6,598 research outputs found
Interpolation by Linear Functions on an -Dimensional Ball
By we denote the Euclidean ball in given by
the inequality . Here ,
. We mean by the space of
continuous functions with the norm
and by
the set of polynomials in variables of degree , i.e., linear
functions on . Let be the vertices
of -dimensional nondegenerate simplex . The interpolation
projector
corresponding to is defined by the
equalities We obtain the
formula to compute the norm of as an operator from into via
, and coefficients of basic Lagrange polynomials of . In more
details we study the case when is a regular simplex inscribed into
.Comment: 17 pages, 6 figures, 1 tabl
FitSKIRT: genetic algorithms to automatically fit dusty galaxies with a Monte Carlo radiative transfer code
We present FitSKIRT, a method to efficiently fit radiative transfer models to
UV/optical images of dusty galaxies. These images have the advantage that they
have better spatial resolution compared to FIR/submm data. FitSKIRT uses the
GAlib genetic algorithm library to optimize the output of the SKIRT Monte Carlo
radiative transfer code. Genetic algorithms prove to be a valuable tool in
handling the multi- dimensional search space as well as the noise induced by
the random nature of the Monte Carlo radiative transfer code. FitSKIRT is
tested on artificial images of a simulated edge-on spiral galaxy, where we
gradually increase the number of fitted parameters. We find that we can recover
all model parameters, even if all 11 model parameters are left unconstrained.
Finally, we apply the FitSKIRT code to a V-band image of the edge-on spiral
galaxy NGC4013. This galaxy has been modeled previously by other authors using
different combinations of radiative transfer codes and optimization methods.
Given the different models and techniques and the complexity and degeneracies
in the parameter space, we find reasonable agreement between the different
models. We conclude that the FitSKIRT method allows comparison between
different models and geometries in a quantitative manner and minimizes the need
of human intervention and biasing. The high level of automation makes it an
ideal tool to use on larger sets of observed data.Comment: 14 pages, 10 figures; accepted for publication in Astronomy and
Astrophysic
Rokhlin dimension for actions of residually finite groups
We introduce the concept of Rokhlin dimension for actions of residually
finite groups on C*-algebras, extending previous notions of Rokhlin dimension
for actions of finite groups and the integers, as introduced by Hirshberg,
Winter and the third author. If the group has a box space of finite asymptotic
dimension, then actions with finite Rokhlin dimension preserve the property of
having finite nuclear dimension, when passing to the crossed product
C*-algebra. A detailed study of the asymptotic dimension of box spaces shows
that finitely generated, virtually nilpotent groups have box spaces with finite
asymptotic dimension, providing a reasonably large class of examples. We then
establish a relation between Rokhlin dimension of residually finite groups
acting on compact metric spaces and amenability dimension of the action in the
sense of Guentner, Willett and Yu. We show that for free actions of infinite,
finitely generated, nilpotent groups on finite dimensional spaces, both these
dimensional values are finite. In particular, the associated transformation
group C*-algebras have finite nuclear dimension. This extends an analogous
result about -actions by the first author. We also provide some
results concerning the genericity of finite Rokhlin dimension, and permanence
properties with respect to the absorption of a strongly self-absorbing
C*-algebra.Comment: 64 pages; some small corrections and added remark
Reconstruction methods for single-shot diffractive imaging of free nanostructures with ultrashort x-ray and XUV laser pulses
With x-ray and XUV single-shot diffractive imaging on free nanoparticles it is possible to investigate structure and shape of the particles. The scattering image of the nanoparticle only contains the intensity distribution but not the phase of the scattered light. Thus, numerical methods are required to infer information from experimental data. In the thesis, different reconstruction methods are implemented, advanced and applied to different scattering scenarios to characterize diffraction patterns for different laser parameters, reconstruct optical properties and 3d shapes of nanotargets.Mit Einzelschussmessungen an freien Nanoteilchen mit Hilfe von Röntgenstrahlung ist es möglich, die Teilchenstruktur zu untersuchen. Damit die Information aus Experimenten gewonnen werden kann, sind numerische Methoden notwendig, da das Streubild der Nanoteilchen nur die Inensitätsverteilung des gestreuten Lichtes, aber nicht die Phase bereitstellt. Es werden verschiedene Rekonstruktionsmethoden implementiert, weiterentwickelt und auf verschiedene Streuszenarien angewandt, um Streubilder für verschiedene Laserparameter zu charakterisieren, optische Eigenschaften und 3D-Formen zu rekonstruieren
GMOSS: All-sky model of spectral radio brightness based on physical components and associated radiative processes
We present Global MOdel for the radio Sky Spectrum (GMOSS) -- a novel,
physically motivated model of the low-frequency radio sky from 22 MHz to 23
GHz. GMOSS invokes different physical components and associated radiative
processes to describe the sky spectrum over 3072 pixels of
resolution. The spectra are allowed to be convex, concave or of more complex
form with contributions from synchrotron emission, thermal emission and
free-free absorption included. Physical parameters that describe the model are
optimized to best fit four all-sky maps at 150 MHz, 408 MHz, 1420 MHz and 23
GHz and two maps at 22 MHz and 45 MHz generated using the Global Sky Model of
de Oliveira-Costa et al. (2008). The fractional deviation of model to data has
a median value of and is less than for of the pixels.
Though aimed at modeling of foregrounds for the global signal arising from the
redshifted 21-cm line of Hydrogen during Cosmic Dawn and Epoch of Reionization
(EoR) - over redshifts , GMOSS is well suited for any
application that requires simulating spectra of the low-frequency radio sky as
would be observed by the beam of any instrument. The complexity in spectral
structure that naturally arises from the underlying physics of the model
provides a useful expectation for departures from smoothness in EoR foreground
spectra and hence may guide the development of algorithms for EoR signal
detection. This aspect is further explored in a subsequent paper.Comment: 19 pages, 7 figure
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