Showing papers by "University of Marne-la-Vallée published in 2008"

Simulations of the 2004 lava flow at Etna volcano using the magflow cellular automata model

Abstract: Since the mechanical properties of lava change over time, lava flows represent a challenge for physically based modeling. This change is ruled by a temperature field which needs to be modeled. MAGFLOW Cellular Automata (CA) model was developed for physically based simulations of lava flows in near real-time. We introduced an algorithm based on the Monte Carlo approach to solve the anisotropic problem. As transition rule of CA, a steady-state solution of Navier-Stokes equations was adopted in the case of isothermal laminar pressure-driven Bingham fluid. For the cooling mechanism, we consider only the radiative heat loss from the surface of the flow and the change of the temperature due to mixture of lavas between cells with different temperatures. The model was applied to reproduce a real lava flow that occurred during the 2004–2005 Etna eruption. The simulations were computed using three different empirical relationships between viscosity and temperature.

Combinatorics on Words: Christoffel Words and Repetitions in Words

Abstract: The two parts of this text are based on two series of lectures delivered by Jean Berstel and Christophe Reutenauer in March 2007 at the Centre de Recherches Mathematiques, Montreal, Canada. Part I represents the first modern and comprehensive exposition of the theory of Christoffel words. Part II presents numerous combinatorial and algorithmic aspects of repetition-free words stemming from the work of Axel Thue - a pioneer in the theory of combinatorics on words. A beginner to the theory of combinatorics on words will be motivated by the numerous examples, and the large variety of exercises, which make the book unique at this level of exposition. The clean and streamlined exposition and the extensive bibliography will also be appreciated. After reading this book, beginners should be ready to read modern research papers in this rapidly growing field and contribute their own research to its development. Experienced readers will be interested in the finitary approach to Sturmian words that Christoffel words offer, as well as the novel geometric and algebraic approach chosen for their exposition. They will also appreciate the historical presentation of the Thue - Morse word and its applications, and the novel results on Abelian repetition-free words.

A Nonlinear Stein-Based Estimator for Multichannel Image Denoising

Abstract: The use of multicomponent images has become widespread with the improvement of multisensor systems having increased spatial and spectral resolutions. However, the observed images are often corrupted by an additive Gaussian noise. In this paper, we are interested in multichannel image denoising based on a multiscale representation of the images. A multivariate statistical approach is adopted to take into account both the spatial and the intercomponent correlations existing between the different wavelet subbands. More precisely, we propose a new parametric nonlinear estimator which generalizes many reported denoising methods. The derivation of the optimal parameters is achieved by applying Stein's principle in the multivariate case. Experiments performed on multispectral remote sensing images clearly indicate that our method outperforms conventional wavelet denoising techniques.

Some extensions of the uncertainty principle

Abstract: We study the formulation of the uncertainty principle in quantum mechanics in terms of entropic inequalities, extending results recently derived by Bialynicki-Birula [I. Bialynicki-Birula, Formulation of the uncertainty relations in terms of the Renyi entropies, Physical Review A 74 (5) (2006) 052101] and Zozor et al. [S. Zozor, C. Vignat, On classes of non-Gaussian asymptotic minimizers in entropic uncertainty principles, Physica A 375 (2) (2007) 499–517]. Those inequalities can be considered as generalizations of the Heisenberg uncertainty principle, since they measure the mutual uncertainty of a wave function and its Fourier transform through their associated Renyi entropies with conjugated indices. We consider here the general case where the entropic indices are not conjugated, in both cases where the state space is discrete and continuous: we discuss the existence of an uncertainty inequality depending on the location of the entropic indices α and β in the plane ( α , β ) . Our results explain and extend a recent study by Luis [A. Luis, Quantum properties of exponential states, Physical Review A 75 (2007) 052115], where states with quantum fluctuations below the Gaussian case are discussed at the single point (2,2).

A positivity conjecture for Jack polynomials

Abstract: We present a positivity conjecture for the coefficients of the development of Jack polynomials in terms of power sums. This extends Stanley's ex-conjecture about normalized characters of the symmetric group. We prove this conjecture for partitions having a rectangular shape.

Increasing functions and inverse Santaló inequality for unconditional functions

Abstract: Let $$\phi: {\mathbb{R}}^n\to {\mathbb{R}}\cup\{+\infty\}$$ be a convex function and $$\mathcal{L}\phi$$ be its Legendre tranform. It is proved that if $$\phi$$ is invariant by changes of signs, then $$\int e^{-\phi}\int e^{-\mathcal{L}\phi} \ge 4^n$$ . This is a functional version of the inverse Santalo inequality for unconditional convex bodies due to J. Saint Raymond. The proof involves a general result on increasing functions on $$\mathbb{R}^{n} \times \mathbb{R}^n$$ together with a functional form of Lozanovskii’s lemma. In the last section, we prove that for some c > 0, one has always $$\int e^{-\phi}\int e^{-\mathcal{L}\phi} \ge c^n$$ . This generalizes a result of B. Klartag and V. Milman.

Diversity Measurements of a Reconfigurable Antenna With Switched Polarizations and Patterns

Abstract: A reconfigurable parasitic patch array is designed to provide polarization and pattern diversities. The performance of the device is predicted using a commercial simulator including a pin diode modelling and an optimization procedure of the switched loads based on an equivalent flow graph. Measurements of the correlation factor and the diversity gain in an indoor environment are performed with a dedicated platform for two orientations of the antenna.

Bias-reduced estimators of the Weibull tail-coefficient

Abstract: In this paper, we consider the problem of the estimation of the Weibull tail-coefficient θ. In particular, we propose a regression model, from which we derive a bias-reduced estimator of θ. This estimator is based on a least-squares approach. The asymptotic normality of this estimator is established. We also introduce an adaptive selection procedure to determine the number of upper order statistics to be used in the estimator. A simulation study as well as an application to a real data set are provided in order to prove the efficiency of the above mentioned methods.

Trees, functional equations, and combinatorial Hopf algebras

Abstract: One of the main virtues of trees is the representation of formal solutions of various functional equations which can be cast in the form of fixed point problems. Basic examples include differential equations and functional (Lagrange) inversion in power series rings. When analyzed in terms of combinatorial Hopf algebras, the simplest examples yield interesting algebraic identities or enumerative results.

High SNR analysis of the MIMO interference channel

Abstract: The rate region achievable by two transmitter-receiver pairs who wish to communicate over a Gaussian interference channel has been the subject of intense study over the last decades. Recently, the high SNR capacity region of this channel has been completely characterized in a work by Etkin, Tse and Wang (2007). In this paper, we study the effect of adding random fading into the picture, as well as multiple antennas at the transmitters and the receivers. Under the fast fading assumption, we recover a result of the same type as that obtained in the above mentioned paper. Under the slow fading assumption, we obtain an upper bound on the maximally achievable diversity order for a given target rate pair, which we conjecture to be tight.

An explicit formula for the characters of the symmetric group

Abstract: We give an explicit expression of the normalized characters of the symmetric group in terms of the “contents” of the partition labelling the representation.

A Risk Assessment System with Automatic Extraction of Event Types

Abstract: In this article we describe the joint effort of experts in linguistics, information extraction and risk assessment to integrate EventSpotter, an automatic event extraction engine, into ADAC, an automated early warning system. By detecting as early as possible weak signals of emerging risks ADAC provides a dynamic synthetic picture of situations involving risk. The ADAC system calculates risk on the basis of fuzzy logic rules operated on a template graph whose leaves are event types. EventSpotter is based on a general purpose natural language dependency parser, XIP, enhanced with domain-specific lexical resources (Lexicon-Grammar). Its role is to automatically feed the leaves with input data.

Changes in University Governance in France and in Italy.

Abstract: The transformation of higher education systems under the pressure of new needs required by the “society of knowledge” in France and Italy has had a deep effect on the relationship between state and university, and therefore a direct impact on university governance. This article sums up the main results of a research carried out on university Presidents in France and Rettori in Italy, with the aim of understanding their views about the changing process in higher education systems and the related impact on their leading role. There emerges a picture of systems transiting from a traditional centralised Napoleonic structure to a new one where the growing autonomy requires a different kind of governance not previously experienced, and thus not easy to implement mostly because of the resistance coming from universities themselves. The majority of Presidents and Rectors seem aware of the changing process and the related problems even if they do not all agree upon feasible trends to follow. Such a disagreement ap...

GPU rendering for autostereoscopic displays

Abstract: In recent years, stereoscopic technology has advanced from stereoscopic to autostereoscopic displays. These lat ter family involves to display several views of a scene. In the case of real-time computer graphics images, the standard approach consists in rendering every view independently. This paper presents an alternative method to generate multiple views for autostereoscopic displays in a single rende ring pass. Our algorithm is based on the fact that vertices properties remain the same from different viewpoints. Taking advantage of the latest generation of GPUs including geometry shaders, we propose a method that significantly speeds up the rendering process by duplicating and transforming incoming primitives for a defined set of views. Our method involves very few modifications to be used with a standard stereo device.

Linearization Coefficients of Bessel Polynomials and Properties of Student t-Distributions

Abstract: We prove positivity results about linearization and connection coefficients for Bessel polynomials. The proof is based on a recursion formula and explicit formulas for the coefficients in special cases. The result implies that the distribution of a finite convex combination of independent Student t-variables with arbitrary odd degrees of freedom has a density which is a finite convex combination of certain Student t-densities with odd degrees of freedom.

The Five Pillars of Presence: Guidelines to Reach Presence

Abstract: This paper presents our theoretical point of view on presence and the way to reach it. We wish to debunk the dominant idea in virtual reality that the feeling of presence can only be reach thanks to more and more realistic rendering and complex interfaces. We will argue that the key point for the presence is not the realism but the credibility of the proposed experiment. Credibility is central if we want users accept to get caught up in the experience. To succeed in this search for credibility, we can lean on the five following pillars: immersion, interaction, consistency of the sensorimotor loop, emotions and cognitive sciences. The ins and the outs of each pillar will be explaining.

Autocalibrated regularized parallel mri reconstruction in the wavelet domain

Abstract: To reduce the scanning time in some MRI applications, parallel acquisition techniques with multiple coils have been developed. Then, the full Field of View (FOV) image is reconstructed from the resulting registered subsampled k-space data. To this end, several reconstruction techniques have been proposed such as the widely-used SENSE method. However, the reconstructed image generally presents artifacts especially when perturbations occur in both the measured data and in the estimated coil sensitivity maps. In order to alleviate such shortcomings by limiting the distortions, Tikhonov regularization in the image domain has also been investigated. In this paper, we present a novel algorithm for SENSE reconstruction which proceeds with regularization in the wavelet domain, the hyperparameters being estimated from the data. Experiments carried out on real T1-weighted MRI data at 1.5 T indicate that the proposed algorithm generates reconstructed images with reduced artifacts in comparison with conventional reconstruction techniques.

Individual sensitivity to framing effects

Abstract: Surveys are sometimes viewed with suspicion when used to provide economic values, since they are sensitive to framing effects. However, the extent to which those effects may vary between individuals has received little attention. Are some individuals less sensitive to framing effects than others? We use the theory of social representation to assign to each individual a new variable to serve as a proxy for the individual's sensitivity to framing effects. This allows to gather new and relevant information to limit the impact of framing effects. We examine two framing effects, starting-point bias and illingness-to-pay and willingness-to-accept divergence.

Comparison Between Different Criteria for Evaluating Reverberation Chamber Functioning Using a 3-D FDTD Algorithm

Abstract: This paper is devoted to the comparison between different criteria used to evaluate the functioning of a mechanically stirred reverberation chamber. Usual criteria based on power or electric fields are considered, and we also propose a criterion based on the Kolmogorov-Smirnov test, called in this paper ldquosuccess ratio to Kolmogorov-Smirnov testrdquo (SRKS). The SRKS represents the acceptation ratio of this test applied at several points of the working volume, when considering a field component and its associated theoretical distribution. After a presentation of the criteria and of the studied chamber, data issued from 3-D finite-difference time-domain simulations and measurements are used to analyze the chamber functioning by the use of these criteria. This study is performed on a large frequency band, in the lossless case as well as with a lossy chamber, and for several stirrer shapes, to emphasize the influence of these parameters. The comparison of the usual criteria with the SRKS shows the SKRS is an accurate test to estimate the lowest usable frequency of a given reverberation chamber.