Estimating the speed-up of Adaptively Restrained Langevin Dynamics

We consider Adaptively Restrained Langevin dynamics, in which the kinetic energy function vanishes for small velocities. Properly parameterized, this dynamics makes it possible to reduce the computational complexity of updating inter-particle forces, and to accelerate the computation of ergodic averages of molecular simulations. In this paper, we analyze the influence of the method parameters on the total achievable speed-up. In particular, we estimate both the algorithmic speed-up, resulting from incremental force updates, and the influence of the change of the dynamics on the asymptotic variance. This allows us to propose a practical strategy for the parametrization of the method. We validate these theoretical results by representative numerical experiments

Error Analysis of Modified Langevin Dynamics

We consider Langevin dynamics associated with a modified kinetic energy vanishing for small momenta. This allows us to freeze slow particles, and hence avoid the re-computation of inter-particle forces, which leads to computational gains. On the other hand, the statistical error may increase since there are a priori more correlations in time. The aim of this work is first to prove the ergodicity of the modified Langevin dynamics (which fails to be hypoelliptic), and next to analyze how the asymptotic variance on ergodic averages depends on the parameters of the modified kinetic energy. Numerical results illustrate the approach, both for low-dimensional systems where we resort to a Galerkin approximation of the generator, and for more realistic systems using Monte Carlo simulations

Nanotechnologie : conception à l'échelle atomique par Stéphane Redon. Dynamique moléculaire interactive, entretien avec Serge Crouzy, propos recueillis par Dominique Chouchan.

National audienceL'exploration et la manipulation de l'infiniment petit passeront sans doute par la conception assistée par ordinateur : le prototypage virtuel d'objets nanoscopiques concerne un grand nombre d'applications industrielles, pharmacologiques..

Ciència, veritat i epistemologia antropològica. Una síntesi històrica de les crítiques a la universalitat de la ciència

Aquest treball pretén fer una síntesi de les aportacions epistemològiques fonamentals que al llarg del segle XX han criticat la vella concepció positivista del coneixement científic. Aquesta concepció no només perdura encara hui dia, sinó que és la concepció dominant internacionalment al món de les ciències socials. Sembla llavors indispensable traçar una crítica en profunditat d'aquesta qüestió. El present treball no pretén ésser aquesta crítica, necessàriament complexa, sinó que intenta aportar-ne les línies mestres de manera sintètica.This work aims to make a synthesis of the main epistemological contributions that have criticized, along the 20th century, the old positivist conception of scientific knowledge. This conception is not only still alive nowadays, but is the dominant conception in the international world of social sciences. It seems then necessary to sketch a deep critic concerning this question. This works doesn't pretend to be this critic, which would be necessarily complex, but tries to give the main lines towards this direction in a synthetic way

Risk-hedging using options for an upgrading investment in a data network

In this paper, we illustrate how a mobile data network operator can plan an upgrading investment to anticipate explosions of the demand, taking into account the expected generated profit and the customers satisfaction. The former parameter grows with the demand, whereas the latter sinks if the demand is too high as throughput may collapse. As the equipment price decreases with time, it may be interesting to wait rather than to invest at once. We then propose a real option strategy to hedge against the risk that the investment has to take place earlier than expected. At last, we price this option with a backward dynamic programming approach, using recent improvements based on least-squares estimations.

Heartwater surveillance in Guadeloupe: a model of partnership between research and surveillance for the Caribbean

Heartwater, a tick-borne disease of ruminants transmitted by Amblyomma ticks is present in 3 Caribbean islands: Guadeloupe, Marie-Galante and Antigua, representing a threat for neighboring islands and North America. Despite the availability of efficient acaricides, no significant improvement has been seen in vector and disease control. The Ticks and tick borne disease working group of the Caribbean animal health network (CaribVET) recommended Guadeloupe the conduct of (1) a sociologic study to understand farmers' reluctance to adopt efficient treatment; and (2) heartwater surveillance. A passive surveillance network monitoring ruminant neurological syndromes, RESPANG, was set up in July 2010. RESPANG objectives are to assess the burden of heartwater and sensitize farmers. Private veterinarians collect blood and ticks after farmer reporting of clinical suspicion. Diagnostic for heartwater, babesiosis and anaplasmosis is performed at CIRAD. An online database displays results on interactive maps allowing the identification of areas where communication campaigns by farmer association can be focused. Leaflets and key messages were developed, based on the recommendations of the sociological survey. Out of 238 suspicions, 30.5% were positive for heartwater all along the year. Analysis of the factors associated with heartwater is currently ongoing. RESPANG shows excellent partner involvement and interaction. Surveillance performance indicators will improve network operation and coordination. Long-term data set will enable to assess the impact of communication campaign and possibly to detect introduction of diseases with similar clinical signs. In parallel, pathogen and vector genetic characterization is being developed using RESPANG samples. This shows the tight link between surveillance network and research activities. CaribVET supports the establishment of similar networks for other diseases in other Caribbean islands. (Texte intégral

A three-state model for the regulation of telomerase by TERRA and hnRNPA1

Telomeres, the physical ends of eukaryotic chromosomes, are transcribed into telomeric repeat-containing RNA (TERRA), a large non-coding RNA, which forms an integral part of telomeric heterochromatin. In vitro, naked TERRA molecules are efficient inhibitors of human telomerase, base-pairing via their 5′-UUAGGG-3′ repeats with the template sequence of telomerase RNA, in addition to contacting the telomerase reverse transcriptase protein subunit. In vivo, however, TERRA-mediated inhibition of telomerase can be prevented by unknown mechanisms. Also, heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) has been implicated in telomere length control. In vivo, TERRA is partially associated with hnRNPA1, and hnRNPA1 is also detected at telomeres. We demonstrate that on binding of TERRA, hnRNPA1 can alleviate the TERRA-mediated inhibition of telomerase. However, when in excess over TERRA, hnRNPA1 becomes itself an inhibitor of telomere extension, on binding of the telomeric DNA substrate. Yet, hnRNPA1 has no notable direct effects on the telomerase catalysis. Our in vitro results suggest that TERRA-mediated telomerase inhibition may be prevented by hnRNPA1 in vivo. Telomere extension by telomerase may require balanced levels of TERRA and hnRNPA1 at telomeres. Thus, TERRA and hnRNPA1 can function as a bimolecular regulator to turn telomerase and the telomere on and of

Connaissances naturalisées dans le champ du numérique à l'articulation école maternelle / école primaire

This paper is dedicated to the numerical knowledge at pre-elementary and elementary school (pupils 4 to 7 years old. At these levels, observing pupils' numerical activities is quite difficult, because often the knowledge at stake is not recognised as mathematical knowledge. Following Briand, we will study different situations, focusing on the counting of movable objects. This study leads to the determination of two different spaces: the space attached to the collection and the space attached to the process.Les travaux pionniers de Joël Briand et Guy Brousseau ont montré l'existence d'un savoir : l'énumération, qui n'est pas visible pour les professeurs et l'institution scolaire et qui pourtant intervient dans les connaissances nécessaires au dénombrement des collections, dans tous les aspects de ce dénombrement, de l'école maternelle à l'université. Pour la deuxième année, notre groupe de travail Démathé (Développement des Mathématiques à l'école élémentaire) a revisité les travaux concernant l'énumération pour construire un document à destination des professeurs permettant de mieux comprendre les difficultés des élèves dans les situations de dénombrement et plus généralement dans les situations qui nécessitent des connaissances énumératives. Nous commencerons par travailler la notion d'énumération, dans une acception un peu plus large que celle de Briand, que nous nommons l'organisation des collections, car elle comprend pour nous certaines relations liées à la l'articulation entre l'espace et le temps dans le cadre de la mesure de la numérosité d'une collection. Ceci nous permettra d'analyser très finement un certain nombre d'activités très banales (dans le sens de très présentes dans de très nombreuses classes ordinaires) dont la complexité n'est souvent pas perçue par les professeurs eux-mêmes, mais vécue de plein fouet par les élèves. Ces activités, choisies pour leurs relations avec l'énumération et l'organisation des collections, seront le plus souvent du niveau cycle 2 de l'école (élèves de 5 à 7 ans). Certaines sont considérées comme des activités typiquement mathématiques par les professeurs, alors que d'autres se situent institutionnellement dans d'autre champs (pré lecture, notamment). Il s'agira d'en dégager les variables pour mieux en comprendre la complexité pour les élèves et l'intervention de connaissances numériques naturalisées. Le problème se posera alors à nous en termes de formation. Les professeurs peuvent-ils et doivent-ils mieux comprendre ces phénomènes, certaines connaissances pourraient-elles être intégrées dans les pratiques professionnelles ? Pour travailler ces questions, nous montrerons tout d'abord comment certains phénomènes de différenciation sociale à l'articulation entre les institutions scolaires maternelles et élémentaires peuvent être rendu visibles par nos analyses. Nous chercherons ensuite à repérer les ressources et les contraintes de l'activité des professeurs dans l'institution école maternelle. Cette dernière partie s'appuiera à la fois sur des observations de classe réalisées en ZEP en GS en 2004-2005 et sur des entretiens hors classe avec des professeurs des écoles concernant leurs pratiques en mathématiques. Nous concluerons sur la discussion des moyens possibles pour la formation, moyens qui incluent pour nous la diffusion de documents de type traité (au sens de Robert Neyret

Interactive Chemical Reactivity Exploration

Elucidating chemical reactivity in complex molecular assemblies of a few hundred atoms is, despite the remarkable progress in quantum chemistry, still a major challenge. Black-box search methods to find intermediates and transition-state structures might fail in such situations because of the high-dimensionality of the potential energy surface. Here, we propose the concept of interactive chemical reactivity exploration to effectively introduce the chemist's intuition into the search process. We employ a haptic pointer device with force-feedback to allow the operator the direct manipulation of structures in three dimensions along with simultaneous perception of the quantum mechanical response upon structure modification as forces. We elaborate on the details of how such an interactive exploration should proceed and which technical difficulties need to be overcome. All reactivity-exploration concepts developed for this purpose have been implemented in the Samson programming environment.Comment: 36 pages, 14 figure

Allison Brown, The Return of Lucretius to Renaissance Florence, Cambridge, Harvard University Press, 2010

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