12,914 research outputs found
Global pointwise estimates for Green's matrix of second order elliptic systems
We establish global pointwise bounds for the Green's matrix for divergence
form, second order elliptic systems in a domain under the assumption that weak
solutions of the system vanishing on a portion of the boundary satisfy a
certain local boundedness estimate. Moreover, we prove that such a local
boundedness estimate for weak solutions of the system is equivalent to the
usual global pointwise bound for the Green's matrix. In the scalar case, such
an estimate is a consequence of De Giorgi-Moser-Nash theory and holds for
equations with bounded measurable coefficients in arbitrary domains. In the
vectorial case, one need to impose certain assumptions on the coefficients of
the system as well as on domains to obtain such an estimate. We present a
unified approach valid for both the scalar and vectorial cases and discuss
several applications of our result.Comment: 17 pages, accepted in J. Differential Equations, references adde
Equilibrium Initialization and Stability of Three-Dimensional Gas Disks
We present a new systematic way of setting up galactic gas disks based on the
assumption of detailed hydrodynamic equilibrium. To do this, we need to specify
the density distribution and the velocity field which supports the disk. We
first show that the required circular velocity has no dependence on the height
above or below the midplane so long as the gas pressure is a function of
density only. The assumption of disks being very thin enables us to decouple
the vertical structure from the radial direction. Based on that, the equation
of hydrostatic equilibrium together with the reduced Poisson equation leads to
two sets of second-order non-linear differential equation, which are easily
integrated to set-up a stable disk. We call one approach `density method' and
the other one `potential method'. Gas disks in detailed balance are especially
suitable for investigating the onset of the gravitational instability. We
revisit the question of global, axisymmetric instability using fully
three-dimensional disk simulations. The impact of disk thickness on the disk
instability and the formation of spontaneously induced spirals is studied
systematically with or without the presence of the stellar potential. In our
models, the numerical results show that the threshold value for disk
instability is shifted from unity to 0.69 for self-gravitating thick disks and
to 0.75 for combined stellar and gas thick disks. The simulations also show
that self-induced spirals occur in the correct regions and with the right
numbers as predicted by the analytic theory.Comment: 17 pages, 10 figures, accepted by MNRA
Origin of superconductivity transition broadening in MgB2
We report resistivity and magnetization of single crystal MgB2, focusing on
the broadening of superconducting (SC) transition in magnetic fields. In-plane
and out-of-plane resistivity indicate that the broadening of superconducting
transition is independent of Lorentz force and that it is merely dependent on
the magnetic field direction. In magnetization, diamagnetic signal begins to
appear at almost the same temperature as the onset temperature of resistivity
transition. These results suggest that the broadening is attributed not to the
surface superconductivity but to the superconducting fluctuation or the
vortex-liquid picture, owing to the short coherence length and the high
transition temperature of MgB2.Comment: 8pages, 6 figures, to be published in Physica
The Lie Algebraic Significance of Symmetric Informationally Complete Measurements
Examples of symmetric informationally complete positive operator valued
measures (SIC-POVMs) have been constructed in every dimension less than or
equal to 67. However, it remains an open question whether they exist in all
finite dimensions. A SIC-POVM is usually thought of as a highly symmetric
structure in quantum state space. However, its elements can equally well be
regarded as a basis for the Lie algebra gl(d,C). In this paper we examine the
resulting structure constants, which are calculated from the traces of the
triple products of the SIC-POVM elements and which, it turns out, characterize
the SIC-POVM up to unitary equivalence. We show that the structure constants
have numerous remarkable properties. In particular we show that the existence
of a SIC-POVM in dimension d is equivalent to the existence of a certain
structure in the adjoint representation of gl(d,C). We hope that transforming
the problem in this way, from a question about quantum state space to a
question about Lie algebras, may help to make the existence problem tractable.Comment: 56 page
The pseudo scalar form factor of the nucleon, the sigma-like term, and the amplitude for charged pion electro-production near threshold
The pseudo scalar form factor, which represents the pseudo scalar quark
density distribution due to finite quark masses on the nucleon, is shown to
manifest itself with the induced pseudo scalar form factor in the
amplitude for the charged pion electro-production. Both form factors show their
own peculiar momentum dependence. Under the approximation on which the
Goldberg-Treimann relation holds, a sum of both form factors' contributions
accounts for the t-channel contribution in the charged pion electro-production
near threshold.Comment: 10 page
Neural Image-based Avatars: Generalizable Radiance Fields for Human Avatar Modeling
We present a method that enables synthesizing novel views and novel poses of
arbitrary human performers from sparse multi-view images. A key ingredient of
our method is a hybrid appearance blending module that combines the advantages
of the implicit body NeRF representation and image-based rendering. Existing
generalizable human NeRF methods that are conditioned on the body model have
shown robustness against the geometric variation of arbitrary human performers.
Yet they often exhibit blurry results when generalized onto unseen identities.
Meanwhile, image-based rendering shows high-quality results when sufficient
observations are available, whereas it suffers artifacts in sparse-view
settings. We propose Neural Image-based Avatars (NIA) that exploits the best of
those two methods: to maintain robustness under new articulations and
self-occlusions while directly leveraging the available (sparse) source view
colors to preserve appearance details of new subject identities. Our hybrid
design outperforms recent methods on both in-domain identity generalization as
well as challenging cross-dataset generalization settings. Also, in terms of
the pose generalization, our method outperforms even the per-subject optimized
animatable NeRF methods. The video results are available at
https://youngjoongunc.github.io/ni
Metastability, Mode Coupling and the Glass Transition
Mode coupling theory (MCT) has been successful in explaining the observed
sequence of time relaxations in dense fluids. Previous expositions of this
theory showing this sequence have required the existence of an ideal glass
transition temperature . Recent experiments show no evidence of . We
show here how the theory can be reformulated, in a fundamental way, such that
one retains this sequence of relaxation behaviors but with a smooth temperature
dependence and without any indication of . The key ingredient in the
reformulation is the inclusion of the metastable nature of the glass transition
problem through a coupling of the mass density to the defect density. A main
result of our theory is that the exponents governing the sequence of time
relaxations are weak functions of the temperature in contrast to the results
from conventional MCT.Comment: 14 pages (2 figures upon request), REVTEX
A New Satellite-Based Retrieval of Low-Cloud Liquid-Water Path Using Machine Learning and Meteosat SEVIRI Data
Clouds are one of the major uncertainties of the climate system. The study of cloud processes requires information on cloud physical properties, in particular liquid water path (LWP). This parameter is commonly retrieved from satellite data using look-up table approaches. However, existing LWP retrievals come with uncertainties related to assumptions inherent in physical retrievals. Here, we present a new retrieval technique for cloud LWP based on a statistical machine learning model. The approach utilizes spectral information from geostationary satellite channels of Meteosat Spinning-Enhanced Visible and Infrared Imager (SEVIRI), as well as satellite viewing geometry. As ground truth, data from CloudNet stations were used to train the model. We found that LWP predicted by the machine-learning model agrees substantially better with CloudNet observations than a current physics-based product, the Climate Monitoring Satellite Application Facility (CM SAF) CLoud property dAtAset using SEVIRI, edition 2 (CLAAS-2), highlighting the potential of such approaches for future retrieval developments
Le tourbillon des sentiments amoureux dans la poésie de Marceline Desbordes-Valmore ou la poétisation d’une histoire amoureuse
Oubliées à tort pendant plusieurs siècles, les femmes de lettres, dont Christine de Pisan, Louise Labé, Madelaine et Catherine des Roches, Marceline Desbordes-Valmore et beaucoup d’autres, prêtaient à maintes reprises leur plume prolixe au service des épanchements du cœur. La voix féminine de la poésie courtoise, Christine de Pisan, s’est inscrite dans l’histoire comme la première Française à avoir jamais vécu de ses écrits. D’origine vénitienne, elle s’est permis, à la fin du XIVe, début du XVe siècle, de célébrer le mariage et chanter, en son propre nom, l’amour pour son mari et le délice de la nuit de noces afin de montrer du doigt le concept erroné de l’amour courtois, vanté par les troubadours. Néanmoins, ce n’est qu’à la Renaissance que la scène littéraire assiste à « l’avènement » de la figure de la femme écrivaine/poétesse, incarnée par-dessus tout dans le personnage de Louise Labé, femme dotée de la liberté d’esprit, qui assumait pleinement sa féminité dans ses vers. À l’aube du XIXe siècle, le cercle s’agrandit avec Marceline Desbordes-Valmore, connue de nos jours comme prédécesseure de la poésie romantique
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