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Software Supply Chain Attacks
(2023)CSS Cyberdefense ReportsMost elements constituting modern life, from the economy to social habits, are now characterized by using digital technologies and the consumption of goods and services that depend on complex, interconnected, transnational, and, at times, vulnerable, supply chains. Critical dependencies and heightened (cyber) threats combined with strategic competitiveness are increasingly turning the issue of supply chain security into matters of national ...Report -
National Risk Assessments of Cross-Border Risks
(2023)CSS Risk and Resilience ReportsThis report compares the assessment of nine European countries of five cross-border risks: An electricity supply shortage, a nuclear accident, a pandemic, severe space weather, and a volcanic outbreak. Even though the selected countries have correlated risk profiles for these hazards, the report finds some significant differences in their likelihood and impact estimations. For example, for a severe nuclear accident scenario the likelihood ...Report -
Semi-Lagrangian Finite-Element Exterior Calculus for Incompressible Flows
(2023)SAM Research ReportWe develop a mesh-based semi-Lagrangian discretization of the time-dependent incompressible Navier-Stokes equations with free boundary conditions recast as a non-linear transport problem for a momentum 1-form. A linearly implicit fully discrete version of the scheme enjoys excellent stability properties in the vanishing viscosity limit and is applicable to incviscid incompressible Euler flows. Conservation of energy and helicity are ...Report -
A mathematical theory of super-resolution and diffraction limit
(2023)SAM Research ReportThis paper is devoted to elucidating the essence of super-resolution and deals mainly with the stability of super-resolution and the diffraction limit. The first discovery is two location-amplitude identities characterizing the relations between source locations and amplitudes in the super-resolution problem. These identities allow us to directly derive the super-resolution capability for number, location, and amplitude recovery in the ...Report -
A mathematical theory of resolution limits for super-resolution of positive sources
(2023)SAM Research ReportThe superresolving capacity for number and location recoveries in the super-resolution of positive sources is analyzed in this work. Specifically, we introduce the computational resolution limit for respectively the number detection and location recovery in the one-dimensional super-resolution problem and quantitatively characterize their dependency on the cutoff frequency, signal-to-noise ratio, and the sparsity of the sources. As a ...Report -
Transmission properties of time-dependent one-dimensional metamaterials
(2023)SAM Research ReportWe solve the wave equation with periodically time-modulated material parameters in a one-dimensional high-contrast resonator structure in the subwavelength regime exactly, for which we compute the subwavelength quasifrequencies numerically using Muller's method. We prove a formula in the form of an ODE using a capacitance matrix approximation. Comparison of the exact results with the approximations reveals that the method of capacitance ...Report -
Multi-Level Markov Chain Monte Carlo for Bayesian Elliptic Inverse Problems with Besov Random Tree Priors
(2023)SAM Research ReportWe propose a multilevel Markov Chain Monte Carlo-FEM algorithm to solve elliptic Bayesian inverse problems with "Besov random tree prior". These priors are given by a wavelet series with stochastic coefficients, and certain terms in the expansion vanishing at random, according to the law of so-called Galton-Watson trees. This allows to incorporate random fractal structures and large deviations in the log-diffusion, which occur naturally ...Report -
Spectral convergence of defect modes in large finite resonator arrays
(2023)SAM Research ReportWe show that defect modes in infinite systems of resonators have corresponding modes in finite systems which converge as the size of the system increases. We study the generalized capacitance matrix as a model for three-dimensional coupled resonators with long-range interactions and consider defect modes that are induced by compact perturbations. If such a mode exists, then there are elements of the discrete spectrum of the corresponding ...Report -
A-posteriori QMC-FEM error estimation for Bayesian inversion and optimal control with entropic risk measure
(2023)SAM Research ReportWe propose a novel a-posteriori error estimation technique where the target quantities of interest are ratios of high-dimensional integrals, as occur e.g. in PDE constrained Bayesian inversion and PDE constrained optimal control subject to an entropic risk measure. We consider in particular parametric, elliptic PDEs with affine-parametric diffusion coefficient, on high-dimensional parameter spaces. We combine our recent a-posteriori ...Report -
Edge modes in subwavelength resonators in one dimension
(2023)SAM Research ReportWe present the mathematical theory of one-dimensional infinitely periodic chains of subwavelength resonators. We analyse both Hermitian and non-Hermitian systems. Subwavelength resonances and associated modes can be accurately predicted by a finite dimensional eigenvalue problem involving a capacitance matrix. We are able to compute the Hermitian and non-Hermitian Zak phases, showing that the former is quantised and the latter is not. ...Report