5,091 research outputs found
Multiple radial positive solutions of semilinear elliptic problems with Neumann boundary conditions
Assuming is a ball in , we analyze the positive
solutions of the problem that branch out from the constant solution as grows from to
. The non-zero constant positive solution is the unique positive
solution for close to . We show that there exist arbitrarily many
positive solutions as (in particular, for supercritical exponents)
or as for any fixed value of , answering partially a
conjecture in [Bonheure-Noris-Weth]. We give the explicit lower bounds for
and so that a given number of solutions exist. The geometrical properties
of those solutions are studied and illustrated numerically. Our simulations
motivate additional conjectures. The structure of the least energy solutions
(among all or only among radial solutions) and other related problems are also
discussed.Comment: 37 pages, 24 figure
Characteristic Functions Describing the Power Absorption Response of Periodic Structures to Partially Coherent Fields
Many new types of sensing or imaging surfaces are based on periodic thin
films. It is explained how the response of those surfaces to partially coherent
fields can be fully characterized by a set of functions in the wavenumber
spectrum domain. The theory is developed here for the case of 2D absorbers with
TE illumination and arbitrary material properties in the plane of the problem,
except for the resistivity which is assumed isotropic. Sum and difference
coordinates in both spatial and spectral domains are conveniently used to
represent the characteristic functions, which are specialized here to the case
of periodic structures. Those functions can be either computed or obtained
experimentally. Simulations rely on solvers based on periodic-boundary
conditions, while experiments correspond to Energy Absorption Interferometry
(EAI), already described in the literature. We derive rules for the convergence
of the representation versus the number of characteristic functions used, as
well as for the sampling to be considered in EAI experiments. Numerical
examples are given for the case of absorbing strips printed on a semi-infinite
substrate.Comment: Submitted to JOSA
Characterization of Power Absorption Response of Periodic 3D Structures to Partially Coherent Fields
In many applications of absorbing structures it is important to understand
their spatial response to incident fields, for example in thermal solar panels,
bolometric imaging and controlling radiative heat transfer. In practice, the
illuminating field often originates from thermal sources and is only spatially
partially coherent when reaching the absorbing device. In this paper, we
present a method to fully characterize the way a structure can absorb such
partially coherent fields. The method is presented for any 3D material and
accounts for the partial coherence and partial polarization of the incident
light. This characterization can be achieved numerically using simulation
results or experimentally using the Energy Absorption Interferometry (EAI) that
has been described previously in the literature. The absorbing structure is
characterized through a set of absorbing functions, onto which any partially
coherent field can be projected. This set is compact for any structure of
finite extent and the absorbing function discrete for periodic structures
Atmospheric Stellar Parameters from Cross-Correlation Functions
The increasing number of spectra gathered by spectroscopic sky surveys and
transiting exoplanet follow-up has pushed the community to develop automated
tools for atmospheric stellar parameters determination. Here we present a novel
approach that allows the measurement of temperature (),
metallicity () and gravity () within a few seconds
and in a completely automated fashion. Rather than performing comparisons with
spectral libraries, our technique is based on the determination of several
cross-correlation functions (CCFs) obtained by including spectral features with
different sensitivity to the photospheric parameters. We use literature stellar
parameters of high signal-to-noise (), high-resolution HARPS
spectra of FGK Main Sequence stars to calibrate , and as a function of CCFs parameters. Our technique is validated
using low spectra obtained with the same instrument. For FGK
stars we achieve a precision of K, and at , while the precision for observation with
and the overall accuracy are constrained by the
literature values used to calibrate the CCFs. Our approach can be easily
extended to other instruments with similar spectral range and resolution, or to
other spectral range and stars other than FGK dwarfs if a large sample of
reference stars is available for the calibration. Additionally, we provide the
mathematical formulation to convert synthetic equivalent widths to CCF
parameters as an alternative to direct calibration. We have made our tool
publicly available.Comment: Accepted by MNRAS. 12 pages, 12 figures. The code to retrieve the
atmospheric stellar parameters from HARPS and HARPS-N spectra is available
"at this url, https://github.com/LucaMalavolta/CCFpams
Réutilisation d'un corpus pour une nouvelle analyse des réseaux sociaux grâce à l'adaptation de l'outil KSV
In the field of interaction analysis in computer supported collaborative learning, this paper revisits a previous analysis by applying a new tool on a corpus now available. The goal is to analyze cliques (Social Network Analysis) by traveling along the dimensions of intensity and time, thanks to an adapted version of KSV: Knowledge Space Visualizer. The theory invoked is SNA. This new exploration alows us to suggest new forms of indicators for cohesion and espescially to compare cohesion of different groups. This work also made possible some improvements for KSV that gives it new perspectives.Dans le champ de l'analyse des interactions en situation d'apprentissage collaboratif en ligne, nous revisitons un corpus connu et accessible pour en reprendre l'analyse avec un outil qui offre de nouvelles dimensions d'exploration : l'intensité et le temps. Le cadre d'analyse est ici celui des réseaux sociaux. Cette nouvelle exploration des données nous permet de proposer de nouveaux indicateurs pour comparer la cohésion des groupes au regard de l'intensité de communication. Cette rencontre entre un corpus et un outil a aussi permis d'étendre les capacités de traitement et de visualisation de l'outil KSV, qui lui offrent de nouvelles perspectives
Destabilization of a flow focused suspension of magnetotactic bacteria
Active matter is a new class of material, intrinsically out-of equilibrium
with intriguing properties. So far, the recent upsurge of studies has mostly
focused on the spontaneous behavior of these systems --in the absence of
external constraints or driving--. Yet, many real life systems evolve under
constraints, being both submitted to flow and various taxis. In the present
work, we demonstrate a new experimental system which opens up the way for
quantitative investigations, and discriminating examinations, of the
challenging theoretical description of such systems. We explore the behavior of
magnetotactic bacteria as a particularly rich and versatile class of driven
matter, which behavior can be studied under contrasting and contradicting
stimuli. In particular we demonstrate that the competing driving of an
orienting magnetic field and hydrodynamic flow lead not only to jetting, but
also unveils a new pearling instability. This illustrates new structuring
capabilities of driven active matter
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