious solutions encountered in Helmholtz scattering resonance computations in Rd with applications to nano-photonics and acoustics2021In: Journal of Computational Physics, ISSN 0021-9991, E-ISSN 1090-2716, Vol. 429, p. 1-20, article id 110024Article in journal (Refereed)
    Abstract [en]

    In this paper, we consider a sorting scheme for potentially spurious scattering resonant pairs in one- and two-dimensional electromagnetic problems and in three-dimensional acoustic problems. The novel sorting scheme is based on a Lippmann-Schwinger type of volume integral equation and can, therefore, be applied to structures with graded materials as well as to configurations including piece-wise constant material properties. For TM/TE polarized electromagnetic waves and for acoustic waves, we compute first approximations of scattering resonances with finite elements. Then, we apply the novel sorting scheme to the computed eigenpairs and use it to mark potentially spurious solutions in electromagnetic and acoustic scattering resonances computations at a low computational cost. Several test cases with Drude-Lorentz dielectric resonators as well as with graded material properties are considered.

  • 23.
    Araújo Cabarcas, Juan Carlos
    et al.
    Umeå University, Sweden.
    Engström, Christian
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    On spurious solutions encountered in Helmholtz scattering resonance computations in Rd with applications to nano-photonics and acoustics2019Manuscript (preprint) (Other academic)
    Abstract [en]

    In this paper, we consider a sorting scheme for potentially spurious scattering resonant pairs in one- and two-dimensional electromagnetic problems and in three-dimensional acoustic problems. The novel sorting scheme is based on a Lippmann-Schwinger type of volume integral equation and can, therefore, be applied to structures with graded materials as well as to configurations including piece-wise constant material properties. For TM/TE polarized electromagnetic waves and for acoustic waves, we compute first approximations of scattering resonances with finite elements. Then, we apply the novel sorting scheme to the computed eigenpairs and use it to mark potentially spurious solutions in electromagnetic and acoustic scattering resonances computations at a low computational cost. Several test cases with Drude-Lorentz dielectric resonators as well as with graded material properties are considered.

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  • 24.
    Aref’eva, I. Ya.
    et al.
    Russian Academy of Sciences, Russia.
    Djordjevic, G. S.
    University of Niš, Serbia.
    Khrennikov, Andrei
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Kozyrev, S. V.
    Russian Academy of Sciences, Russia.
    Rakic, Z.
    University of Belgrade, Serbia.
    Volovich, I. V.
    Russian Academy of Sciences, Russia.
    p-Adic mathematical physics and B. Dragovich research2017In: P-Adic Numbers, Ultrametric Analysis, and Applications, ISSN 2070-0466, E-ISSN 2070-0474, Vol. 9, no 1, p. 82-85Article in journal (Refereed)
    Abstract [en]

    We present a brief review of some parts of p-adic mathematical physics related to the scientific work of Branko Dragovich on the occasion of his 70th birthday.

  • 25.
    Asekritova, Irina
    et al.
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Nilsson, Börje
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Rydström, Sara
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Diffractive Index Determination by Tikhonov Regularization on Forced String Vibration Data2009In: Mathematical modelling of wave phenomena: 3rd Conference on Mathematical Modelling of Wave Phenomena, Växjö, Sweden, 9-13 June 2008, Melville, New York: American Institute of Physics , 2009, p. 224-232Conference paper (Refereed)
    Abstract [en]

    Wave analysis is efficient for investigating the interior of objects. Examples are ultra sound examination of humans and radar using elastic and electromagnetic waves. A common procedure is inverse scattering where both transmitters and receivers are located outside the object or on its boundary. A variant is when both transmitters and receivers are located on the scattering object. The canonical model is a finite inhomogeneous string driven by a harmonic point force. The inverse problem for the determination of the diffractive index of the string is studied. This study is a first step to the problem for the determination of the mechanical strength of wooden logs. An inverse scattering theory is formulated incorporating two regularizing strategies. The results of simulations using this theory show that the suggested method works quite well and that the regularization methods based on the couple of spaces (L2; H1 ) could be very useful in such problems.

     

     

  • 26.
    Baladron, Carlos
    et al.
    Universidad de Valladolid, Spain.
    Khrennikov, Andrei
    Linnaeus University, Faculty of Technology, Department of Mathematics. National Research University of Information Technologies, Russia.
    At the Crossroads of Three Seemingly Divergent Approaches to Quantum Mechanics2018In: Quantum Foundations, Probability and Information / [ed] A. Khrennikov, B. Toni, Springer, 2018, p. 13-21Chapter in book (Refereed)
    Abstract [en]

    Several concepts stemming from three apparently divergent approaches to quantum mechanics—Bohmian Mechanics, QBism, and Time-Symmetric Quantum Mechanics—are interwoven in an information-theoretic Darwinian scheme applied to fundamental physical systems that might contribute to shed light on some long-standing quantum mechanical conundrums.

  • 27.
    Basieva, Irina
    et al.
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Khrennikov, Andrei
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Decision-making and cognition modeling from the theory of mental instruments2017In: The Palgrave Handbook of Quantum Models in Social Science: Applications and Grand Challenges / [ed] Emmanuel Haven, Andrei Khrennikov, Palgrave Macmillan, 2017, p. 75-93Chapter in book (Other academic)
    Abstract [en]

    The authors present the theory of quantum measurements in a humanities friendly way. The most general process of decision-making is represented with the aid of the formalism of quantum apparatuses and instruments. This measurement formalism generalizes the standard one based on the von Neumann–Lüders projection postulate. Generalized quantum observables are mathematically represented as positive operator valued measures (POVMs) and state transformers resulting from the feedback of measurements to the states of systems that are given by quantum instruments. The quantum scheme of indirect measurements (a special realization of quantum instruments) is applied to model decision-making as resulting from the interaction between the belief and decision states. The authors analyze the specific features of quantum instruments which are important for cognitive and social applications. In particular, the state transformers given by quantum instruments are in general less invasive than the state projections. Thus quantum-like decision-making need not be viewed as a kind of state collapse.

  • 28.
    Beermann, Marie
    et al.
    Linnaeus University, Faculty of Health, Social Work and Behavioural Sciences, School of Education, Psychology and Sport Science.
    Eriksson, Kerstin
    Linnaeus University, Faculty of Health, Social Work and Behavioural Sciences, School of Education, Psychology and Sport Science.
    Vardagsmatematik: Pedagogers gruppsamtal om vardagsmatematiken i de tidigare skolåren2011Independent thesis Advanced level (professional degree), 10 credits / 15 HE creditsStudent thesis
    Abstract [sv]

    Syftet med vår studie var att synliggöra hur pedagoger som undervisar i grundskolans tidiga år samtalar om undervisningen av matematikens användning i vardagen utifrån den socio­kulturella teorin. Gruppsamtalen utgick från rubrikerna konkretisering, individ­uali­se­ring, mate­matik och språk samt matematiksvårigheter kopplade till vardagsmatematiken. Vi använ­de oss av fokusgrupper som undersökningsinstrument. I studien ingick 13 peda­goger som alla undervisade i matematik på skolans lågstadium. Deltagarna i studien var verk­samma på fyra olika skolor i två olika kommuner.

    Undersökningens resultat bearbetades utifrån en hermeneutisk innehållsanalys. I resultatet synliggörs en omfattande och medveten undervisning kring vardagsmatematiken hos de med­­verkande pedagogerna. Det som upplevs som hinder eller skapande av möjligheter är resurstilldelningen både materiellt och personellt. Gruppstorlek och gruppsammansättning upp­levs också som avgörande för en undervisning som lever upp till pedagogernas ambi­tioner. Konkretiseringen ges en stor betydelse för att fånga och förklara den abstrakta mate­matiken. Individualiseringen ses av pedagogerna som nödvändig för att få med sig alla elev­erna i matematikundervisningens gemenskap och för att alla elever ska nå målen. Den sam­talande matematiken beskrivs som ovärderlig för elevernas tänkande och därmed för­ståelse och lärandet. Det framgår att för elever med matematiska svårigheter är det konkreta materi­alet, tid tillsammans med pedagog och ett varierat undervisningssätt faktorer som skapar för­utsättningar för lärande. 

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    Vardagsmatematik
  • 29.
    Bock, Wolfgang
    et al.
    Technische Universität Kaiserslautern, Germany.
    Bracke, Martin
    Technische Universität Kaiserslautern, Germany.
    Applied school mathematics: Made in kaiserslautern2015In: Currents in Industrial Mathematics: From Concepts to Research to Education / [ed] Helmut Neunzert, Dieter Prätzel-Wolters, Springer, 2015, p. 403-437Chapter in book (Refereed)
    Abstract [en]

    The role of mathematics in our society has undergone a dramatic change in recent years. In general, however, mathmatics instruction in schools has not kept pace with this change. Here, we describe the activities and events with which the Felix Klein Center for Mathematics, in Kaiserslautern, has tried to address this deficit. We first look briefly at mathematical modeling and illustrate how it fits into mathematics instruction. We then present examples of various didactic activities.

  • 30.
    Bock, Wolfgang
    et al.
    University of Kaiserslautern, Germany.
    Götz, Thomas
    University of Koblenz, Germany.
    Grothaus, Martin
    University of Kaiserslautern, Germany.
    Liyanage, Uditha Prabhath
    University of Kaiserslautern, Germany.
    Parameter estimation from occupation times: a white noise approach2014In: Communcations on Stocastic Analysis, ISSN 2688-6669, Vol. 8, no 4, article id 5Article in journal (Refereed)
    Abstract [en]

    We derive an equation to compute directly the expected occupation time of the centered Ornstein–Uhlenbeck process. This allows us toidentify the parameters of the Ornstein–Uhlenbeck process for available occupation times via a standard least squares minimization. To test the method,we generate occupation times via Monte–Carlo simulations and recover theparameters with the above mentioned procedure.

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  • 31.
    Bock, Wolfgang
    et al.
    University of Kaiserslautern, Germany.
    Götz, Thomas
    University Koblenz–Landau, Germany.
    Liyanage, Uditha Prabhath
    University of Kelaniya, Sri Lanka.
    Parameter estimation of fiber lay-down in nonwoven production: an occupation time approach2018In: International Journal of Advances in Engineering Sciences and Applied Mathematics, ISSN 0975-0770, E-ISSN 0975-5616, Vol. 10, no 1, p. 2-8Article in journal (Refereed)
    Abstract [en]

    In this paper we investigate the parameter estimation of the fiber lay-down process in the production of nonwovens. The parameter estimation is based on the mass per unit area data, which is available at least on an industrial scale. We introduce a stochastic model to represent the fiber lay-down and through the model’s parameters we characterize this fiber lay-down. Based on the occupation time, which is the equivalent quantity for the mass per unit area in the context of stochastic dynamical systems, an optimization procedure is formulated that estimates the parameters of the model. The optimization procedure is tested using occupation time data given by Monte–Carlo simulations. The feasibility of the optimization procedure on an industrial level is tested using the fiber paths simulated by the industrial software FYDIST.

  • 32.
    Bock, Wolfgang
    et al.
    Technical University Kaiserslautern, Germany.
    Jayathunga, Yashika
    University of Koblenz-Landau, Germany.
    Götz, Thomas
    University of Koblenz-Landau, Germany.
    Rockenfeller, Robert
    University of Koblenz-Landau, Germany.
    Are the upper bounds for new SARS-CoV-2 infections in Germany useful?2021In: Computational and Mathematical Biophysis, ISSN 2544-7297, Vol. 9, no 1, p. 242-260Article in journal (Refereed)
    Abstract [en]

    At the end of 2019, an outbreak of a new coronavirus, called SARS-CoV-2, was reported in China and later in other parts of the world. First infection reported in Germany by the end of January 2020 and on March 16th, 2020 the federal government announced a partial lockdown in order to mitigate the spread. Since the dynamics of new infections started to slow down, German states started to relax the confinement measures as to May 6th, 2020. As a fall back option, a limit of 50 new infections per 100,000 inhabitants within seven days was introduced for each district in Germany. If a district exceeds this limit, measures to control the spread of the virus should be taken. Based on a multi-patch SEAIRD-type model, we will simulate the effect of choosing a specific upper limit for new infections. We investigate, whether the politically motivated bound is low enough to detect new outbreaks at an early stage. Subsequently, we introduce an optimal control problem to tackle the multi-criteria problem of finding a bound for new infections that is low enough to avoid new outbreaks, which might lead to an overload of the health care system, but is large enough to curb the expected economic losses.

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  • 33.
    Chen, Yuanyuan
    et al.
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Signahl, Mikael
    University of Agder, Norway.
    Toft, Joachim
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Hilbert space embeddings for Gelfand–Shilov and Pilipović spaces2017In: Generalized Functions and Fourier Analysis: Dedicated to Stevan Pilipović on the Occasion of his 65th Birthday / [ed] Michael Oberguggenberger, Joachim Toft, Jasson Vindas, Patrik Wahlberg, Springer, 2017, p. 31-44Chapter in book (Refereed)
    Abstract [en]

    We consider quasi-Banach spaces that lie between a Gelfand–Shilov space, or more generally, Pilipovi´c space, H, and its dual, H′. We prove that for such quasi-Banach space B, there are convenient Hilbert spaces, Hk, k=1,2ss, with normalized Hermite functions as orthonormal bases and such that B lies between H1 and H1, and the latter spaces lie between H and H′.

  • 34.
    Doerr, Benjamin
    et al.
    Max-Planck-Institut für Informatik, Saarbrücken,.
    Fischer, Paul
    Technical University of Denmark.
    Hilbert, Astrid
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Witt, Carsten
    Technical University of Denmark.
    Evolutionary Algorithms for the Detection of Structural Breaks in Time Series: extended abstract2013In: Proceedings of the 15th annual conference on Genetic and evolutionary computation, ACM Press, 2013, p. 119-120Conference paper (Refereed)
    Abstract [en]

    Detecting structural breaks is an essential task for the statistical analysis of time series, for example,  for fitting parametric models to it. In short, structural breaks  are points in time at which the behavior of the time series changes. Typically, no solid background knowledge of the time series under consideration is available. Therefore, a black-box optimization approach is our method of choice for detecting structural breaks. We describe a \ea framework which easily adapts to a large number of statistical settings. The experiments on artificial and real-world time series show that the algorithm detects break points with high precision and is computationally very efficient.

    A reference implementation is availble at the following address:

    http://www2.imm.dtu.dk/\~\/pafi/SBX/launch.html

  • 35.
    Dower, Catrin
    et al.
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Berg, Pia
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Individualisering i matematik: En studie om matematikundervisningens individualisering, f-6, i jämförelse med Lpo 94.2006Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [sv]

    Syftet med vår studie var att se om lärarna i år f - 6 i sin matematikundervisning följde de intentioner som Lpo 94 ger angående individualisering. Vi ville även se vilken innebörd ordet individualisering hade för olika lärare och om de ansåg att det fanns några svårigheter med att individualisera sin matematikundervisning. För att få svar på våra frågeställningar gjorde vi fyra intervjuer och en enkätundersökning med lärare på åtta olika skolor i södra Sverige. Enkätundersökningen byggde vi utifrån svaren vi fått från våra intervjuer för att få bekräftelse på om åsikterna var gällande för flertalet lärare. Vi redovisar vad tidigare forskning säger om kunskap, inlärning och individualisering. Vi definierar ordet individualisering och talar om vilka riktlinjer som Lpo 94 ger angående detta. Resultatet visar att de flesta lärare försöker följa intentionerna i Lpo 94 men att det finns svårigheter som gör att de inte lyckas anpassa undervisningen för den enskilde individen. Många lärare använder fortfarande samma läromedel till samtliga elever och hastighetsindividualiserar sin undervisning.

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  • 36.
    Dragovich, B.
    et al.
    University of Belgrade, Serbia;Serbian Academy of Sciences and Arts, Serbia.
    Khrennikov, Andrei
    Linnaeus University, Faculty of Technology, Department of Mathematics. National Research University of Information Technologies, Russia.
    Kozyrev, S. V.
    Steklov Mathematical Institute of the Russian Academy of Sciences, Russia.
    Volovich, I. V.
    Steklov Mathematical Institute of the Russian Academy of Sciences, Russia.
    Zelenov, E. I.
    Steklov Mathematical Institute of the Russian Academy of Sciences, Russia.
    p-Adic mathematical physics: the first 30 years2017In: P-Adic Numbers, Ultrametric Analysis, and Applications, ISSN 2070-0466, E-ISSN 2070-0474, Vol. 9, no 2, p. 87-121Article in journal (Refereed)
    Abstract [en]

    p-Adic mathematical physics is a branch of modern mathematical physics based on the application of p-adic mathematical methods in modeling physical and related phenomena. It emerged in 1987 as a result of efforts to find a non-Archimedean approach to the spacetime and string dynamics at the Planck scale, but then was extended to many other areas including biology. This paper contains a brief review of main achievements in some selected topics of p-adic mathematical physics and its applications, especially in the last decade. Attention is mainly paid to developments with promising future prospects.

  • 37.
    Effenberger, Cedric
    et al.
    ETH Zurich, Switzerland.
    Kressner, Daniel
    ETH Zurich, Switzerland.
    Engström, Christian
    ETH Zurich, Switzerland.
    Linearization techniques for band structure calculations in absorbing photonic crystals2012In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 89, no 2, p. 180-191Article in journal (Refereed)
    Abstract [en]

    Band structure calculations for photonic crystals require the numerical solution of eigenvalue problems. In this paper, we consider crystals composed of lossy materials with frequency-dependent permittivities. Often, these frequency dependencies are modeled by rational functions, such as the Lorentz model, in which case the eigenvalue problems are rational in the eigenvalue parameter. After spatial discretization using an interior penalty discontinuous Galerkin method, we employ a recently developed linearization technique to deal with the resulting rational matrix eigenvalue problems. In particular, the efficient implementation of Krylov subspace methods for solving the linearized eigenvalue problems is investigated in detail. Numerical experiments demonstrate that our new approach is considerably cheaper in terms of memory and computing time requirements compared with the naive approach of turning the rational eigenvalue problem into a polynomial eigenvalue problem and applying standard linearization techniques. Copyright © 2011 John Wiley & Sons, Ltd.

  • 38.
    El Fatini, Mohamed
    et al.
    Ibn Tofail Univ, Morocco.
    El Khalifi, Mohamed
    Ibn Tofail Univ, Morocco.
    Lahrouz, Aadil
    FST, Morocco.
    Pettersson, Roger
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Settati, Adel
    FST, Morocco.
    The effect of stochasticity with respect to reinfection and nonlinear transition states for some diseases with relapse2020In: Mathematical methods in the applied sciences, ISSN 0170-4214, E-ISSN 1099-1476, Vol. 43, no 18, p. 10659-10670Article in journal (Refereed)
    Abstract [en]

    In this paper, we consider a stochastic epidemic model with relapse, reinfection, and a general incidence function. Using stochastic tools, we establish a stochastic thresholdRs0and prove the extinction of the disease when its value is equal or less than unity. We also show the persistence in mean of the disease whenRs0>1.Moreover, we prove the existence and uniqueness of a stationary distribution. Finally, numerical simulations are presented to show the effectiveness of theoretical results.

  • 39.
    El Fatini, Mohamed
    et al.
    Université Ibn Tofail, Morocco.
    Pettersson, Roger
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Sekkak, Idriss
    Université Ibn Tofail, Morocco.
    Taki, Regragui
    Chouaib Doukkali University EST Sidi Bennour, Morocco.
    A stochastic analysis for a triple delayed SIQR epidemic model with vaccination and elimination strategies2020In: Journal of Applied Mathematics and Computing, ISSN 1598-5865, E-ISSN 1865-2085, Vol. 64, p. 781-805Article in journal (Refereed)
    Abstract [en]

    In this paper, a delayed SIQR epidemic model with vaccination and elimination hybrid strategies is analysed under a white noise perturbation. We prove the existence and the uniqueness of a positive solution. Afterwards, we establish a stochastic threshold R-s in order to study the extinction and persistence in mean of the stochastic epidemic system. Then we investigate the existence of a stationary distribution for the delayed stochastic model. Finally, some numerical simulations are presented to support our theoretical results.

  • 40.
    Eliasson, Peter
    et al.
    FOI, Swedish Defence Research Agency.
    Eriksson, Sofia
    Uppsala university.
    Nordström, Jan
    Uppsala university ; FOI, Swedish Defence Research Agency.
    The influence of weak and strong solid wall boundary conditions on the convergence to steady-state of the Navier-Stokes equations2009In: 19th AIAA Computational Fluid Dynamics Conference 2009, American Institute of Aeronautics and Astronautics, 2009, article id 3551Conference paper (Refereed)
    Abstract [en]

    In the present paper we study the influence of weak and strong no-slip solid wall boundary conditions on the convergence to steady-state. Our Navier-Stokes solver is edge based and operates on unstructured grids. The two types of boundary conditions are applied to no-slip adiabatic walls. The two approaches are analyzed for a simplified model problem and the reason for the different convergence rates are discussed in terms of the theoretical findings for the model problem. Numerical results for a 2D viscous steady state low Reynolds number problem show that the weak boundary conditions often provide faster convergence. It is shown that strong boundary conditions can prevent the steady state convergence. It is also demonstrated that the two approaches converge to the same solution. Similar results are obtained for high Reynolds number flow in two and three dimensions.

  • 41.
    Engström, Christian
    ETH Zurich, Switzerland.
    On a high-order discontinuous Galerkin method applied to canonical scattering problems2010In: International Symposium on Electromagnetic Theory: EMTS, 2010 URSI, IEEE, 2010, p. 752-755Conference paper (Refereed)
    Abstract [en]

    A high-order interior penalty method for scattering problems in two-dimensions is presented. Results for perfectly conducting objects illustrate the high accuracy of the method at low computational cost.

  • 42.
    Engström, Christian
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Spectra of Gurtin-Pipkin type of integro-differential equations and applications to waves in graded viscoelastic structures2020Manuscript (preprint) (Other academic)
    Abstract [en]

    In this paper, we study spectral properties and spectral enclosures for the Gurtin-Pipkin type of integro-differential equations in several dimensions. The analysis is based on an operator function and we consider the relation between the studied operator function and other formulations of the spectral problem. The theory is applied to wave equations with Boltzmann damping.

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  • 43.
    Engström, Christian
    et al.
    Umeå university, Sweden.
    Giani, Stefano
    Durham University, UK.
    Grubisic, Luka
    University of Zagreb, Croatia.
    Efficient and reliable hp-FEM estimates for quadratic eigenvalue problems and photonic crystal applications2016In: Computers and Mathematics with Applications, ISSN 0898-1221, E-ISSN 1873-7668, Vol. 72, no 4, p. 952-973Article in journal (Refereed)
    Abstract [en]

    We present a-posteriori analysis of higher order finite element approximations (hp-FEM) for quadratic Fredholm-valued operator functions. Residual estimates for approximations of the algebraic eigenspaces are derived and we reduce the analysis of the estimator to the analysis of an associated boundary value problem. For the reasons of robustness we also consider approximations of the associated invariant pairs. We show that our estimator inherits the efficiency and reliability properties of the underlying boundary value estimator. As a model problem we consider spectral problems arising in analysis of photonic crystals. In particular, we present an example where a targeted family of eigenvalues cannot be guaranteed to be semisimple. Numerical experiments with hp-FEM show the predicted convergence rates. The measured effectivities of the estimator compare favorably with the performance of the same estimator on the associated boundary value problem. We also present a benchmark estimator, based on the dual weighted residual (DWR) approach, which is more expensive to compute but whose measured effectivities are close to one. 

  • 44.
    Engström, Christian
    et al.
    Linnaeus University, Faculty of Technology, Department of Mathematics.
    Giani, Stefano
    Durham University, UK.
    Grubišić, Luka
    University of Zagreb, Croatia.
    A spectral projection based method for the numerical solution of wave equations with memory2022In: Applied Mathematics Letters, ISSN 0893-9659, E-ISSN 1873-5452, Vol. 127, article id 107844Article in journal (Refereed)
    Abstract [en]

    In this paper, we compare two approaches to numerically approximate the solution of second-order Gurtin-Pipkin type of integro-differential equations. Both methods are based on a high-order Discontinous Galerkin approximation in space and the numerical inverse Laplace transform. In the first approach, we use functional calculus and the inverse Laplace transform to represent the solution. The spectral projections are then numerically computed and the approximation of the solution of the time-dependent problem is given by a summation of terms that are the product of projections of the data and the inverse Laplace transform of scalar functions. The second approach is the standard inverse Laplace transform technique. We show that the approach based on spectral projections can be very efficient when several time points are computed, and it is particularly interesting for parameter-dependent problems where the data or the kernel depends on a parameter.

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  • 45.
    Engström, Christian
    et al.
    Umeå University, Sweden.
    Grubisic, Luka
    University of Zagreb, Croatia.
    A subspace iteration algorithm for Fredholm valued functions2015In: Mathematical problems in engineering (Print), ISSN 1024-123X, E-ISSN 1563-5147, Vol. 2015, p. 1-14, article id 459895Article in journal (Refereed)
    Abstract [en]

    We present an algorithm for approximating an eigensubspace of a spectral component of an analytic Fredholm valued function. Our approach is based on numerical contour integration and the analytic Fredholm theorem. The presented method can be seen as a variant of the FEAST algorithm for infinite dimensional nonlinear eigenvalue problems. Numerical experiments illustrate the performance of the algorithm for polynomial and rational eigenvalue problems.

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  • 46.
    Engström, Christian
    et al.
    Lund University, Sweden.
    Sjöberg, Daniel
    Lund University, Sweden.
    A Comparison of two numerical methods for homogenization of Maxwell's equations2004In: Computational Electromagnetics- Methods and Applications (EMB04), Gothenburg, Sweden, October 18-19, 2004, 2004, p. 50-57Conference paper (Refereed)
    Abstract [en]

    When the wavelength is much larger than the typical scale of the microstructure in a material, it is possible to define effective or homogenized material coefficients. The classical way of determination of the homogenized coefficients consists of solving an elliptic problem in a unit cell. This method and the Floquet-Bloch method, where an eigenvalue problem is solved, are numerically compared with respect to accuracy and contrast sensitivity. The Floquet-Bloch method is shown to be a good alternative to the classical homogenization method, when the contrast is modest.

  • 47.
    Engström, Christian
    et al.
    Lund University, Sweden.
    Sjöberg, Daniel
    Lund University, Sweden.
    On two numerical methods for homogenization of Maxwell's equations2007In: Journal Electromagnetic Waves and Applications, ISSN 0920-5071, E-ISSN 1569-3937, Vol. 21, no 13, p. 1845-1856Article in journal (Refereed)
    Abstract [en]

    When the wavelength is much larger than the typical scale of the microstructure in a material, it is possible to define effective or homogenized material coefficients. The classical way of determination of the homogenized coefficients consists of solving an elliptic problem in a unit cell. This method and the Floquet-Bloch method, where an eigenvalue problem is solved, are numerically compared with respect to accuracy and contrast sensitivity. Moreover, we provide numerical bounds on the effective permittivity. The Floquet-Bloch method is shown to be a good alternative to the classical homogenization method, when the contrast is modest.

  • 48.
    Engström, Christian
    et al.
    ETH Zurich, Switzerland.
    Wang, Mengyu
    ETH Zurich, Switzerland.
    Complex dispersion relation calculations with the applications to absorptive photonic crystals2010In: International Journal for Numerical Methods in Engineering, ISSN 0029-5981, E-ISSN 1097-0207, Vol. 84, p. 849-863Article in journal (Refereed)
    Abstract [en]

    A high-order discontinuous Galerkin method for calculations of complex dispersion relations of two-dimensional photonic crystals is presented. The medium is characterized by a complex-valued permittivityand we relate for this absorptive system the spectral parameter to the time frequency. We transform thenon-linear eigenvalue problem for a Lorentz material in air into a non-Hermitian linear eigenvalue problemand uses a Krylov space method to compute approximate eigenvalues. Moreover, we study the impact ofthe penalty term numerically and illustrate the high convergence rate of the method.

  • 49.
    Eriksson, Camilla
    et al.
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Skalleberg, Tina
    Växjö University, Faculty of Mathematics/Science/Technology, School of Mathematics and Systems Engineering.
    Problemlösning i form av räknesagor2009Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [sv]

    Abstrakt

    Syftet med arbetet är att undersöka hur barn funderar ut lösningar på matematiska problem som de stöter på när de arbetar med räknesagor. Fokus har legat på vilka strategier eleverna använder sig av när de löser räknesagor och vilken betydelse kommunikationen har för hur eleverna utvecklar sina strategier. Den första kontakten med matematik kan vara avgörande för det fortsatta intresset. Läraren har därför en stor uppgift att fylla genom att introducera matematiken på ett intressant och lustfyllt sätt. Det är därför angeläget att det finns en balans mellan teori och praktik. Barn behöver uppleva matematiken genom flera olika tillvägagångssätt för att förstå den, där av vårt val av ämne.

    Den metod vi valde i denna fallstudie var observationer enskilt och i grupp när barn gjorde räknesagor, för att finna svar på vårt syfte. Även intervjufrågor ställdes i samband med de enskilda räknesagorna.

    Resultatet visade att barnen använder sig av olika lösningsstrategier. De vanligaste strategierna i de enskilda arbetena var att de ritade bilder medan de föredrog att använda siffror i grupparbetena. Flera av barnen hade god användning av sina bilder när de löste sina räknesagor. Detta kunde vi se genom att de hela tiden behövde gå tillbaka till föregående bild, när de skulle rita nästa. Resultatet i vår undersökning av räknesagor visade även att barnen inte var vana att kommunicera med varandra. Detta visade sig genom att det oftast var ett barn som tog initiativet och utförde räknesagan själv utan att fråga de andra.

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  • 50.
    Eriksson, Sofia
    Technische Universität Darmstadt, Germany.
    A Dual Consistent Finite Difference Method with Narrow Stencil Second Derivative Operators2018In: Journal of Scientific Computing, ISSN 0885-7474, E-ISSN 1573-7691, Vol. 75, no 2, p. 906-940Article in journal (Refereed)
    Abstract [en]

    We study the numerical solutions of time-dependent systems of partial differential equations, focusing on the implementation of boundary conditions. The numerical method considered is a finite difference scheme constructed by high order summation by parts operators, combined with a boundary procedure using penalties (SBP-SAT). Recently it was shown that SBP-SAT finite difference methods can yield superconvergent functional output if the boundary conditions are imposed such that the discretization is dual consistent. We generalize these results so that they include a broader range of boundary conditions and penalty parameters. The results are also generalized to hold for narrow-stencil second derivative operators. The derivations are supported by numerical experiments.

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