65 research outputs found
Einfluss der BearbeitungsintensitÀt beim Umbruch von Leguminosen-Grasgemengen auf die N-Mineralisierung zur Folge-frucht Winterweizen
Einleitung
Leguminosen-Grasgemenge sind als N-Quelle in Fruchtfolgen des Organischen Landbaus von zentraler Bedeutung. Nach meist mehrjĂ€hrigem Anbau wird der von den Leguminosen assimilierte Stickstoff mit dem Umbruch der Narbe durch die Mineralisation der organischen Substanz wieder freigesetzt. In der Praxis steht fĂŒr eine Steuerung der Mineralisierung ausschlieĂlich das Instrument der Bodenbearbei-tung zur VerfĂŒgung. Ziel dieser Untersuchungen war es, die Beeinflussbarkeit der N Mineralisierung durch eine Variation des Narbenumbruchs zu quantifizieren, um so eine Datengrundlage fĂŒr die Modellierung der N-Dynamik in AbhĂ€ngigkeit von der Bodenbearbeitung zu schaffen
Generative Sliced MMD Flows with Riesz Kernels
Maximum mean discrepancy (MMD) flows suffer from high computational costs in
large scale computations. In this paper, we show that MMD flows with Riesz
kernels , have exceptional properties which
allow for their efficient computation. First, the MMD of Riesz kernels
coincides with the MMD of their sliced version. As a consequence, the
computation of gradients of MMDs can be performed in the one-dimensional
setting. Here, for , a simple sorting algorithm can be applied to reduce
the complexity from to for two empirical
measures with and support points. For the implementations we
approximate the gradient of the sliced MMD by using only a finite number of
slices. We show that the resulting error has complexity , where
is the data dimension. These results enable us to train generative models
by approximating MMD gradient flows by neural networks even for large scale
applications. We demonstrate the efficiency of our model by image generation on
MNIST, FashionMNIST and CIFAR10
Learning-based approaches for reconstructions with inexact operators in nanoCT applications
Imaging problems such as the one in nanoCT require the solution of an inverse
problem, where it is often taken for granted that the forward operator, i.e.,
the underlying physical model, is properly known. In the present work we
address the problem where the forward model is inexact due to stochastic or
deterministic deviations during the measurement process. We particularly
investigate the performance of non-learned iterative reconstruction methods
dealing with inexactness and learned reconstruction schemes, which are based on
U-Nets and conditional invertible neural networks. The latter also provide the
opportunity for uncertainty quantification. A synthetic large data set in line
with a typical nanoCT setting is provided and extensive numerical experiments
are conducted evaluating the proposed methods
MultiTalent: A Multi-Dataset Approach to Medical Image Segmentation
The medical imaging community generates a wealth of datasets, many of which
are openly accessible and annotated for specific diseases and tasks such as
multi-organ or lesion segmentation. Current practices continue to limit model
training and supervised pre-training to one or a few similar datasets,
neglecting the synergistic potential of other available annotated data. We
propose MultiTalent, a method that leverages multiple CT datasets with diverse
and conflicting class definitions to train a single model for a comprehensive
structure segmentation. Our results demonstrate improved segmentation
performance compared to previous related approaches, systematically, also
compared to single dataset training using state-of-the-art methods, especially
for lesion segmentation and other challenging structures. We show that
MultiTalent also represents a powerful foundation model that offers a superior
pre-training for various segmentation tasks compared to commonly used
supervised or unsupervised pre-training baselines. Our findings offer a new
direction for the medical imaging community to effectively utilize the wealth
of available data for improved segmentation performance. The code and model
weights will be published here: [tba]Comment: Accepted for Miccai 2023 and selected for an ora
On the Doppler effect for light from orbiting sources in Kerr-type metrics
A formula is derived for the combined motional and gravitational Doppler
effect in general stationary axisymmetric metrics for a photon emitted parallel
or antiparallel to the assumed circular orbital motion of its source. The same
formula is derived from eikonal approximation and Killing vector approaches to
elucidate connections between observational astronomy and modern Relativity.
The formula yields expected results in the limits of a moving or stationary
source in the exterior Kerr and Schwarzschild metrics and a moving source in
flat space.Comment: Accepted for publication in in Monthly Notices of the Royal
Astronomical Society Main Journal 1.23.15. This version has substantially
shortened and clarified derivations and added content regarding applicability
of the derivation
Motion of charged particles around a rotating black hole in a magnetic field
We study the effects of an external magnetic field, which is assumed to be
uniform at infinity, on the marginally stable circular motion of charged
particles in the equatorial plane of a rotating black hole. We show that the
magnetic field has its greatest effect in enlarging the region of stability
towards the event horizon of the black hole. Using the Hamilton-Jacobi
formalism we obtain the basic equations governing the marginal stability of the
circular orbits and their associated energies and angular momenta. As
instructive examples, we review the case of the marginal stability of the
circular orbits in the Kerr metric, as well as around a Schwarzschild black
hole in a magnetic field. For large enough values of the magnetic field around
a maximally rotating black hole we find the limiting analytical solutions to
the equations governing the radii of marginal stability. We also show that the
presence of a strong magnetic field provides the possibility of relativistic
motions in both direct and retrograde innermost stable circular orbits around a
Kerr black hole.Comment: 25 pages, 2 figure
Black-Hole Spin Dependence in the Light Curves of Tidal Disruption Events
A star orbiting a supermassive black hole can be tidally disrupted if the
black hole's gravitational tidal field exceeds the star's self gravity at
pericenter. Some of this stellar tidal debris can become gravitationally bound
to the black hole, leading to a bright electromagnetic flare with bolometric
luminosity proportional to the rate at which material falls back to pericenter.
In the Newtonian limit, this flare will have a light curve that scales as
t^-5/3 if the tidal debris has a flat distribution in binding energy. We
investigate the time dependence of the black-hole mass accretion rate when
tidal disruption occurs close enough the black hole that relativistic effects
are significant. We find that for orbits with pericenters comparable to the
radius of the marginally bound circular orbit, relativistic effects can double
the peak accretion rate and halve the time it takes to reach this peak
accretion rate. The accretion rate depends on both the magnitude of the
black-hole spin and its orientation with respect to the stellar orbit; for
orbits with a given pericenter radius in Boyer-Lindquist coordinates, a maximal
black-hole spin anti-aligned with the orbital angular momentum leads to the
largest peak accretion rate.Comment: 16 pages, 15 figures, 1 table, PRD published versio
RecycleNet: Latent Feature Recycling Leads to Iterative Decision Refinement
Despite the remarkable success of deep learning systems over the last decade,
a key difference still remains between neural network and human
decision-making: As humans, we cannot only form a decision on the spot, but
also ponder, revisiting an initial guess from different angles, distilling
relevant information, arriving at a better decision. Here, we propose
RecycleNet, a latent feature recycling method, instilling the pondering
capability for neural networks to refine initial decisions over a number of
recycling steps, where outputs are fed back into earlier network layers in an
iterative fashion. This approach makes minimal assumptions about the neural
network architecture and thus can be implemented in a wide variety of contexts.
Using medical image segmentation as the evaluation environment, we show that
latent feature recycling enables the network to iteratively refine initial
predictions even beyond the iterations seen during training, converging towards
an improved decision. We evaluate this across a variety of segmentation
benchmarks and show consistent improvements even compared with top-performing
segmentation methods. This allows trading increased computation time for
improved performance, which can be beneficial, especially for safety-critical
applications.Comment: Accepted at 2024 Winter Conference on Applications of Computer Vision
(WACV
Spintronics: Fundamentals and applications
Spintronics, or spin electronics, involves the study of active control and
manipulation of spin degrees of freedom in solid-state systems. This article
reviews the current status of this subject, including both recent advances and
well-established results. The primary focus is on the basic physical principles
underlying the generation of carrier spin polarization, spin dynamics, and
spin-polarized transport in semiconductors and metals. Spin transport differs
from charge transport in that spin is a nonconserved quantity in solids due to
spin-orbit and hyperfine coupling. The authors discuss in detail spin
decoherence mechanisms in metals and semiconductors. Various theories of spin
injection and spin-polarized transport are applied to hybrid structures
relevant to spin-based devices and fundamental studies of materials properties.
Experimental work is reviewed with the emphasis on projected applications, in
which external electric and magnetic fields and illumination by light will be
used to control spin and charge dynamics to create new functionalities not
feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes
from the published versio
Gravitating discs around black holes
Fluid discs and tori around black holes are discussed within different
approaches and with the emphasis on the role of disc gravity. First reviewed
are the prospects of investigating the gravitational field of a black
hole--disc system by analytical solutions of stationary, axially symmetric
Einstein's equations. Then, more detailed considerations are focused to middle
and outer parts of extended disc-like configurations where relativistic effects
are small and the Newtonian description is adequate.
Within general relativity, only a static case has been analysed in detail.
Results are often very inspiring, however, simplifying assumptions must be
imposed: ad hoc profiles of the disc density are commonly assumed and the
effects of frame-dragging and completely lacking. Astrophysical discs (e.g.
accretion discs in active galactic nuclei) typically extend far beyond the
relativistic domain and are fairly diluted. However, self-gravity is still
essential for their structure and evolution, as well as for their radiation
emission and the impact on the environment around. For example, a nuclear star
cluster in a galactic centre may bear various imprints of mutual star--disc
interactions, which can be recognised in observational properties, such as the
relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be
available from http://www.iop.org/EJ/journal/CQ
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