391 research outputs found
Minimal error momentum Bregman-Kaczmarz
The Bregman-Kaczmarz method is an iterative method which can solve strongly
convex problems with linear constraints and uses only one or a selected number
of rows of the system matrix in each iteration, thereby making it amenable for
large-scale systems. To speed up convergence, we investigate acceleration by
heavy ball momentum in the so-called dual update. Heavy ball acceleration of
the Kaczmarz method with constant parameters has turned out to be difficult to
analyze, in particular no accelerated convergence for the L2-error of the
iterates has been proven to the best of our knowledge. Here we propose a way to
adaptively choose the momentum parameter by a minimal-error principle similar
to a recently proposed method for the standard randomized Kaczmarz method. The
momentum parameter can be chosen to exactly minimize the error in the next
iterate or to minimize a relaxed version of the minimal error principle. The
former choice leads to a theoretically optimal step while the latter is cheaper
to compute. We prove improved convergence results compared to the
non-accelerated method. Numerical experiments show that the proposed methods
can accelerate convergence in practice, also for matrices which arise from
applications such as computational tomography
Collaboration in the offshore wind farm decommissioning supply chain
The purpose of this study is to explore how actors in the offshore wind farm decommissioning supply chain collaborate to manage collective uncertaintiesâexternal supply chain uncertainties shared among several supply chain actors. Semi-structured expert interviews with ten companies active in five different segments of the decommissioning supply chain in the Netherlands, Germany, and Belgium are conducted. Businesses currently utilize collaborative communication, information sharing, and joint knowledge creation to mitigate the adverse effects of collective uncertainties. However, collaboration is severely impeded by the presence of barriers such as intellectual property or the lack of resources. Nevertheless, opportunities to lower these hurdles and improve collaboration across the supply chain exist. This study contributes to the literature on collective uncertainties and supply chain collaboration in offshore wind decommissioning. Additionally, this paper provides practitioner-oriented insights to leverage supply chain collaboration in dealing with the decommissioning challenge
A Bregman-Kaczmarz method for nonlinear systems of equations
We propose a new randomized method for solving systems of nonlinear
equations, which can find sparse solutions or solutions under certain simple
constraints. The scheme only takes gradients of component functions and uses
Bregman projections onto the solution space of a Newton equation. In the
special case of euclidean projections, the method is known as nonlinear
Kaczmarz method. Furthermore, if the component functions are nonnegative, we
are in the setting of optimization under the interpolation assumption and the
method reduces to SGD with the recently proposed stochastic Polyak step size.
For general Bregman projections, our method is a stochastic mirror descent with
a novel adaptive step size. We prove that in the convex setting each iteration
of our method results in a smaller Bregman distance to exact solutions as
compared to the standard Polyak step. Our generalization to Bregman projections
comes with the price that a convex one-dimensional optimization problem needs
to be solved in each iteration. This can typically be done with globalized
Newton iterations. Convergence is proved in two classical settings of
nonlinearity: for convex nonnegative functions and locally for functions which
fulfill the tangential cone condition. Finally, we show examples in which the
proposed method outperforms similar methods with the same memory requirements
Mechanism of peptide-induced mast cell degranulation: translocation and patch clamp studies.
Substance P and other polycationic peptides are thought to stimulate mast cell degranulation via direct activation of G proteins. We investigated the ability of extracellularly applied substance P to translocate into mast cells and the ability of intracellularly applied substance P to stimulate degranulation. In addition, we studied by reverse transcription--PCR whether substance P-specific receptors are present in the mast cell membrane. To study translocation, a biologically active and enzymatically stable fluorescent analogue of substance P was synthesized. A rapid, substance P receptor- and energy-independent uptake of this peptide into pertussis toxin-treated and -untreated mast cells was demonstrated using confocal laser scanning microscopy. The peptide was shown to localize preferentially on or inside the mast cell granules using electron microscopic autoradiography with 125I-labeled all-D substance P and 3H-labeled substance P. Cell membrane capacitance measurements using the patch-clamp technique demonstrated that intracellularly applied substance P induced calcium transients and activated mast cell exocytosis with a time delay that depended on peptide concentration (delay of 100-500 s at concentrations of substance P from 50 to 5 microM). Degranulation in response to intracellularly applied substance P was inhibited by GDPbetaS and pertussis toxin, suggesting that substance P acts via G protein activation. These results support the recently proposed model of a receptor-independent mechanism of peptide-induced mast cell degranulation, which assumes a direct interaction of peptides with G protein alpha subunits subsequent to their translocation across the plasma membrane
Development of breast cancer mortality considering the implementation of mammography screening programs â a comparison of western European countries
Background: Triggered by the successive implementation of organized mammography screening programs (MSPs) throughout western European countries over the last decades, there is an ongoing debate questioning their effectiveness. Since it is difficult to assess the effect of MSPs on a population level, we rather aim to assess the impact of the implementation itself on breast cancer mortality rates utilizing an ecological study design.
Methods: We analyzed age group-specific (50â59, 60â69 and 70â79âyears) female breast cancer mortality rates in 14 western European countries between 1980 and 2017 using Joinpoint regression, interrupted time series (ITS) regression and multivariable Poisson regression.
Results: The Joinpoint analysis demonstrated decreasing trends resulting in annual percentage changes ranging from ââ1.5% to ââ5.4% (50â59), ââ0.2% to ââ8.1% (60â69) and 0% to ââ7.1% (70â79) depending on the country within 3 years after MSP implementation. The ITS analysis results in highly significant interaction terms (calendar year * binary MSP indicator) for all age groups. The multivariable regression using âcalendar yearâ, âyear of MSP implementationâ and âyears with MSPâ as independent variables yielded a significant yearly decrease for âyears with MSPâ ranging from 0.9 to 1.2%.
Conclusions: The results of this study suggest a positive association between the implementation of MSPs and the (accelerated) reduction of breast cancer mortality rates. Measuring and quantifying the isolated effect of MSPs on a population level will require additional studies using individual data
Role of Reaction Intermediate Diffusion on the Performance of Platinum Electrodes in Solid Acid Fuel Cells
Understanding the reaction pathways for the hydrogen oxidation reaction (HOR) and the oxygen reduction reaction (ORR) is the key to design electrodes for solid acid fuel cells (SAFCs). In general, electrochemical reactions of a fuel cell are considered to occur at the triple-phase boundary where an electrocatalyst, electrolyte and gas phase are in contact. In this concept, diffusion processes of reaction intermediates from the catalyst to the electrolyte remain unconsidered. Here, we unravel the reaction pathways for open-structured Pt electrodes with various electrode thicknesses from 15 to 240 nm. These electrodes are characterized by a triple-phase boundary length and a thickness-depending double-phase boundary area. We reveal that the double-phase boundary is the active catalytic interface for the HOR. For Pt layers †60 nm, the HOR rate is rate-limited by the processes at the gas/catalyst and/or the catalyst/electrolyte interface while the hydrogen surface diffusion step is fast. For thicker layers (>60 nm), the diffusion of reaction intermediates on the surface of Pt be-comes the limiting process. For the ORR, the predominant reaction pathway is via the triple-phase boundary. The double-phase boundary contributes additionally with a diffusion length of a few nanometers. Based on our results, we propose that the molecular reaction mechanism at the electrode interfaces based upon the triple-phase boundary concept may need to be extended to an effective area near the triple-phase boundary length to include all catalytically relevant diffusion processes of the reaction intermediates. © 2021 by the authors. Licensee MDPI, Basel, Switzerland
Measurement of Rashba and Dresselhaus spin-orbit magnetic fields
Spin-orbit coupling is a manifestation of special relativity. In the
reference frame of a moving electron, electric fields transform into magnetic
fields, which interact with the electron spin and lift the degeneracy of
spin-up and spin-down states. In solid-state systems, the resulting spin-orbit
fields are referred to as Dresselhaus or Rashba fields, depending on whether
the electric fields originate from bulk or structure inversion asymmetry,
respectively. Yet, it remains a challenge to determine the absolute value of
both contributions in a single sample. Here we show that both fields can be
measured by optically monitoring the angular dependence of the electrons' spin
precession on their direction of movement with respect to the crystal lattice.
Furthermore, we demonstrate spin resonance induced by the spin-orbit fields. We
apply our method to GaAs/InGaAs quantum-well electrons, but it can be used
universally to characterise spin-orbit interactions in semiconductors,
facilitating the design of spintronic devices
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