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Geometric multiscale decompositions of dynamic low-rank matrices
(2012)SAM Research ReportThe present paper is concerned with the study of manifold-valued multiscale transforms with a focus on the Stiefel manifold. For this specific geometry we derive several formulas and algorithms for the computation of geometric means which will later enable us to construct multiscale transforms of wavelet type. As an application we study compression of piecewise smooth families of low-rank matrices both for synthetic data and also real-world ...Report -
hp-DG-QTT solution of high-dimensional degenerate diffusion equations
(2012)SAM Research ReportWe consider the discretization of degenerate, time-inhomogeneous Fokker-Planck equations for diffusion problems in high-dimensional domains. Well-posedness of the problem in time-weighted Bochner spaces is established. Analytic regularity of the time-dependence of the solution in countably normed, weighted Sobolev spaces is established. Time discretization by the hp-discontinuous Galerkin method is shown to converge exponentially. The ...Report -
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Phosphate nutrition of crops in lixisols from semi-arid West Africa
(2012)Annual report 2011 North-South CentreReport -
A Time-Splitting for the Semiclassical Schrödinger Equation
(2012)SAM Research ReportWe propose a new algorithm for solving the semiclassical time–dependent Schr¨odinger equation. The algorithm is based on semiclassical wavepackets. Convergence is proved to be quadratic in the time step and linear in the semiclassical parameter !. To the the authors’ best knowledge, all earlier algorithms had errors that grew as the semiclassical parameter was decreased.Report -
Evaluating User Privacy in Bitcoin
(2012)Cryptology ePrint ArchiveBitcoin is quickly emerging as a popular digital payment system. However, in spite of its reliance on pseudonyms, Bitcoin raises a number of privacy concerns due to the fact that all of the transactions that take place are publicly announced in the system. In this paper, we investigate the privacy guarantees of Bitcoin in the setting where Bitcoin is used as a primary currency for the daily transactions of individuals. More specifically, ...Report -
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Fukushima und die Grenzen der Risikoanalyse
(2012)CSS Risk and Resilience ReportsWas können Risikoanalysen erfassen und was nicht, wie vertrauenswürdig können sie sein, und was für Aussagen über die Zukunft können aus ihnen hergeleitet werden? Dieses Factsheet thematisiert die Grenzen der Risikoanalyse. Anhand verschiedener Beispiele aus dem Kernenergiebereich untersucht es, wie die Definition von Risiken, die Methodologie ihrer Erfassung sowie die Interpretation von Resultaten in der probabilistischen Risikoanalyse ...Report -
Fast Private Set Operations with SEPIA
(2012)TIK ReportPrivate set operations allow correlation of sensitive data from multiple data owners. Although intensely researched, current solutions still exhibit limited scalability in terms of the supported maximum set size and number of sets. To address these issues, we propose a new approach to private set operations based on a combination of efficient secure multiparty computation and bloom filters, a space-efficient probabilistic data structure ...Report