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Modeling the optical properties of self-organized arrays of liquid crystal defects
No full textInternational audienceLocal full Mueller matrix measurements in the Fourier plane of a microscope lens were used to determine the internal anisotropic ordering in periodic linear arrays of smectic liquid crystal defects, known as 'oily streaks'. We propose a single microstructure-dependent model taking into account the anisotropic dielectric function of the liquid crystal that reproduces the smectic layers orientation and organization in the oily streaks. The calculated Mueller matrix elements are compared to the measured data to reveal the anchoring mechanism of the smectic oily streaks on the substrate and evidence the presence of new type of defect arrangement. Beyond the scientific inquiry, the understanding and control of the internal structure of such arrays offer technological opportunities for developing liquid-crystal based sensors and self-assembled nanostructures
Modelling the dynamics of the piano action: is apparent success real?
No full textInternational audienceThe kinematics and the dynamics of the piano action mechanism have been much studied in the last 50 years and fairly sophisticated models have been proposed in the last decade. Surprisingly, simple as well as sophisticated models seem to yield very valuable simulations when compared to measurements. We propose here a very simple model, with only 1-degree of freedom, and compare its outcome with force and motion measurements obtained by playing a real piano mechanism. The model, purposely chosen as obviously too simple to be predictive of the dynamics of the grand piano action, appears either as very good or as very bad, depending on which physical quantities are used as the input and output. We discuss the sensitivity of the simulation results to the initial conditions and to noise and the sensitivity of the experimental/simulation comparisons to the chosen dynamical model. It is shown that force-driven simulations with position comparisons, as they are proposed in the literature, do not validate the dynamical models of the piano action. It is suggested that these models be validated with position-driven simulations and force comparisons
Interferon beta-1b reduces black holes in a randomised trial of clinically isolated syndrome.
No full textTRIAL REGISTRATION NUMBER: NCT00544037International audienc
On the asymptotic behaviour of the posterior distribution in hidden Markov Models with unknown number of states
No full textInternational audienceWe consider finite state space stationary hidden Markov models (HMMs) in the situation where the number of hidden states is unknown. We provide a frequentist asymptotic evaluation of Bayesian analysis methods. Our main result gives posterior concentration rates for the marginal densities, that is for the density of a fixed number of consecutive observations. Using conditions on the prior, we are then able to define a consistent Bayesian estimator of the number of hidden states. It is known that the likelihood ratio test statistic for overfitted HMMs has a non standard behaviour and is unbounded. Our conditions on the prior may be seen as a way to penalize parameters to avoid this phenomenon. Inference of parameters is a much more difficult task than inference of marginal densities, we still provide a precise description of the situation when the observations are i.i.d. and we allow for 2 possible hidden states
Improvements in Glucose Metabolism Early After Gastric Bypass Surgery Are Not Explained by Increases in Total Bile Acids and Fibroblast Growth Factor 19 Concentrations
No full textInternational audienc
Modélisation numérique du couplage thermique photoélectrique pour des modules photovoltaïques sous faible concentration
No full textLGEP 2014 ID = 1655National audienc
Agrégation PAC-bayésienne d'estimateurs par projection
No full textInternational audienceAggregating estimators using exponential weights depending on their risk performs well in expectation, but sadly not in probability. A way to overcome this issue is considering exponential weights of a penalized risk. In this case, an oracle inequality can be obtained in probability, but is not sharp. Taking into account the estimated function's norm in the penalty offers a sharp inequality.L'agrégation d'estimateur a l'aide de poids exponentiels dépendant de leur risque offre de bonnes performances en moyenne. Malheureusement, il est impossible d'obtenir un aussi bon contrôle du risque de l'estimateur agrégé en probabilité. Pour contourner ce problème, nous considérons des poids exponentiels du risque pénalisé. Cette technique permet d'obtenir une inégalité oracle inexacte en probabilité. En surpénalisant, avec une prise en compte de la norme de la fonction estimée, une inégalité exacte est accessible
Minimal time trajectories for two-level quantum systems with two bounded controls
No full textInternational audienceIn this paper we consider the minimum time population transfer problem for a two level quantum system driven by two external fields with bounded amplitude. The controls are modeled as real functions and we do not use the Rotating Wave Approximation. After projection on the Bloch sphere, we treat the time-optimal control problem with techniques of optimal synthesis on 2D manifolds. Based on the Pontryagin Maximum Principle, we characterize a restricted set of candidate optimal trajectories. Properties on this set, crucial for complete optimal synthesis, are illustrated by numerical simulations. Furthermore, when the two controls have the same bound and this bound is small with respect to the difference of the two energy levels, we get a complete optimal synthesis up to a small neighborhood of the antipodal point of the initial condition
Direct Construction of Recursive MDS Diffusion Layers using Shortened BCH Codes
No full textBest paper awardInternational audienceMDS matrices allow to build optimal linear diffusion layers in block ciphers. However, MDS matrices cannot be sparse and usually have a large description, inducing costly software/hardware implementations. Recursive MDS matrices allow to solve this problem by focusing on MDS matrices that can be computed as a power of a simple companion matrix, thus having a compact description suitable even for constrained environ- ments. However, up to now, finding recursive MDS matrices required to perform an exhaustive search on families of companion matrices, thus limiting the size of MDS matrices one could look for. In this article we propose a new direct construction based on shortened BCH codes, al- lowing to efficiently construct such matrices for whatever parameters. Unfortunately, not all recursive MDS matrices can be obtained from BCH codes, and our algorithm is not always guaranteed to find the best matrices for a given set of parameters