@book{ElsaesserKlebinggatKuhnetal.2024, author = {Elsaesser, Evelyn and Klebinggat, Michael and Kuhn, Wilfried and Michielsens, Constant and Pauels, Willibert and Popkes, Enno E. and Schneider, Elke and Laack, Walter van and Warven, Rinus van}, title = {Schnittstelle Tod - Ist die Menschheit zu retten ohne Vertrauen auf ein Danach}, editor = {Laack, Walter van}, publisher = {van Laack Buchverlag}, address = {Aachen}, isbn = {978-3-936624-58-8}, pages = {156 Seiten}, year = {2024}, language = {de} } @article{PieronekKleefeld2024, author = {Pieronek, Lukas and Kleefeld, Andreas}, title = {On trajectories of complex-valued interior transmission eigenvalues}, series = {Inverse problems and imaging : IPI}, volume = {18}, journal = {Inverse problems and imaging : IPI}, number = {2}, publisher = {AIMS}, address = {Springfield, Mo}, issn = {1930-8337 (Print)}, doi = {10.3934/ipi.2023041}, pages = {480 -- 516}, year = {2024}, abstract = {This paper investigates the interior transmission problem for homogeneous media via eigenvalue trajectories parameterized by the magnitude of the refractive index. In the case that the scatterer is the unit disk, we prove that there is a one-to-one correspondence between complex-valued interior transmission eigenvalue trajectories and Dirichlet eigenvalues of the Laplacian which turn out to be exactly the trajectorial limit points as the refractive index tends to infinity. For general simply-connected scatterers in two or three dimensions, a corresponding relation is still open, but further theoretical results and numerical studies indicate a similar connection.}, language = {en} } @techreport{BarnatArntzBerneckeretal.2024, type = {Working Paper}, author = {Barnat, Miriam and Arntz, Kristian and Bernecker, Andreas and Fissabre, Anke and Franken, Norbert and Goldbach, Daniel and H{\"u}ning, Felix and J{\"o}rissen, J{\"o}rg and Kirsch, Ansgar and Pettrak, J{\"u}rgen and Rexforth, Matthias and Josef, Rosenkranz and Terstegge, Andreas}, title = {Strategische Gestaltung von Studieng{\"a}ngen f{\"u}r die Zukunft: Ein kollaborativ entwickeltes Self-Assessment}, series = {Hochschulforum Digitalisierung - Diskussionspapier}, journal = {Hochschulforum Digitalisierung - Diskussionspapier}, publisher = {Stifterverband f{\"u}r die Deutsche Wissenschaft}, address = {Berlin}, issn = {2365-7081}, pages = {16 Seiten}, year = {2024}, abstract = {Das Diskussionspapier beschreibt einen Prozess an der FH Aachen zur Entwicklung und Implementierung eines Self-Assessment-Tools f{\"u}r Studieng{\"a}nge. Dieser Prozess zielte darauf ab, die Relevanz der Themen Digitalisierung, Internationalisierung und Nachhaltigkeit in Studieng{\"a}ngen zu st{\"a}rken. Durch Workshops und kollaborative Entwicklung mit Studiendekan:innen entstand ein Fragebogen, der zur Reflexion und strategischen Weiterentwicklung der Studieng{\"a}nge dient.}, language = {de} } @article{HarrisKleefeld2018, author = {Harris, Isaac and Kleefeld, Andreas}, title = {The inverse scattering problem for a conductive boundary condition and transmission eigenvalues}, series = {Applicable Analysis}, volume = {99}, journal = {Applicable Analysis}, number = {3}, publisher = {Taylor \& Francis}, address = {London}, issn = {1563-504X}, doi = {10.1080/00036811.2018.1504028}, pages = {508 -- 529}, year = {2018}, abstract = {In this paper, we consider the inverse scattering problem associated with an inhomogeneous media with a conductive boundary. In particular, we are interested in two problems that arise from this inverse problem: the inverse conductivity problem and the corresponding interior transmission eigenvalue problem. The inverse conductivity problem is to recover the conductive boundary parameter from the measured scattering data. We prove that the measured scatted data uniquely determine the conductivity parameter as well as describe a direct algorithm to recover the conductivity. The interior transmission eigenvalue problem is an eigenvalue problem associated with the inverse scattering of such materials. We investigate the convergence of the eigenvalues as the conductivity parameter tends to zero as well as prove existence and discreteness for the case of an absorbing media. Lastly, several numerical and analytical results support the theory and we show that the inside-outside duality method can be used to reconstruct the interior conductive eigenvalues.}, language = {en} } @article{KleefeldPieronek2020, author = {Kleefeld, Andreas and Pieronek, J.}, title = {Elastic transmission eigenvalues and their computation via the method of fundamental solutions}, series = {Applicable Analysis}, volume = {100}, journal = {Applicable Analysis}, number = {16}, publisher = {Taylore \& Francis}, address = {London}, issn = {1563-504X}, doi = {10.1080/00036811.2020.1721473}, pages = {3445 -- 3462}, year = {2020}, abstract = {A stabilized version of the fundamental solution method to catch ill-conditioning effects is investigated with focus on the computation of complex-valued elastic interior transmission eigenvalues in two dimensions for homogeneous and isotropic media. Its algorithm can be implemented very shortly and adopts to many similar partial differential equation-based eigenproblems as long as the underlying fundamental solution function can be easily generated. We develop a corroborative approximation analysis which also implicates new basic results for transmission eigenfunctions and present some numerical examples which together prove successful feasibility of our eigenvalue recovery approach.}, language = {en} } @article{BreussKleefeld2020, author = {Breuß, Michael and Kleefeld, Andreas}, title = {Implicit monotone difference methods for scalar conservation laws with source terms}, series = {Acta Mathematica Vietnamica}, volume = {45}, journal = {Acta Mathematica Vietnamica}, publisher = {Springer Singapore}, address = {Singapore}, issn = {2315-4144}, doi = {10.1007/s40306-019-00354-1}, pages = {709 -- 738}, year = {2020}, abstract = {In this article, a concept of implicit methods for scalar conservation laws in one or more spatial dimensions allowing also for source terms of various types is presented. This material is a significant extension of previous work of the first author (Breuß SIAM J. Numer. Anal. 43(3), 970-986 2005). Implicit notions are developed that are centered around a monotonicity criterion. We demonstrate a connection between a numerical scheme and a discrete entropy inequality, which is based on a classical approach by Crandall and Majda. Additionally, three implicit methods are investigated using the developed notions. Next, we conduct a convergence proof which is not based on a classical compactness argument. Finally, the theoretical results are confirmed by various numerical tests.}, language = {en} } @article{AsanteAsamaniKleefeldWade2020, author = {Asante-Asamani, E.O. and Kleefeld, Andreas and Wade, B.A.}, title = {A second-order exponential time differencing scheme for non-linear reaction-diffusion systems with dimensional splitting}, series = {Journal of Computational Physics}, volume = {415}, journal = {Journal of Computational Physics}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0021-9991}, doi = {10.1016/j.jcp.2020.109490}, year = {2020}, abstract = {A second-order L-stable exponential time-differencing (ETD) method is developed by combining an ETD scheme with approximating the matrix exponentials by rational functions having real distinct poles (RDP), together with a dimensional splitting integrating factor technique. A variety of non-linear reaction-diffusion equations in two and three dimensions with either Dirichlet, Neumann, or periodic boundary conditions are solved with this scheme and shown to outperform a variety of other second-order implicit-explicit schemes. An additional performance boost is gained through further use of basic parallelization techniques.}, language = {en} } @incollection{Kleefeld2020, author = {Kleefeld, Andreas}, title = {Numerical calculation of interior transmission eigenvalues with mixed boundary conditions}, series = {Computational and Analytic Methods in Science and Engineering}, booktitle = {Computational and Analytic Methods in Science and Engineering}, editor = {Constanda, Christian}, publisher = {Birkh{\"a}user}, address = {Cham}, isbn = {978-3-030-48185-8 (Hardcover)}, doi = {10.1007/978-3-030-48186-5_9}, pages = {173 -- 195}, year = {2020}, abstract = {Interior transmission eigenvalue problems for the Helmholtz equation play an important role in inverse wave scattering. Some distribution properties of those eigenvalues in the complex plane are reviewed. Further, a new scattering model for the interior transmission eigenvalue problem with mixed boundary conditions is described and an efficient algorithm for computing the interior transmission eigenvalues is proposed. Finally, extensive numerical results for a variety of two-dimensional scatterers are presented to show the validity of the proposed scheme.}, language = {en} } @article{MartinVaqueroKleefeld2020, author = {Mart{\´i}n-Vaquero, J. and Kleefeld, Andreas}, title = {Solving nonlinear parabolic PDEs in several dimensions: Parallelized ESERK codes}, series = {Journal of Computational Physics}, journal = {Journal of Computational Physics}, number = {423}, publisher = {Elsevier}, address = {Amsterdam}, issn = {0021-9991}, doi = {10.1016/j.jcp.2020.109771}, year = {2020}, abstract = {There is a very large number of very important situations which can be modeled with nonlinear parabolic partial differential equations (PDEs) in several dimensions. In general, these PDEs can be solved by discretizing in the spatial variables and transforming them into huge systems of ordinary differential equations (ODEs), which are very stiff. Therefore, standard explicit methods require a large number of iterations to solve stiff problems. But implicit schemes are computationally very expensive when solving huge systems of nonlinear ODEs. Several families of Extrapolated Stabilized Explicit Runge-Kutta schemes (ESERK) with different order of accuracy (3 to 6) are derived and analyzed in this work. They are explicit methods, with stability regions extended, along the negative real semi-axis, quadratically with respect to the number of stages s, hence they can be considered to solve stiff problems much faster than traditional explicit schemes. Additionally, they allow the adaptation of the step length easily with a very small cost. Two new families of ESERK schemes (ESERK3 and ESERK6) are derived, and analyzed, in this work. Each family has more than 50 new schemes, with up to 84.000 stages in the case of ESERK6. For the first time, we also parallelized all these new variable step length and variable number of stages algorithms (ESERK3, ESERK4, ESERK5, and ESERK6). These parallelized strategies allow to decrease times significantly, as it is discussed and also shown numerically in two problems. Thus, the new codes provide very good results compared to other well-known ODE solvers. Finally, a new strategy is proposed to increase the efficiency of these schemes, and it is discussed the idea of combining ESERK families in one code, because typically, stiff problems have different zones and according to them and the requested tolerance the optimum order of convergence is different.}, language = {en} } @article{Kleefeld2021, author = {Kleefeld, Andreas}, title = {The hot spots conjecture can be false: some numerical examples}, series = {Advances in Computational Mathematics}, volume = {47}, journal = {Advances in Computational Mathematics}, publisher = {Springer}, address = {Dordrecht}, issn = {1019-7168}, doi = {10.1007/s10444-021-09911-5}, year = {2021}, abstract = {The hot spots conjecture is only known to be true for special geometries. This paper shows numerically that the hot spots conjecture can fail to be true for easy to construct bounded domains with one hole. The underlying eigenvalue problem for the Laplace equation with Neumann boundary condition is solved with boundary integral equations yielding a non-linear eigenvalue problem. Its discretization via the boundary element collocation method in combination with the algorithm by Beyn yields highly accurate results both for the first non-zero eigenvalue and its corresponding eigenfunction which is due to superconvergence. Additionally, it can be shown numerically that the ratio between the maximal/minimal value inside the domain and its maximal/minimal value on the boundary can be larger than 1 + 10- 3. Finally, numerical examples for easy to construct domains with up to five holes are provided which fail the hot spots conjecture as well.}, language = {en} } @misc{BurgethKleefeldNaegeletal.2020, author = {Burgeth, Bernhard and Kleefeld, Andreas and Naegel, Beno{\^i}t and Perret, Benjamin}, title = {Editorial — Special Issue: ISMM 2019}, series = {Mathematical Morphology - Theory and Applications}, volume = {4}, journal = {Mathematical Morphology - Theory and Applications}, number = {1}, publisher = {De Gruyter}, address = {Warschau}, issn = {2353-3390}, doi = {10.1515/mathm-2020-0200}, pages = {159 -- 161}, year = {2020}, abstract = {This editorial presents the Special Issue dedicated to the conference ISMM 2019 and summarizes the articles published in this Special Issue.}, language = {en} } @article{KleefeldZimmermann2022, author = {Kleefeld, Andreas and Zimmermann, M.}, title = {Computing Elastic Interior Transmission Eigenvalues}, series = {Integral Methods in Science and Engineering}, journal = {Integral Methods in Science and Engineering}, editor = {Constanda, Christian and Bodmann, Bardo E.J. and Harris, Paul J.}, publisher = {Birkh{\"a}user}, address = {Cham}, isbn = {978-3-031-07171-3}, doi = {10.1007/978-3-031-07171-3_10}, pages = {139 -- 155}, year = {2022}, abstract = {An alternative method is presented to numerically compute interior elastic transmission eigenvalues for various domains in two dimensions. This is achieved by discretizing the resulting system of boundary integral equations in combination with a nonlinear eigenvalue solver. Numerical results are given to show that this new approach can provide better results than the finite element method when dealing with general domains.}, language = {en} } @inproceedings{BurgethKleefeldZhangetal.2022, author = {Burgeth, Bernhard and Kleefeld, Andreas and Zhang, Eugene and Zhang, Yue}, title = {Towards Topological Analysis of Non-symmetric Tensor Fields via Complexification}, series = {Discrete Geometry and Mathematical Morphology}, booktitle = {Discrete Geometry and Mathematical Morphology}, editor = {Baudrier, {\´E}tienne and Naegel, Beno{\^i}t and Kr{\"a}henb{\"u}hl, Adrien and Tajine, Mohamed}, publisher = {Springer}, address = {Cham}, isbn = {978-3-031-19897-7}, doi = {10.1007/978-3-031-19897-7_5}, pages = {48 -- 59}, year = {2022}, abstract = {Fields of asymmetric tensors play an important role in many applications such as medical imaging (diffusion tensor magnetic resonance imaging), physics, and civil engineering (for example Cauchy-Green-deformation tensor, strain tensor with local rotations, etc.). However, such asymmetric tensors are usually symmetrized and then further processed. Using this procedure results in a loss of information. A new method for the processing of asymmetric tensor fields is proposed restricting our attention to tensors of second-order given by a 2x2 array or matrix with real entries. This is achieved by a transformation resulting in Hermitian matrices that have an eigendecomposition similar to symmetric matrices. With this new idea numerical results for real-world data arising from a deformation of an object by external forces are given. It is shown that the asymmetric part indeed contains valuable information.}, language = {en} } @article{HarrisKleefeld2022, author = {Harris, Isaac and Kleefeld, Andreas}, title = {Analysis and computation of the transmission eigenvalues with a conductive boundary condition}, series = {Applicable Analysis}, volume = {101}, journal = {Applicable Analysis}, number = {6}, publisher = {Taylor \& Francis}, address = {London}, issn = {1563-504X}, doi = {10.1080/00036811.2020.1789598}, pages = {1880 -- 1895}, year = {2022}, abstract = {We provide a new analytical and computational study of the transmission eigenvalues with a conductive boundary condition. These eigenvalues are derived from the scalar inverse scattering problem for an inhomogeneous material with a conductive boundary condition. The goal is to study how these eigenvalues depend on the material parameters in order to estimate the refractive index. The analytical questions we study are: deriving Faber-Krahn type lower bounds, the discreteness and limiting behavior of the transmission eigenvalues as the conductivity tends to infinity for a sign changing contrast. We also provide a numerical study of a new boundary integral equation for computing the eigenvalues. Lastly, using the limiting behavior we will numerically estimate the refractive index from the eigenvalues provided the conductivity is sufficiently large but unknown.}, language = {en} } @article{ClausnitzerKleefeld2024, author = {Clausnitzer, Julian and Kleefeld, Andreas}, title = {A spectral Galerkin exponential Euler time-stepping scheme for parabolic SPDEs on two-dimensional domains with a C² boundary}, series = {Discrete and Continuous Dynamical Systems - Series B}, volume = {29}, journal = {Discrete and Continuous Dynamical Systems - Series B}, number = {4}, publisher = {AIMS}, address = {Springfield}, issn = {1531-3492}, doi = {10.3934/dcdsb.2023148}, pages = {1624 -- 1651}, year = {2024}, abstract = {We consider the numerical approximation of second-order semi-linear parabolic stochastic partial differential equations interpreted in the mild sense which we solve on general two-dimensional domains with a C² boundary with homogeneous Dirichlet boundary conditions. The equations are driven by Gaussian additive noise, and several Lipschitz-like conditions are imposed on the nonlinear function. We discretize in space with a spectral Galerkin method and in time using an explicit Euler-like scheme. For irregular shapes, the necessary Dirichlet eigenvalues and eigenfunctions are obtained from a boundary integral equation method. This yields a nonlinear eigenvalue problem, which is discretized using a boundary element collocation method and is solved with the Beyn contour integral algorithm. We present an error analysis as well as numerical results on an exemplary asymmetric shape, and point out limitations of the approach.}, language = {en} } @article{SchierenKleinschmidtSchmutzetal.2019, author = {Schieren, Mark and Kleinschmidt, Joris and Schmutz, Axel and Loop, Torsten and Gatzweiler, Karl-Heinz and Staat, Manfred and Wappler, Frank and Defosse, Jerome}, title = {Comparison of forces acting on maxillary incisors during tracheal intubation with different laryngoscopy techniques: a blinded manikin study}, series = {Anaesthesia}, volume = {74}, journal = {Anaesthesia}, number = {12}, publisher = {Wiley-Blackwell}, address = {Oxford}, isbn = {1365-2044}, doi = {10.1111/anae.14815}, year = {2019}, language = {en} } @incollection{Laack2020, author = {Laack, Walter van}, title = {Twee Kanten van {\´e}{\´e}n Medaille}, series = {Het Geheim van Elysion : 45 Jaar Studies naar Nabij-de-Dood-Ervaringen over Bewustzijn in Liefde zonder Waarheen}, booktitle = {Het Geheim van Elysion : 45 Jaar Studies naar Nabij-de-Dood-Ervaringen over Bewustzijn in Liefde zonder Waarheen}, publisher = {Van Warven}, address = {Kampen}, isbn = {978-94-93175-44-0}, pages = {97 -- 105}, year = {2020}, language = {nl} } @incollection{EngelmannShashaSlabu2021, author = {Engelmann, Ulrich M. and Shasha, Carolyn and Slabu, Ioana}, title = {Magnetic nanoparticle relaxation in biomedical application: focus on simulating nanoparticle heating}, series = {Magnetic nanoparticles in human health and medicine}, booktitle = {Magnetic nanoparticles in human health and medicine}, publisher = {Wiley-Blackwell}, address = {Hoboken, New Jeersey}, isbn = {978-1-119-75467-1}, pages = {327 -- 354}, year = {2021}, language = {en} } @unpublished{BornheimGriegerBlanecketal.2023, author = {Bornheim, Tobias and Grieger, Niklas and Blaneck, Patrick Gustav and Bialonski, Stephan}, title = {Preprint: Speaker attribution in German parliamentary debates with QLoRA-adapted large language models}, series = {Journal for Language Technology and Computational Linguistics}, journal = {Journal for Language Technology and Computational Linguistics}, doi = {10.48550/arXiv.2309.09902}, pages = {8 Seiten}, year = {2023}, abstract = {The growing body of political texts opens up new opportunities for rich insights into political dynamics and ideologies but also increases the workload for manual analysis. Automated speaker attribution, which detects who said what to whom in a speech event and is closely related to semantic role labeling, is an important processing step for computational text analysis. We study the potential of the large language model family Llama 2 to automate speaker attribution in German parliamentary debates from 2017-2021. We fine-tune Llama 2 with QLoRA, an efficient training strategy, and observe our approach to achieve competitive performance in the GermEval 2023 Shared Task On Speaker Attribution in German News Articles and Parliamentary Debates. Our results shed light on the capabilities of large language models in automating speaker attribution, revealing a promising avenue for computational analysis of political discourse and the development of semantic role labeling systems.}, language = {en} } @inproceedings{BornheimGriegerBialonski2021, author = {Bornheim, Tobias and Grieger, Niklas and Bialonski, Stephan}, title = {FHAC at GermEval 2021: Identifying German toxic, engaging, and fact-claiming comments with ensemble learning}, series = {Proceedings of the GermEval 2021 Workshop on the Identification of Toxic, Engaging, and Fact-Claiming Comments : 17th Conference on Natural Language Processing KONVENS 2021}, booktitle = {Proceedings of the GermEval 2021 Workshop on the Identification of Toxic, Engaging, and Fact-Claiming Comments : 17th Conference on Natural Language Processing KONVENS 2021}, publisher = {Heinrich Heine University}, address = {D{\"u}sseldorf}, doi = {10.48415/2021/fhw5-x128}, pages = {105 -- 111}, year = {2021}, language = {en} }