684 research outputs found
Hydrogen Stark broadened Brackett lines
Stark broadened lines of the hydrogen Brackett series are computed for the
conditions of stellar atmospheres and circumstellar envelopes. The computation
is performed within the Model Microfield Method, which includes the ion dynamic
effects and makes the bridge between the impact limit at low density and the
static limit at high density and in the line wings. The computation gives the
area normalized line shape, from the line core up to the static line wings.Comment: 13 pages - 7 figures, to be published in International Journal of
Spectroscopy (IJS
Decoding by Sampling: A Randomized Lattice Algorithm for Bounded Distance Decoding
Despite its reduced complexity, lattice reduction-aided decoding exhibits a
widening gap to maximum-likelihood (ML) performance as the dimension increases.
To improve its performance, this paper presents randomized lattice decoding
based on Klein's sampling technique, which is a randomized version of Babai's
nearest plane algorithm (i.e., successive interference cancelation (SIC)). To
find the closest lattice point, Klein's algorithm is used to sample some
lattice points and the closest among those samples is chosen. Lattice reduction
increases the probability of finding the closest lattice point, and only needs
to be run once during pre-processing. Further, the sampling can operate very
efficiently in parallel. The technical contribution of this paper is two-fold:
we analyze and optimize the decoding radius of sampling decoding resulting in
better error performance than Klein's original algorithm, and propose a very
efficient implementation of random rounding. Of particular interest is that a
fixed gain in the decoding radius compared to Babai's decoding can be achieved
at polynomial complexity. The proposed decoder is useful for moderate
dimensions where sphere decoding becomes computationally intensive, while
lattice reduction-aided decoding starts to suffer considerable loss. Simulation
results demonstrate near-ML performance is achieved by a moderate number of
samples, even if the dimension is as high as 32
IRIS: A Generic Three-Dimensional Radiative Transfer Code
We present IRIS, a new generic three-dimensional (3D) spectral radiative
transfer code that generates synthetic spectra, or images. It can be used as a
diagnostic tool for comparison with astrophysical observations or laboratory
astrophysics experiments. We have developed a 3D short-characteristic solver
that works with a 3D nonuniform Cartesian grid. We have implemented a piecewise
cubic, locally monotonic, interpolation technique that dramatically reduces the
numerical diffusion effect. The code takes into account the velocity gradient
effect resulting in gradual Doppler shifts of photon frequencies and subsequent
alterations of spectral line profiles. It can also handle periodic boundary
conditions. This first version of the code assumes Local Thermodynamic
Equilibrium (LTE) and no scattering. The opacities and source functions are
specified by the user. In the near future, the capabilities of IRIS will be
extended to allow for non-LTE and scattering modeling. IRIS has been validated
through a number of tests. We provide the results for the most relevant ones,
in particular a searchlight beam test, a comparison with a 1D plane-parallel
model, and a test of the velocity gradient effect. IRIS is a generic code to
address a wide variety of astrophysical issues applied to different objects or
structures, such as accretion shocks, jets in young stellar objects, stellar
atmospheres, exoplanet atmospheres, accretion disks, rotating stellar winds,
cosmological structures. It can also be applied to model laboratory
astrophysics experiments, such as radiative shocks produced with high power
lasers.Comment: accepted for publication in A&A; 17 pages, 9 figures, 2 table
Gender homophily from spatial behavior in a primary school: a sociometric study
We investigate gender homophily in the spatial proximity of children (6 to 12
years old) in a French primary school, using time-resolved data on face-to-face
proximity recorded by means of wearable sensors. For strong ties, i.e., for
pairs of children who interact more than a defined threshold, we find
statistical evidence of gender preference that increases with grade. For weak
ties, conversely, gender homophily is negatively correlated with grade for
girls, and positively correlated with grade for boys. This different evolution
with grade of weak and strong ties exposes a contrasted picture of gender
homophily
Worst-Case Hermite-Korkine-Zolotarev Reduced Lattice Bases
The Hermite-Korkine-Zolotarev reduction plays a central role in strong
lattice reduction algorithms. By building upon a technique introduced by Ajtai,
we show the existence of Hermite-Korkine-Zolotarev reduced bases that are
arguably least reduced. We prove that for such bases, Kannan's algorithm
solving the shortest lattice vector problem requires
d^{\frac{d}{2\e}(1+o(1))} bit operations in dimension . This matches the
best complexity upper bound known for this algorithm. These bases also provide
lower bounds on Schnorr's constants and that are
essentially equal to the best upper bounds. Finally, we also show the existence
of particularly bad bases for Schnorr's hierarchy of reductions
Modelling of mercury isotope separation in CP stellar atmospheres: results and problems
Formation of anomalous isotope abundances in the atmospheres of chemically
peculiar (CP) stars can be explained by light-induced drift (LID). This effect
is additional to the radiative acceleration and appears due to systematic
asymmetry of radiative flux in partly overlapping isotopic spectral line
profiles. LID causes levitation of an isotope with a red-shifted spectral line
and sinking of an isotope with a blue-shifted line, generating thus diffusive
separation of isotopes. We have studied diffusion of mercury as a typical
well-studied isotope-rich heavy metal. Our model computations show that in
mercury-rich quiescent atmospheres of CP stars LID causes levitation of the
heavier mercury isotopes and sinking of the lighter ones. Precise quantitative
modelling of the process of isotope separation demands very high-resolution
computations and the high-precision input data, including data on hyperfine and
isotopic splitting of spectral lines, adequate line profiles and impact
cross-sections. Presence of microturbulence and weak stellar winds can
essentially reduce the effect of radiative-driven diffusion.Comment: 8 pages, 4 figures. Manuscript accepted for publication in New
Astronomy Reviews (proceedings of the 7th Serbian Conference on Spectral Line
Shapes in Astrophysics, Zrenjanin, Serbia, June 15-19 2009
Chiffrement avancé à partir du problème Learning With Errors
National audienceLe problèmeLearning With Errors (LWE) est algorithmiquement difficile pour des instances aléatoires. Il a été introduit par Oded Regev en 2005 et, depuis lors, il s'est avéré très utile pour construire des primitives cryptographiques, pour assurer la confidentialité de l'information. Dans ce chapitre, nous présenterons le problème LWE et illustrerons sa richesse, en décrivant des schémas de chiffrement avancés pouvant être prouvés au moins aussi sûrs que LWE est difficile. Nous rappellerons le concept fondamental de chiffrement, puis nous nous focaliserons sur les notions de chiffrement fondé sur l'identité et de chiffrement par attributs
Analyse numérique et réduction de réseaux
29 pagesNational audienceL'algorithmique des réseaux euclidiens est un outil fréquemment utilisé en informatique et en mathématiques. Elle repose essentiellement sur la réduction LLL qu'il est donc important de rendre aussi efficace que possible. Une approche initiée par Schnorr consiste à effectuer des calculs approchés pour estimer les orthogonalisations de Gram-Schmidt sous-jacentes. Sans approximations, ces calculs dominent le coût de la réduction. Récemment, des outils classiques d'analyse numérique ont été revisités et améliorés, pour exploiter plus systématiquement l'idée de Schnorr et réduire les coûts. Nous décrivons ces développements, notamment comment l'algorithmique en nombres flottants peut être introduite à plusieurs niveaux dans la réduction
Sanitization of FHE ciphertexts
By definition, fully homomorphic encryption (FHE) schemes support homomorphic decryption, and all known FHE constructions are bootstrapped from a Somewhat Homomorphic Encryption (SHE) scheme via this technique. Additionally, when a public key is provided, ciphertexts are also re-randomizable, e.g., by adding to them fresh encryptions of 0. From those two operations we devise an algorithm to sanitize a ciphertext, by making its distribution canonical. In particular, the distribution of the ciphertext does not depend on the circuit that led to it via homomorphic evaluation, thus providing circuit privacy in the honest-but-curious model. Unlike the previous approach based on noise flooding, our approach does not degrade much the security/efficiency trade-off of the underlying FHE. The technique can be applied to all lattice-based FHE proposed so far, without substantially affecting their concrete parameters
Perturbation Analysis of the QR Factor R in the Context of LLL Lattice Basis Reduction
... \ud
computable notion of reduction of basis of a Euclidean lattice that is now commonly referred to as LLLreduction. The precise definition involves the R-factor of the QR factorisation of the basis matrix. A natural mean of speeding up the LLL reduction algorithm is to use a (floating-point) approximation to the R-factor. In the present article, we investigate the accuracy of the factor R of the QR factorisation of an LLL-reduced basis. The results we obtain should be very useful to devise LLL-type algorithms relying on floating-point approximations
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