312 research outputs found
Observables de premier passage de marches aléatoires à renforcement
No full textThis thesis seeks to quantify the time to first passage over a target (FPT) of a class of random walks for which the walker is endowed with long-range memory effects that emerge from the interaction of the walker with the territory he has visited at previous times. The first part is devoted to the study of the links between aging and persistence exponent. We present examples of aging processes that are asymptotically diffusive and that are characterized by anomalous persistence exponents. In a second part, we establish that the attractively reinforced walks fall into different categories for which we have characterized their aging and first-pass properties. We highlight the importance of such reinforcement effects in the analysis of real cell trajectories. In a third part, we show that these long-range memory effects can induce non-trivial aging, persistence and transience exponents in both infinite and confined geometries. In particular, we quantify the dependence of the FPT distribution in confinement on different geometric parameters. In a fourth part, we study the first-pass properties of a walker following a generalized Langevin equation in the presence of a harmonic potential. From the position of the walker in the future of the FPT, we predict the values of the mean time of first passage. Finally, we address the question of the joint distribution of the first-pass time and the number of distinct sites visited at the FPT time.Cette thèse cherche à quantifier le temps de premier passage sur une cible (FPT) d'une classe de marches aléatoires pour lesquelles le marcheur est doté d'effets de mémoire à longue portée qui émergent de l'interaction de celui-ci avec le territoire qu'il a visité à des temps antérieurs. La première partie est consacrée à l'étude des liens entre vieillissement et exposant de persistance. Nous y présentons des exemples de processus vieillissants, asymptotiquement diffusifs, et qui sont caractérisés par des exposants de persistance anormaux. Dans une seconde partie, nous établissons que les marches à renforcement attractif se répartissent selon différentes catégories pour lesquelles nous avons caractérisé leurs propriétés de vieillissement et de premier passage. Nous soulignons l'importance de tels effets de renforcement dans l'analyse de trajectoires cellulaires réelles. Dans une troisième partie, nous montrons que ces effets de mémoire à longue portée peuvent induire un vieillissement et des exposants de persistance et de transience non triviaux à la fois dans des géométries infinies et confinées. Nous quantifions notamment la dépendance de la distribution du FPT en confinement envers les différents paramètres géométriques. Dans une quatrième partie, nous entamons l'étude des propriétés de premier passage d'un marcheur suivant une équation de Langevin généralisée en présence d'un potentiel harmonique. À partir de la position du marcheur dans le futur du FPT, nous prédisons les valeurs du temps moyen de premier passage. Enfin, nous abordons la question de la distribution jointe du temps de premier passage et du nombre de sites distincts visités à l'instant du FPT
First passage time of self-reinforced walks
No full textCette thèse cherche à quantifier le temps de premier passage sur une cible (FPT) d'une classe de marches aléatoires pour lesquelles le marcheur est doté d'effets de mémoire à longue portée qui émergent de l'interaction de celui-ci avec le territoire qu'il a visité à des temps antérieurs. La première partie est consacrée à l'étude des liens entre vieillissement et exposant de persistance. Nous y présentons des exemples de processus vieillissants, asymptotiquement diffusifs, et qui sont caractérisés par des exposants de persistance anormaux. Dans une seconde partie, nous établissons que les marches à renforcement attractif se répartissent selon différentes catégories pour lesquelles nous avons caractérisé leurs propriétés de vieillissement et de premier passage. Nous soulignons l'importance de tels effets de renforcement dans l'analyse de trajectoires cellulaires réelles. Dans une troisième partie, nous montrons que ces effets de mémoire à longue portée peuvent induire un vieillissement et des exposants de persistance et de transience non triviaux à la fois dans des géométries infinies et confinées. Nous quantifions notamment la dépendance de la distribution du FPT en confinement envers les différents paramètres géométriques. Dans une quatrième partie, nous entamons l'étude des propriétés de premier passage d'un marcheur suivant une équation de Langevin généralisée en présence d'un potentiel harmonique. À partir de la position du marcheur dans le futur du FPT, nous prédisons les valeurs du temps moyen de premier passage. Enfin, nous abordons la question de la distribution jointe du temps de premier passage et du nombre de sites distincts visités à l'instant du FPT.This thesis seeks to quantify the time to first passage over a target (FPT) of a class of random walks for which the walker is endowed with long-range memory effects that emerge from the interaction of the walker with the territory he has visited at previous times. The first part is devoted to the study of the links between aging and persistence exponent. We present examples of aging processes that are asymptotically diffusive and that are characterized by anomalous persistence exponents. In a second part, we establish that the attractively reinforced walks fall into different categories for which we have characterized their aging and first-pass properties. We highlight the importance of such reinforcement effects in the analysis of real cell trajectories. In a third part, we show that these long-range memory effects can induce non-trivial aging, persistence and transience exponents in both infinite and confined geometries. In particular, we quantify the dependence of the FPT distribution in confinement on different geometric parameters. In a fourth part, we study the first-pass properties of a walker following a generalized Langevin equation in the presence of a harmonic potential. From the position of the walker in the future of the FPT, we predict the values of the mean time of first passage. Finally, we address the question of the joint distribution of the first-pass time and the number of distinct sites visited at the FPT time
Sally Barbier-Dorrance : Drawings and Sculpture
No full textThe author notes Barbier-Dorrance's use of materials, images and mood in her drawings and clay sculptures. Biographical notes
Structural change, marginal land and economic development in Latin America and the Caribbean
No full textEmpirical evidence indicates that in Latin America and the Caribbean, households on less favored, or marginal, agricultural land form a "residual" pool of rural labor. Although the modern sector may be the source of dynamic growth through learning-by-doing and knowledge spillovers, patterns of labor, land and other natural resources use in the rural economy matter in the overall dynamics of structural change. The concentration of rural populations on marginal land is essentially a barometer of economy-wide development. As long as there is abundant marginal land for cultivation, they serve to absorb rural migrants, increased population, and displaced unskilled labor from elsewhere in the economy. Moreover, the economy is vulnerable to the "Dutch disease" effects of a booming primary products sector. As a consequence, productivity increases and expansion in the commercial primary production sector will cause manufacturing employment and output to contract, until complete specialization occurs. Avoiding such an outcome and combating the inherent dualism of the economy require both targeted polices for the modern sector and traditional agriculture on marginal land.Erratum to Lat Am Econ Rev (2014) 23:3, DOI: 10.1007/s40503-014-0003-5, Author: Edward B. Barbier: The article displays incorrect author information. Please note the correct author information: The sole author of this paper is Edward B. Barbier, "John S. Bugas Professor of Economics", Department of Economics and Finance, University of Wyoming. The publisher regrets this mistake (http://dx.doi.org/10.1007/s40503-014-0007-1)
Les cellules, « Petits Poucets » des tissus vivants
No full textInternational audienceNo abstract availabl
Computer algebra and transputers applied to the finite element method
No full textRecent developments in computing technology have opened new prospects for computationally intensive numerical methods such as the finite element method. More complex and refined problems can be solved, for example increased number and order of the elements improving accuracy. The power of Computer Algebra systems and parallel processing techniques is expected to bring significant improvement in such methods. The main objective of this work has been to assess the use of these techniques in the finite element method. The generation of interpolation functions and element matrices has been investigated using Computer Algebra. Symbolic expressions were obtained automatically and efficiently converted into FORTRAN routines. Shape functions based on Lagrange polynomials and mapping functions for infinite elements were considered. One and two dimensional element matrices for bending problems based on Hermite polynomials were also derived. Parallel solvers for systems of linear equations have been developed since such systems often arise in numerical methods. Both symmetric and asymmetric solvers have been considered. The implementation was on Transputer-based machines. The speed-ups obtained are good. An analysis by finite element method of a free surface flow over a spillway has been carried out. Computer Algebra was used to derive the integrand of the element matrices and their numerical evaluation was done in parallel on a Transputer-based machine. A graphical interface was developed to enable the visualisation of the free surface and the influence of the parameters. The speed- ups obtained were good. Convergence of the iterative solution method used was good for gated spillways. Some problems experienced with the non-gated spillways have lead to a discussion and tests of the potential factors of instability
Information approximée pour des stratégies d'exploration-exploitation efficaces
No full textInternational audienceThis paper addresses the exploration-exploitation dilemma inherent in decision-making, focusing on multi-armed bandit problems. The problems involve an agent deciding whether to exploit current knowledge for immediate gains or explore new avenues for potential long-term rewards. We here introduce a novel algorithm, approximate information maximization (AIM), which employs an analytical approximation of the entropy gradient to choose which arm to pull at each point in time. AIM matches the performance of Infomax and Thompson sampling while also offering enhanced computational speed, determinism, and tractability. Empirical evaluation of AIM indicates its compliance with the Lai & Robbins asymptotic bound and demonstrates its robustness for a range of priors. Its expression is tunable, which allows for specific optimization in various settings
Information maximization for a broad variety of multi-armed bandit games
No full textInformation and free-energy maximization are physics principles that provide general rules for an agent to optimize actions in line with specific goals and policies. These principles are the building blocks for designing decision-making policies capable of efficient performance with only partial information. Notably, the information maximization principle has shown remarkable success in the classical bandit problem and has recently been shown to yield optimal algorithms for Gaussian and sub-Gaussian reward distributions. This article explores a broad extension of physics-based approaches to more complex and structured bandit problems. To this end, we cover three distinct types of bandit problems, where information maximization is adapted and leads to strong performance. Since the main challenge of information maximization lies in avoiding over-exploration, we highlight how information is tailored at various levels to mitigate this issue, paving the way for more efficient and robust decision-making strategies
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