Hasard

Résumé

vignette|Les jeux de dés sont des symboles du hasard (jeux de hasard). vignette|Tyché ou Fortuna et sa corne d'abondance (fortune, hasard, en grec ancien, sort en latin) déesse allégorique gréco-romaine de la chance, des coïncidences, de la fortune, de la prospérité, de la destinée…|alt= Le hasard est le principe déclencheur d'événements non liés à une cause connue. Il peut être synonyme de l'« imprévisibilité », de l'« imprédictibilité », de fortune ou de destin. Étymologie On attribue l'origine du mot hasard à l'arabe « az-zahr, الزهر » signifiant à l'origine « dés » et ayant pris la signification de « chance », car il désigna jusqu'au un jeu de dés, mais aussi par métaphore tous les domaines relevant de la « science de la Chance » (Averroès). Cependant, le Trésor de la langue française signale que le terme « az-zahr » dans le sens de « dé à jouer » est relativement moderne et propose l'étymologie « yasara » (« jouer aux dés ») dont l'existence est attestée en arabe cl

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A decentralized system is one that works when no single party is in charge or fully trusted. This course teaches decentralized systems principles while guiding students through the development and testing of their own decentralized system incorporating messaging, encryption, and blockchain concepts.

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Safety assessments of road bridges to braking events combine the braking force, acting along the longitudinal axis of the deck, with a vertical load that accounts for the vertical component of the traffic action. In modern design standards the vertical load models result from probabilistic calibration procedures targeting predefined return periods. On the contrary, the braking force was derived from a deterministic characterization of the vehicle configurations and of the braking process. Therefore, the return period of the braking force is unclear and may not be consistent with that of the vertical load model. Significant deviations from the target return period might lead to either uneconomical decisions, e.g. uncalled-for retrofitting interventions, or to inaccurate structural safety verifications. This thesis presents an original stochastic model to compute site-specific values of the braking force as a function of the return period. The developed stochastic model takes into account the length of the bridge deck and its dynamic properties for vibrations in the longitudinal direction, as well as different sources of randomness related to braking events, all of which comply with real-world measurements, including: - vehicle configurations, resorting to a time-history of crossing vehicles; - driver response times, randomly generated from probability distributions defined in the scope of this project; - deceleration profiles of the vehicles, resampled from catalogues of realistic deceleration profiles. The stochastic model uses Monte Carlo simulation of braking events and computes the maximum of the dynamic response of the bridge to each event. The computed maxima are collected in an empirical distribution function of the braking force. In the end, the model returns the quantile of this distribution that is suitable for safety assessments. This value of braking force is specific to the bridge given properties, to the traffic characteristics, and to the target return period. An additional novelty of this research work is the estimation of a rate of occurrence on motorways of braking events per vehicle-distance travelled. This parameter enables the estimation of the period of time covered by the simulations of braking events as a function of traffic flow and of the total number of braking events simulated. This step is fundamental to determine the value of the braking force that has a given return period. The braking forces returned by the stochastic model show significant dependence on the bridge length, the natural vibration period of the deck in the longitudinal direction, and the number of directions of traffic on the deck. On the contrary, damping ratio, traffic on the fast-lane or on weekends, and an augmentation of traffic in 20% show no substantial influence on the braking force. Moreover, the two motorway locations considered as sources of traffic data, Denges and Monte Ceneri, both in Switzerland, yielded braking forces with similar magnitudes, despite the significant differences in traffic characteristics. Finally, the results compiled served to calibrate an updated braking force that depends explicitly on the parameters found relevant, as well as on the return period so that it can be adopted by different standards even if they enforce different safety targets. This updated expression evidences that the braking forces of current codes tend to be conservative and, hence, can be improved based on the findings of this project.

EPFL2016

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Reducing Randomness in Matrix Models for Wireless Communication

Marc Desgroseilliers

Pushed by the proliferation of antennas and of multiuser scenarios, matrices with random entries are appearing more and more frequently in information theory. This leads to the study of matrix channels, where the capacity depends on the distribution of the matrix's eigenvalues. These eigenvalues are complicated functionals of the entries of the matrix and the challenge lies therein. It is often the case that in order to better model different communication scenarios, one is driven away from matrix models typically studied in pure mathematics and physics. One cannot simply resort to the standard tools developed over the years in these fields and must come up with new approaches. In this thesis, our goal is to obtain results in scenarios where the randomness is limited by the nature of the channel, in order to widen applicability in real life scenarios.

EPFL2015

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Learning from survey propagation: a neural network for MAX-E-3-SAT

Raffaele Marino

Many natural optimization problems are NP-hard, which implies that they are probably hard to solve exactly in the worst-case. However, it suffices to get reasonably good solutions for all (or even most) instances in practice. This paper presents a new algorithm for computing approximate solutions in Theta(N) for the maximum exact 3-satisfiability (MAX-E-3-SAT) problem by using supervised learning methodology. This methodology allows us to create a learning algorithm able to fix Boolean variables by using local information obtained by the Survey Propagation algorithm. By performing an accurate analysis, on random conjunctive normal form instances of the MAX-E-3-SAT with several Boolean variables, we show that this new algorithm, avoiding any decimation strategy, can build assignments better than a random one, even if the convergence of the messages is not found. Although this algorithm is not competitive with state-of-the-art maximum satisfiability solvers, it can solve substantially larger and more complicated problems than it ever saw during training.

IOP PUBLISHING LTD2021

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EPFL2016