Mathématiques et Interactions à Nice     

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				René Lozi

				(LJAD - UNSA)

				Emergence du pseudo-hasard à partir du chaos dans des systèmes dynamiques
couplés

				Le terme chaos, pour désigner le comportement de certains systèmes dynamiques non linéaires, a semble-t-il, été introduit par James Yorke et son
étudiant Tien-Yen Li dans leur fameux article « Period three implies chaos » publié en 1974.
Depuis, 40 années se sont écoulées et les milliers d’études effectuées dans de nombreuses disciplines (physique, informatique, mathématiques, biologie, chimie, théorie du contrôle…) sur les systèmes dynamiques chaotiques ont permis d’explorer de façon très approfondie ce champ de recherche.
La génération de nombres chaotiques (c’est-à-dire dont on ne perçoit pas directement le rapport entre eux) est très simple à obtenir avec un ordinateur,
cependant elle ne satisfait pas les besoins de plus en plus nombreux de générer
des nombres au hasard (plus exactement des nombres pseudo-aléatoires) pour les simulations numériques, les loteries en ligne, les méthodes d’intégration
numériques pour des intégrales multiples (méthode de Monte-Carlo ou quasi-Monte Carlo) des problèmes économiques…
Nous présenterons dans cet exposé tout d’abord rapidement quelques exemples d’utilisation actuelle des nombres chaotiques (comme pour le réglage des
dispositifs PID dans l’industrie via l’optimisation chaotique), puis nous montrerons comment on peut en couplant de façon particulière des générateurs de
nombres chaotiques, obtenir des générateurs de nombres pseudo-aléatoires dont
les propriétés sont comparables à celles des meilleurs générateurs actuels basés
sur d’autres branches des mathématiques, tout en étant ultra-rapides (plusieurs
dizaines de millions de nombres à la seconde).
En outre ces générateurs sont multi-dimensionnels, c’est-à-dire qu’ils produisent des séries simultanées de nombres pseudo-aléatoires non corrélés, ce qui permet de construire des algorithmes de cryptage symétriques.

29-03-2013 - Léa Cot (INSA Toulouse),

Structural airframe maintenance strategy comparison

Résumé :

 In recent years, airline maintenance cost has steadily increased and significant investigations are made to improve it. Currently, preventive maintenance approach based on predetermined inspections over all the life of the aircraft, including airframe and engine maintenance, is used. Our purpose focuses on structural airframe maintenance. Regarding the structural airframe maintenance, damages that could in a near future threaten the integrity of the structure are detected at these scheduled maintenances. In-service, an aircraft is subject to a sequence of cycles with each cycle consisting of a flight phase, followed by a ground phase (which then precedes the next flight). Consequently, the fuselage undergoes frequent pressurization/depressurization cycles leading repeated loading and unloading. During any phase, fatigue damage may occur and a crack can initiate and propagate in fuselage panels. Scheduled inspection intervals are calculated to ensure the safety of the structure, that means such that no crack can grow to critical size before the next inspection. Accordingly, a threshold corresponding to panel replacement has been defined and each panel with a crack reaching this threshold is repaired or replaced. The maintenance intervals and the threshold affect both the safety and lifecycle cost of an aircraft undergoing scheduled maintenance. Alternative condition-based maintenance (CBM) doesn’t required scheduled inspections. In this approach, structural damage is continuously tracked by an on-board system assessing the status of the health of a structure. SHM systems allow to monitor the condition of a structure using embedded sensors and actuators which detect structural damages. The SHM-based conditional maintenance (SHM-CBM) provides a real-time status of a structure. Maintenance is requested (and not scheduled) when a detected crack size exceeds a predetermined threshold different of the scheduled maintenance one. This threshold is chosen according to safety requirements. Our paper proposes a general mathematical framework to compare SHM-CBM maintenance to scheduled maintenance in terms of the average number of fuselage panels replaced over the in-service aircraft operational life. New analytical formulas are demonstrated to express the probability to change a fuselage panel at any cycle. The recursive analytical expressions obtained only depend on both panel crack size with no replacement from the entry into service of the aircraft and thresholds defined before according to strategy maintenances. The average number of panels to be changed after any number of cycles can be straightforwardly deduced, especially at times of scheduled maintenances as well as SHM-CBM maintenances. Based on the average number of panels replaced, the efficiency of the SHM-CBM solution can be quantified compared to results obtained with traditional scheduled maintenances. The fatigue crack growth propagation in the panel is modelled using Paris law. Uncertainties on amplitude fatigue loading, initial crack size and material properties are modelled as random variables. The usefulness of the proposed approach is shown by simulating real-world cases of short range aircraft fuselages provided by Airbus company.

03/04/2013 - Antonio Celani (Institut Pasteur)

The entropic anomaly

Résumé :

Particle motion at the micro-scale is an incessant tug-of-war between thermal  fluctuations and applied forces on one side, and the strong resistance exerted by fluid viscosity on the other. Friction is so strong that completely neglecting inertia – the overdamped approximation – gives an excellent effective description of the actual particle mechanics. In sharp contrast with this result, here we show  that the overdamped approximation dramatically fails when thermodynamic quantities such as the entropy production in the environment is considered, in presence of  temperature gradients. In the limit of vanishingly small, yet finite inertia, we  find that the entropy production features a contribution that is anomalous, i.e.  has no counterpart in the overdamped approximation. This phenomenon, that we call entropic anomaly, is due to a symmetry-breaking that occurs when moving to the  small, finite inertia limit.

 As a consequence of this phenomenon, quasi-static engines, whose efficiency is  maximal in a fluid at uniform temperature, have in fact vanishing efficiency in presence of temperature gradients. For slow cycles the efficiency falls off as the  inverse of the period. The maximum efficiency is reached at a finite value of the cycle period that is inversely proportional to the square root of the gradient intensity. The relative loss in maximal efficiency with respect to the thermally homogeneous case grows as the square root of the gradient. As an illustration of these general results, we construct an explicit, analytically solvable example of a Carnot stochastic engine. In this thought experiment, a Brownian particle is confined by a harmonic trap and immersed in a fluid with a linear temperature profile. This example may serve as a template for the design of real experiments in which the effect of the entropic anomaly can be measured.

12/04/2013 - K. Boehmer (Philipps University Marburg)

Dew drops on spider webs: a symmetry breaking bifurcation for a parabolic differential-algebraic equation

Lines of dew drops on spider webs are frequently observed on cold mornings. In this lecture we present a model explaining their generation. Although dew is supposed to condense somehow evenly along the thread, only lines of drops are observed along the spider thread. What are the reasons for this difference? We try to give an explanation by concentrating on some essential aspects only. This every-day observation is an example of one of the fascinating scenarios of nonlinear problems, symmetry breaking bifurcation. Despite many simplifications the model still provides very interesting mathematical challenges. In fact the necessary mathematical model and the corresponding  numerical methods for this problem are so complicated that in its full complexity it never has been studied before. We analyse and numerically study symmetry breaking  bifurcations for a free boundary value problem of a degenerate parabolic differential-algebraic equation employing a combination of analytical and numerical tools.

This lecture is an appetizer for my two books in OUP:

Numerical Methods for  Nonlinear  Elliptic Differential Equations, A Synopsis, 2010  Numerical Methods for Bifurcation  and Center Manifolds  in  Nonlinear  Elliptic and Parabolic Differential Equations, 2011.

30/04/2013 - Ivan Zelinka (université d'Ostrava, République Tchèque),
On Relations Between Evolutionary Dynamics and Complex Networks

Résumé :

This topic introduces the notion of complex structures synthesis by
means of evolutionary algorithms and develops a new method for complex
systems synthesis.
This method is similar to genetic programming and grammatical evolution
and is applied alongside evolutionary algorithms: differential
evolution, self-organizing migrating, genetic algorithm, simulated
annealing and evolutionary strategies. The aim of this investigation is
to synthesize electronic circuits, complex structures and systems like
a new and “simple” chaotic systems based on some elements contained in
a pre-chosen existing chaotic system and a properly defined cost
function. The investigation consists of two case studies based on the
aforementioned evolutionary algorithms in various versions. For all
algorithms, 100 simulations of chaos synthesis were repeated and then
averaged to guarantee the reliability and robustness of the proposed
method. The most significant results are carefully selected, visualized
and commented in this speech.
The second part will contain three mutually joined parts, namely
introducing a novel approach joining evolutionary dynamics, complex
networks and CML systems exhibiting chaotic behavior. The first part
will discuss a novel method on how dynamics of evolutionary algorithms
can be visualized in the form of complex networks. An analogy between
individuals in the populations in an arbitrary evolutionary algorithm
and the vertices in a complex network will be discussed as well as the
relationship between the communications of individuals in a population
and the edges in a complex network. The second part will discuss the
possibility of how to visualize the dynamics of a complex network by
means of coupled map lattices and to control by means of chaos control
techniques.
The last part will discuss some possibilities on CML systems control,
especially by means of evolutionary algorithms. The spirit of this
keynote speech is to create a closed loop in the following schematic:
evolutionary dynamics --> complex network --> CML system -->
control CML --> control evolutionary dynamics. Real-time simulations
as well as animations and pictures demonstrating the presented ideas
will be presented through this keynote speech.

14/06/2013 - Olivier Dauchot (UMR Gulliver, CNRS and ESPCI-ParisTech)

17/09/2013 - Guillaume Dhont (Univ. du Littoral Côte d'Opale)

Interprétation symbolique de la fonction génératrice en théorie des invariants

Résumé :  

De nombreuses molécules admettent un groupe ponctuel de symétrie G non tri-
vial dans leur configuration d’équilibre. L’étude de tels systèmes amène à construire
des objets appartenant à une représentation irréductible Γ 2 de G à partir de va-
riables élémentaires appartenant à une représentation Γ 1 , réductible en général.
Les objets ainsi générés sont souvent exprimés sous une forme polynomiale. Le
problème posé ici est alors de trouver et caractériser l’ensemble des polynômes
susceptibles d’apparaître dans ce développement polynomial.
La fonction génératrice de Molien est un outil pertinent pour ce travail. Elle se
calcule directement à partir de la représentation matricielle de l’action du groupe
sur les objets élémentaires et des caractères de la représentation irréductible Γ 2 .
Le coefficient de degré n dans le développement en série de cette fonction donne le
nombre de polynômes linéairement indépendants de degré n qui se transforment
selon Γ 2 .
Cette fonction de Molien peut se mettre sous la forme d’une ou plusieurs fonc-
tions rationnelles. Nous discuterons dans cet exposé de l’interprétation symbolique
de cette fonction génératrice à l’aide d’exemples concrets. Nous commencerons par
des rappels sur le cas classique des invariants de groupes finis pour terminer avec
le cas des covariants de groupes continus compacts illustré par le groupe SO(2).

07/01/2014 - Emilie Lebarbier (AgroParisTech)

31/01/2014 - Christophe Pouzat (MAP5, Paris 5)

Analyse de séquences de potentiels d'actions émises par plusieurs
neurones : l'exemple du premier relais olfactif de la blatte, Periplaneta
americana.

Résumé :

La communication entre régions distantes du cerveaux s'effectue
par la propagation de brèves « déviations » du potentiel membranaire
des neurones : les potentiels d'action. Ces potentiels d'action ne
transmettent de l'information que par leur temps d'arrivée et non
par leur forme ou leur amplitude qui sont « toujours » les mêmes.
Une analyse dont le but est de « décoder » l'information portée les
séquences de potentiels d'action peut donc, à priori, se baser sur
une modélisation des temps d'apparition des potentiels d'action,
indépendamment des mécanismes biophysiques qui les génèrent
effectivement ; c'est-à-dire que le formalisme probabiliste des
processus ponctuels / processus de comptage devrait s'appliquer à ce
cas. Poursuivant une désormais longue tradition d'analyse des
séquence de potentiels d'action par ce formalisme, nous présenterons
dans le cadre concret de l'étude du premier relais olfactif d'un
insecte — la blatte, Periplaneta america — une approche non
paramétrique de l'estimation de l'élément fondamental d'un processus
ponctuel : son intensité conditionnelle.

17/04/2014 - Eric Simonnet (INLN, Unice) - [joint seminar with EDP and ProbaStat teams à 14h]

    A splitting method for computing rare events with applications

18/04/2014 - Paul Johnson (University Mc Master, Canada)

    Bethe Ansatz and related wavefunction forms for strong correlation in quantum chemistry

Résumé:

In quantum chemistry, we are tasked with solving the Schrödinger
equation for atoms and molecules: the energies and allowed states are
respectively the eigenvalues and eigenvectors of the physical
Hamiltonian. The main difficulty is in employing a method which is
both sufficiently accurate and computationally tractable.
Conventional approaches are reliable when a single configuration of
the electrons is dominant (weak correlation), but tend to fail when
many configurations contribute to the physical state (strong
correlation). To this end, we have employed the Bethe Ansatz for the
reduced Bardeen-Cooper-Schrieffer Hamiltonian as a trial vector. The
structure is such that a large number of configurations are
represented with a reasonable cost.

I will discuss the usage of the Bethe Ansatz along with generalizations
and present various problems encountered in several approaches.

13/06/2014 - Sylvain Maire (Université du Sud Toulon-Var)

Intégration numérique adaptative en dimension modérée: application au pricing d'options européennes.

Résumé: Le calcul numérique d'intégrales reposent sur deux types de
méthodes:
les méthodes de quadrature de type Gauss-produit utilisables plutot en
petite dimension et les méthodes Monte-Carlo ou quasi-Monte Carlo pour les grandes
dimensions. Nous développons dans un premier temps des quadratures efficaces pour des dimensions
intermédiaires (inférieures à 10) construites à partir d'un ajustement par
moindres carrés d'un modèle polynomial creux sur des données
aléatoires bien choisies. Ces quadratures sont très efficaces pour les
fonctions régulières mais pour des fonctions moins régulières, on doit
procéder à un découpage du domaine afin de localiser les zones de fortes
variations. Nous proposerons deux techniques adaptatives pour effectuer ce
découpage utilisant les quadratures précédentes comme éléments de base.
Enfin nous donnerons des exemples numériques pour le pricing d'options ou la
fonction à intégrer est seulement continue au voisinage d'une surface à
localiser numériquement car elle s'écrit comme une partie positive.

04/07/2014 - Guanrong (Ron) Chen (Center for Chaos and Complex Networks, City University of Hong Kong)

    Looking for best networks for consensus

Résumé & bio : Within a social community of connected people, what is the
best networking relationship that can facilitate their
interactions towards consensus? For an array of chaotic
oscillators, what is the best network topology that promotes the
synchrony of their coupled dynamics? This talk will discuss such
common issues altogether from a unified graph-theoretic
perspective, presenting them as a synchronizability or
consensusability problem of a connected undirected network. The
corresponding mathematical problem is determined by the spectrum
of the Laplacian matrix of the network, which reflects most
topological characteristics of the network such as degree
distribution, path-length, clustering centrality, and so on.
Recently, we found that networks with best possible
synchronizability are in some sense “homogenous” and “symmetric”,
having some common features such as an identical degree sequence,
a longest girth and a shortest path-sum. This talk will present
the motivation and background of the topical issue, explain the
basic idea and methodology in tackling the problem, and verify the
result by using degree-3 regular networks of small sizes, with a
conjecture that the conclusion is true in general.

Professor Chen received the M.Sc. degree in Computer Science from
the Sun Yat-sen University, China and the Ph.D. degree in Applied
Mathematics from Texas A&M University, USA. Currently he is a
Chair Professor and the Director of the Centre for Chaos and
Complex Networks at the City University of Hong Kong. He is
a Fellow of the IEEE since 1997, serving as Editor-in-Chief for
the International Journal of Bifurcation and Chaos. He was
conferred Honorary Doctorate by the Saint Petersburg State
University, Russia in 2011, and is an Honorary Professor at
different ranks in some thirty universities worldwide.

05/09/2014 - Patrick Brest (Institute of Research on Cancer and Ageing of Nice (IRCAN))

   Analyses multi-niveaux (genomique, transcriptomique, proteique):
Outil d'aide à la Décision thérapeutique en médicine personnalisé pour
une meilleure prise en charge du cancer du poumon.

Résumé:

Le cancer du poumon est la première cause de mortalité par cancer en France et dans le monde.
Aujourd'hui, en France, la survie à cinq ans après un diagnostic de cancer du poumon est de 14 %
(13 % chez les hommes, 18 % chez les femmes). La médecine personnalisée est désormais
l’approche privilégiée pour les cancers du poumon à un stade avancé et métastatique, avec
notamment l’apparition de nouvelles stratégies telle que l’immunothérapie qui fait appelle à notre
système immunitaire pour lutter contre le cancer.
Au sein de notre équipe qui regroupe tous les acteurs de la santé (des chercheurs fondamentaux aux
chercheurs cliniciens), nous étudions principalement les signatures prédictives de l’évolution de ces
cancers. Pour cela, nous avons à notre disposition d’énorme quantité de données OMICS
(génomique, transcriptomique, protéomique), mais leur exploitation reste difficile. L'objectif de
cette réunion est de fédérer nos compétences pour fournir au biologiste et au clinicien des
algorithmes lui permettant
*Au niveau fondamental : Modélisation prédictive de la réponse aux drogues. Certains processus
(Apoptose, Autophagie,...) semblent fondamentaux dans les réponses cellulaires aux
chimiothérapies. A l’aide de rapporteurs protéiques, nous sommes capables d’observer certaines
réponses cellulaires en temps réel au niveau de la cellule unique afin d’extraire un grand nombre de
données dynamiques. A partir de ces données, nous utilisons des modèles mathématiques de voies
de signalisation cellulaires pour prédire le devenir des cellules cancéreuse et chercher des cibles
moléculaires.
* Au niveau clinique : a) En analyse en composantes multiples, chaque patient sera, outre le tableau
clinique, associé à un nombre important de données (génomique, transcriptomique et protéomique)
dans un futur proche. Le but de notre étude serait de déterminer si une analyse multiple multi-
niveaux pourrait être envisagée permettant la liaison de l'ensemble de la variabilité individuelle au
transcriptome, au protéome, et idéalement qui pourraient être associées à une mauvaise survie.
b) En transcriptomique, une signature correspond finalement à une somme de sous-composantes
cellulaires. Le but de notre étude serait de déterminer s’il est possible à partir de cette signature de «
déconvoluer » le signal en ces sous composantes cellulaires afin de définir si leur proportion
respectives pourraient être un signe prédictif de l’évolution cancer du poumon et d’autres cancers.
En conclusion, l'enjeu est de hiérarchiser les signatures moléculaires clés du développement tumoral
pour prédire les résistances et les rechutes pour une prise en charge plus précoce et plus efficace des
patients.

05/09/2014 - Yaouen Fily (Brandeis University, USA) [12h00]

   Active particles under strong boundary confinement.

Résumé:

It's been long known that active particles have a tendency to accumulate near walls and get trapped
at corners. More recently, these features have been used to control active suspensions, e.g. direct
bacteria or get them to power micro-devices. However, a general understanding of how boundary
shape affects the properties of active suspensions is still missing. Using a simple model of active
suspension, I will show how one can derive a general relationship between the particle density and
the shape of the box. Despite its limitations (the suspension must be strongly confined and dilute),
the fact that the result holds for arbitrary box shapes makes it a promising tool to better understand
and design confining boxes for active suspensions.

11/09/2014 - Tucker Carrington (Queens University, Canada)

"Pruned basis methods for solving the vibrational Schrödinger equation without approximating the potential"

Spectra are computed by solving the time-independent Schrödinger
equation. If the number of atoms in a molecule is greater than
about 4 the dimensionality of the Schrödinger equation makes solution very
difficult. Standard discretization strategies for large
dimensional systems (D ≥ 9) yield matrices and
vectors that are too large for modern computers. This is the
so-called "curse of dimensionality". Both the basis size
and quadrature grid size are problems. Several ingenious
schemes have been developed in the last decades to reduce the basis
size. We have focused on reducing the size of the quadrature grid. The
Smolyak algorithm allows one to create non-product quadrature grids
with structure. Because of the structure they can be used with
pruned product basis sets and the Lanczos algorithm to compute
spectra. In this talk we shall explain the nature of these grids and why
they are useful for computing spectra.

12/09/2014 - Leon Chua (Imperial College, UK, University of California, Berkeley et Distinguished Professor, Techniche Universitat München)

   Memristors : Device with intelligence

Résumé:

Although postulated in 1971, the memristor has attracted worldwide attention from
academia and industry only after Hewlett-Packard published a working nano-memristor
device in the 1 May 2008 issue of Nature [1]. An acronym for memory resistor, the memristor
is poised to revolutionalize future generations of computers, ipad, smart phones, etc.
Among many applications from numerous disciplines including biology, botany , physics,
mathematics, etc., the memristor can be used to emulate synapses and the Hodgin-Huxley
Equations, and is therefore the right stuff for building intelligent brain-like machines. This
lecture explains what makes the memristor such a versatile device and answers many
questions that memristor researchers are afraid to ask.
Biographie: Leon Chua is a foreign member of Academia Europeia and the Hungarian
Academy of Sciences. He has been awarded 7 patents and 14 Doctor Honoris Causa from
major universities worldwide. He has won many prizes, including the Guggenheim Fellow
and the EC Marie Curie Fellow.

19/09/2014 - Rodrigo Soto (Universidad de Chile, Chili) [12h00]

   Natural and self-assembled swimmers.

Résumé:

Bacteria, like E. coli, constitute a natural example of self-propelled objects. The permanent energy
flux from the fuel, to the swimmers and lately to the fluid, puts these systems in permanent non-
equilibrium conditions. Swimmers constitute paradigmatic examples of active matters and
constitute an interesting playground for non-equilibrium matter. In this talk I will describe the
properties of natural (E. coli) and artificial self-assembled (colloidal) swimmers.
When bacteria are placed in confined environments they are attracted to solid surfaces and swim in
contact with them for long times. In this talk, several aspects related to the motion of bacteria near
surfaces are considered. First, a simple swimmer model will be introduced, which reproduces the
main features of bacteria-surface interaction including the observed circular motion. Second, the
induced diffusion of tracers in presence of a bacterial bath is studied. A kinetic theory combined
with a hydrodynamic approach allow us to compute the resulting diffusivity. It is shown that close
to surfaces, the induced diffusivity is enhanced as a result of the smaller swimming efficiency of
bacteria.
Artificial swimmers are also possible to build. In this talk the case of self-assembled colloidal
particles is described.
Catalytically active colloids maintain non-equilibrium conditions in which they produce and deplete
chemicals. The concentration fields resulting from the chemical activity produce gradients that
attract or repel other colloids depending on their surface chemistry and ambient variables. This
results in a non-equilibrium analogue of ionic systems, but with the remarkable novel feature of
action-reaction symmetry breaking. In dilute conditions they join up to form (colloidal) molecules.
The molecules could be inert or have spontaneous activity in the form of net translational velocity
and spin depending on their symmetry properties and their constituents. Also, oscillatory states are
possible as well as spontaneous transitions from active to passive states, mimicking the run-and-
tumble motion of bacteria.

17/10/2014 - Timothée Masquelier (Institut de la Vision, CNRS, Paris) [14h30 - salle conf.]

   Spike-based computing and learning in brains, machines, and visual systems in particular

Résumé

Using simulations, we have first shown that, thanks to the physiological learning mechanism
referred to as spike timing-dependent plasticity (STDP), neurons can detect and learn repeating
spike patterns, in an unsupervised manner, even when those patterns are embedded in noise [1], [2].
Importantly, the spike patterns do not need to repeat exactly: it also works when only a firing
probability pattern repeats, providing this profile has narrow (10-20ms) temporal peaks [3]. Brain
oscillations may help in getting the required temporal precision [4], in particular when dealing with
slowly changing stimuli. All together, these studies show that some envisaged problems associated
to spike timing codes, in particular noise-resistance, the need for a reference time, or the decoding
issue, might not be as severe as once thought. These generic STDP-based mechanisms are probably
at work in particular the visual system, where they can explain how selectivity to visual primitives
emerges [5], [6]. Finally, these mechanisms are also appealing for neuromorphic engineering: they
can be efficiently implemented in hardware, leading to reactive systems with self-learning abilities
[7].
References
[1] T. Masquelier, R. Guyonneau, and S. J. Thorpe, “Spike timing dependent
plasticity finds the start of repeating patterns in continuous spike trains.,”
PLoS ONE, vol. 3, no. 1, p. e1377, Jan. 2008.
[2] T. Masquelier, R. Guyonneau, and S. J. Thorpe, “Competitive STDP-Based Spike
Pattern Learning.,” Neural Comput, vol. 21, no. 5, pp. 1259–1276, May 2009.
[3] M. Gilson, T. Masquelier, and E. Hugues, “STDP allows fast rate-modulated
coding with Poisson-like spike trains.,” PLoS computational biology, vol. 7, no.
10, p. e1002231, Oct. 2011.
[4] T. Masquelier, E. Hugues, G. Deco, and S. J. Thorpe, “Oscillations, phase-
of-firing coding, and spike timing-dependent plasticity: an efficient learning
scheme.,” The Journal of neuroscience, vol. 29, no. 43, pp. 13484–93, Oct. 2009.
[5] T. Masquelier and S. J. Thorpe, “Unsupervised learning of visual features
through spike timing dependent plasticity.,” PLoS Comput Biol, vol. 3, no. 2, p.
e31, Feb. 2007.
[6] T. Masquelier, “Relative spike time coding and STDP-based orientation
selectivity in the early visual system in natural continuous and saccadic
vision: a computational model.,” Journal of computational neuroscience, vol. 32,
no. 3, pp. 425–41, Jun. 2012.
[7] C. Zamarreño-Ramos, L. A. Camuñas-Mesa, J. A. Pérez-Carrasco, T. Masquelier,
T. Serrano-Gotarredona, and B. Linares-Barranco, “On Spike-Timing-Dependent-
Plasticity, Memristive Devices, and Building a Self-Learning Visual Cortex,”
Frontiers in neuroscience, vol. 5, no. March, p. 22, 2011.

16/01/2015 - Ivan Zelinka  (Ostrava University, République Tchèque) "Evolutionary synthesis"

Résumé:

This talk discusses an alternative approach for symbolic
structures and solutions synthesis and demonstrates a comparison with other
methods, for example Genetic Programming (GP) or Grammatical
Evolution (GE). Generally, there are two well known methods, which can be
used for symbolic structures synthesis by means of computers. The
first one is called GP and the other is GE. Another interesting research
was carried out by Artificial Immune Systems (AIS) or/and
systems, which do not use tree structures like linear GP and other similar
algorithm like Multi Expression Programming (MEP), etc. In this talk,
a different method called Analytic Programming (AP), is
presented. AP is a grammar free algorithmic superstructure, which can be
used by any programming language and also by any arbitrary
Evolutionary Algorithm (EA) or another class of numerical optimization
method. It describes not only theoretical principles of AP, but also
its comparative study with selected well known case examples
from GP as well as applications on synthesis of: controller, systems of
deterministic chaos, electronics circuits, etc. For simulation
purposes, AP has been co-joined with EA's like Differential
Evolution (DE), Self-Organising Migrating Algorithm (SOMA), Genetic
Algorithms (GA) and Simulated Annealing (SA). All case studies has been
carefully prepared and repeated in order to get valid statistical
data for proper conclusions.

23/01/2015 - Tarcísio M. Rocha Filho (Université de Brasília)
"Using Graphics Processing Units in Statistical Mechanics Simulations - The Vlasov equation and Molecuar Dynamics of Many particles system with long-range interactions"

Résumé:

We present the algorithms and implementations of Vlasov equation
integrator for one spatial dimension of systems with
long-range interactions, discussing the caveats in memory access
and some examples of how to choose the most adequate algorithms
for better efficiency of the parallel code in the GPU. Multi-GPU
implementations are illustrated with a Molecular Dynamics code for the
same type of systems. We also present some relevant results obtained
using such codes at the Statystical Physics group at the University
of Brasília.

05/02/2015 - Isabelle Kleiner (Lab. Interuniversitaire des Systèmes Atmosphériques (LISA), CNRS/IPSL et Universités Paris Est-UPEC et Paris Diderot)

"Spectroscopy of internal rotors and applications to astrophysical, prebiotic and biological molecules "

In this talk I will first give a general introduction to the high
resolution spectroscopy techniques in the microwave,
millimetre and infrared spectral range and how they can
contribute to the knowledge of the astrophysical medium or the
planet atmospheres. The topic of our work concerned mainly
molecules containing large amplitude motions and in particular
molecules containing one or two methyl (CH3) internal. One of
the goals of our work is to describe with our theoretical
method and codes the energy levels, and then the line
positions and intensities for this type of molecules.

That way we are able to provide reliable predictions of line positions and
intensities for astrophysical molecules containing one
internal rotor CH3, such as the isotopic species of methyl
formate HCOOCH3, methanol CH3OH, acetic acid CH3COOH,
acetaldehyde CH3CHO or acetamide CH3CONH2. The major new
facilities, the Herschel Space Observatory and the Atacama
Large Millimeter Array (ALMA), in Chili, have now opened the
submillimeter region up to a few THz for astronomical
observations by making investigations with unprecedented high
sensitivity and resolution. Molecules which undergo internal
rotation of a methyl group present thousands of lines in this
spectral range and therefore their spectra are particularly
important to model.

I will show some of the recent results in the microwave or infrared range
for those molecules, and some applications in astrophysics. We
will also present recent results for the study of a two-top
internal rotor, the methyl acetate molecule CH3-O-C(=O)-CH3,
which has been very recently detected in the interstellar
cloud Orion thanks our prediction.

Another goal of our work is to get some knowledge of the structural
properties of small organic molecules or biomimetic molecules.
Fourier transform microwave spectroscopy in the gas phase
coupled with high level quantum chemical calculations or ab
initio calculations has led to the precise and detailed
determination of molecular structures for the lowest energy
conformers of a number of molecules. In this talk, I will show
results from molecules which can be considered as prototypes
of odorant molecules, like isoamyl acetate (so-called “banana
oil”) or linalool, an acyclic mono-terpene present in lavender
and other plants and emitted in the earth atmosphere.

27/03/2015 - Pascal Viot (UPMC) 

"Blocking in single-file particulate flows".

Résumé:

Filtration,  flows in micro/nano­channels and traffic flows are examples
of processes subject to blockingwhen the channel conveying the particles
becomes too crowded. We investigate a concurrent flow model where
particles enter a channel randomly. If at any time two particles are
simultaneously present in the channel, failure occurs. We   obtain   the
  exact   solution   for   the   survival   probability,   the  
distribution   of   the number of particles that pass before failure,
and the instantaneous flux exiting the channel. Generalizations of this
simple model are also considered.

Second, we consider  a counterflow model with two opposing Poisson
streams. There is   no   restriction   on   the   number   of  
particles   passing   in   the   same   direction,   but blockage occurs
if, at any time, two opposing particles are simultaneously present in
the passage. All relevant quantities have been obtained exactly

[1] Non­Markovian Models of Blocking in Concurrent and Countercurrent
Flows, A.

Gabrielli, J. Talbot and P. Viot Phys. Rev. Lett,  110, 170601 (2013)

[2]   Stochastic   model   of   single­file   flow   with   reversible  
blockage,  Chloé   Barré, Julian Talbot and Pascal Viot EPL, 104 60005
(2013)

[3] Irreversible Blocking in Single­File Concurrent and Countercurrent 
articulate Flows, J. Talbot, A. Gabrielli  and P. Viot J. Stat.Mech
P01027 (2015)

10/04/2015 - Guillaume Dhont, (LPCA, Université du Littoral Côte d'Opale)  

Structures combinatoires, algébriques et topologiques en physique 

Résumé:

L'analyse qualitative de systèmes moléculaires vise à obtenir
une description globale de leurs propriétés et amène à étudier
des structures mathématiques qui relèvent de la combinatoire, de
l'algèbre ou de la topologie. En particulier, les fonctions
génératrices sont un outil mathématique efficace pour encoder de
l'information sous une forme compacte. Nous présenterons dans
cet exposé l'application des fonctions génératrices à l'étude des
invariants et des covariants sous l'action du groupe SO(2) d'une
part et au dénombrement des niveaux d'énergie dans les polyades
vibrationnelles de molécules d'autre part.

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Vendredi 20/11/2015  à 11h en salle de conférences

Mohammed-Salah Abdelouahab (Laboratory of Mathematics and their
interactions, Mila University Center, Mila, Algeria)

"Hopf Bifurcation and Chaos in Simplest Fractional-order Memristor-based Electrical Circuit"

Résumé : We investigate the bifurcation and the onset of chaos in a
fractional-order simplest memristor-based electrical circuit
composed of only three circuit elements: a linear passive capacitor,
a linear passive inductor and a nonlinear active memristor with two
degree polynomial memristance and a second order exponent internal
state. It is shown that this fractional circuit can exhibit a very
rich nonlinear dynamics such as a Hopf bifurcation, coexistence of
two, three and four limit cycles, double-scroll chaotic attractor,
four-scroll chaotic attractor, coexistence of one (or two) chaotic
attractors with one limit cycle and new chaotic attractor which is
not observed in the integer case. Finally, the presence of chaos is
confirmed by the application of the recently introduced 0-1 test.
(Email: m.abdelouahab@centre-univ-mila.dz)
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Mardi 24/11/2015  à 11h en salle 3

Tidjani Menacer, (Université Mohamed Khider de Biskra, Algérie)

"Bifurcations cachées dans les attracteurs multi-spiral de type Chua"

Résumé : La recherche « d’attracteurs cachés » dans les systèmes
dynamiques (c’est-à-dire d’attracteurs caractérisés par un bassin
d'attraction très petit qui n’est relié à aucun point d'équilibre,
de sorte qu’il ne peut pas être localisée par des procédures de
calcul standard) a été initiée depuis quelques années par Kuznetsov
et Leonov [1] qui ont proposé une méthode analytique-numérique pour
les localiser (localisation basée sur la méthode des fonctions de
description). Dans le cas des attracteurs chaotiques multi-spiral de
type Chua qui dépendent d’un paramètre discret, nous détournons la
méthode de Kuznetsoz et Leonov pour trouver des « bifurcations
cachées » en faisant apparaître par cette méthode détournée un
paramètre de bifurcation continu.
[1] http://www.worldscientific.com/doi/abs/10.1142/S0218127413300024

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jeudi 10 décembre 2015 à 14h en salle de conférences

soutenance d'Amine ILMANE

" Développements autour de la méthode d'interactions de configurations en champ moyen "

Résumé :  Dans
cette thèse ont été développés  de nouveaux  outils de calcul théorique
de spectres moléculaires rovibrationnels qui permettent de mieux
traiter les états vibrationnels  très excités ainsi que les mouvements
de grandes amplitudes avec la méthode d’interactions de configurations
en champ moyen. Dans un premier temps, nous avons discuté la question du
choix des bases modales et les différents compromis à trouver afin de
pallier aux défauts possibles des surfaces d'énergie potentielle.  Dans
ce cadre nous avons également développé un critère de sélection visant à
améliorer la qualité des fonctions d'ondes rovibrationnelles de base.
Ces approches ont été appliquées avec succés à la molécule de méthane
CH4.Dans un second temps, nous avons  implémenté un algorithme de calcul
formel des opérateurs d'énergie cinétique en coordonnées quelconques
qui permet d'avoir des expressions exactes ainsi que leurs
développements en série de Taylor  ou Fourier, qui exploite au mieux les
potentialité du logiciel MATHEMATICA et a permis d'obtenir des
hamiltoniens rovibrationnels en coordonnées de valence de façon
particulièrement efficace. Enfin, nous avons généralisé la méthode
d’interactions de configurations en champ moyen en ajoutant de façon
perturbative un champ effectif d'ordre deux. Nous avons appliqué cette
généralisation à la molécule de  péroxyde d'hydrogène HOOH, ce qui a
permis de montrer son intérêt tant pour l'amélioration des niveaux
d'énergie que des fonctions d'onde associées, lorsqu'on a affaire à des
groupes de degrés de liberté bien séparés énergétiquement.

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26/02/2016

Magnetic anisotropy in mono- and bi-nuclear complexes: theoretical insight and prospects

Renaud Ruamps;^A R. Maurice;^B C. de Graaf,^C and Nathalie Guihéry^A

^A Université de Toulouse 3, Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France;

^B SUBATECH, UMR CNRS 6457, IN2P3/EMN Nantes/Université de Nantes, 4 rue A. Kastler, BP 20722, 44307 Nantes Cedex 3, France;

^C Universitat Rovira i Virgili, Marcel-li Domingo s:n, 43007 Tarragona, Spain

Magnetic anisotropy is the origin of the single molecule magnet (SMM) behavior
which is manifested by a slow relaxation of the magnetization and a
blocking of the magnetization for low enough temperatures. Since this
bistable behavior may lead to possible technological applications in
the domain of data storage and quantum computing, the understanding
of the microscopic origin of magnetic anisotropy has received a
considerable interest during the last two decades.

Mono-nuclear complexes having exotic coordination of the metal ion have recently
been shown to exhibit a very large magnetic anisotropy. A first study
will be devoted to the rationalization of the magnitude and nature of
single ion anisotropy from theoretical calculations. It will be shown
that the competition between relativistic effects and Jahn Teller
distortion may lead to very large magnetic anisotropy (see figure 1).

The overall magnetic anisotropy of a poly-nuclear complex comes from both
the local anisotropies of paramagnetic ions and their interactions.
The second part of this presentation will focus on the understanding
of synergistic effects between local anisotropies in bi-nuclear
species for which we have both tuned the local anisotropies by
imposing peculiar geometries and combined various types of local
anisotropies (see figure 2, values are in cm^-1).

The method used here was successfully confronted to experimental values in series of mono- and bi-nuclear complexes.[1]

References:

[1] J. of Chem. Theo. Comp., 2009 5, 11 2977; J Chem. Theor. Chem 2010 6 1, 55; J. Chem. Theor. Chem. 2010 6 10, 3092; Phys. Rev. B, 2010 81 21, 214427;  J. Chem. Phys. 2010 133, 084307; Inorg. Chem. 2011 50 13, 6229; Phys. Rev. B 2012, 85, 014409;
Phys. Rev. B 2012, 86, 024411; Chem. Euro. J. 2013, 19, 950; R. Ruamps et al. J. Am. Chem. Soc.
2013, 135, 3017A, Phys. Chem. Chem. Phys., 2013, 15 (43), 18784.

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18/03/2016

Kurusch Ebrahimi-Fard,

"Saving a species in a Lotka-Volterra model via an Hopf algebra antipode"

Abstract: Lotka-Volterra systems can be viewed as nonlinear input-output systems when
time-varying parameters are taken as system inputs and the population
levels are the system outputs. By varying some of these inputs, an
open-loop control problem results, and one objective may be to determine
appropriate inputs to produce desired system profiles. This is known as
the left inversion problem. In this talk we outline the general left
inversion problem for multivariable input-output systems that can be
represented in terms of Fliess operators (aka Chen-Fliess series) using
concepts from combinatorial Hopf algebras. The antipode plays a central
role in this context. This is then applied to a three
species Lotka-Volterra system. The aim is to change the population
dynamics of the top-level predator species in a food chain in such a way
as to prevent extinction.

	Nous avons étudié plusieurs structures neuronales à différentes
échelles allant des  synapses aux réseaux neuronaux. Notre objectif était de développer et analyser des modèles mathématiques, afin de déterminer comment les propriétés des synapses au niveau moléculaire façonnent leur activité, et se propagent au niveau du réseau. Ce changement d’échelle peut être formulé et analysé à l’aide de plusieurs outils tels que les équations aux dérivées partielles, les processus stochastiques ou les simulations numériques. Dans un premier temps, nous avons calculé le temps moyen pour qu’une particule brownienne arrive à une petite ouverture définie comme le cylindre faisant la jonction entre deux sphères tangentes. La méthode repose sur une transformation conforme de Möbius appliquée à l’équation de Laplace. Nous avons également estimé, lorsque la particule se trouve dans un voisinage de l’ouverture, la probabilité d’atteindre l’ouverture avant de quitter le voisinage. De nouveau, cette probabilité est exprimée à l’aide d’une équation de Laplace, avec des conditions aux limites mixtes. En utilisant ces résultats, nous développons un modèle et des simulations stochastiques pour étudier la libération vésiculaire au niveau des synapses, en tenant compte de leur géométrie particulière. En nous appuyant sur ces résultats, nous avons développer un modèle pour le terminal pré-synaptique, qui couple un système d’équations d’action de masse à un ensemble d’équations de Markov, ce qui permet d’obtenir des résultats analytiques et de réaliser des simulations stochastiques rapides.

	The vertebrate body is provided with a repetitive structure of
segments, which originate sequentially -one by one- during embryonic
development, with a definite rhythm. This rhythm is controlled by a
biological clock, known as the vertebrate segmentation clock, which
relies on synchronization of cellular oscillators. It is thought that
each of the cells involved in the process acts as an independent
genetic oscillator. A local communication mechanism between cells
couple their dynamics giving rise to synchronization. Two key
components of this mechanism are communication delays and cell
movement. Intercellular communication involves synthesis and transport
of macromolecules which act as ligands and receptors. This aspect can
be described by incorporating delays in the coupling between
oscillators. Furthermore, cell mobility in the context of coupled
oscillators could promote synchronization by extending the effective
range of the coupling. Motivated by this mechanism we study the
dynamics of a system of coupled oscillators that includes these two
fundamental aspects of cell communication. We find that the system
exhibits complex spatio-temporal patterns on its way towards
synchronization.

Vendredi 2 décembre 2016 en Salle 2 du LJAD à 11 heures

M. Krupa (Inria)

"Heteroclinic chains as a model of priming"

Priming is the ability of the brain to more quickly activate a target concept
in response to a related stimulus (prime). Experiments point to the
existence of an overlap between the populations of the neurons coding
for di erent stimuli. Other experiments show that prime-target relations
arise in the process of long term memory formation. The classical
modelling paradigm is that long term memories correspond to stable
steady states of a Hop eld network with Hebbian connectivity.
Experiments show that short term synaptic depression plays an important
role in the processing of memories. This leads naturally to a
computational model of priming, called latching dynamics; a stable state
(prime) can become unstable and the system may converge to another
transiently stable steady state (target). We show that in the context of
Hop eld networks latching dynamics is closely approximated by (robust)
heteroclinic chains and discuss the role of sparse connectivity,
inhibition and noise in the approximation. Further, we derive necessary
conditions for the existence of heteroclinic chains in Hop eld networks
with synaptic depression and show how a Hop eld network supporting a
heteroclinic chain can be extended to a larger network in which the
weights are derived from a Hebbian learning rule.

Jeudi 9 février 2017 en Salle de Conférences du LJAD à 11 heures

Séminaire commun GAD / Interfaces

Jacques Féjoz (Dauphine)

"Théorème KAM, symétrie et dégénérescence"

La persistence de solutions quasipériodiques dans les systèmes hamiltoniens presque
intégrables dépend de conditions de non-dégénérescences. Nous analyserons comment ces
conditions et leurs conséquences sont modifiées en présence de symétries, avec le problème
des trois corps en guise d'application.

Lundi 27 février 2017 en Salle de Conférences du LJAD à 11 heures

Tarcísio M. Rocha Filho (Brasília)

"Equilibrium and Non-Equilibrium Statistical Mechanics of Systems with Long-Range Interactions"

I
will presente some recent developments on equilibrium and
non-equilibrium Statistical  Mechanics of systems with long-range
interactions. Our results concern mainly kinetic theory, chaos, ensemble
inequivalence and the classical analogue of Goldstone modes excited by
thermal fluctuations

Vendredi 10 mars 2017 en Salle de Conférences du LJAD à 11 heures    

Andreas Deutsch (Dresden)

Collective effects in cancer growth and invasion: insights from
mathematical models

Cancer growth and  invasion can be interpreted as  collective behaviour arising in migrating
 and interacting cell populations. Here, we demonstrate how cellular automaton models allow
for prediction and analysis of emerging properties (e.g. invasion speed) arising from particularly
 chosen cell migration and interaction scenarios (especially go or grow“ and „go and grow“
dynamics). We use our models for the interpretation of data from in vitro cancer cell invasion
assays and will also discuss implications of the „go or grow“ dichotomy for cancer growth and
persistence.

Reference:

A. Deutsch, S. Dormann:  Cellular Automaton Modeling of Biological Pattern Formation: Characterization,
Applications, and Analysis. Birkhäuser, Boston, 2005 (2nd ed. 2016)

Mercredi 10 mai 2017 en Salle de Conférences du LJAD à 11 heures

Antonio Celani (ICTP, Trieste)

"Learning to navigate in dynamic environments: animal behavior and artificial intelligence"

I will discuss two examples of difficult navigation tasks in highly
dynamical environments. The first one is thermal soaring in birds. Birds
 and gliders piggyback ascending currents to fly with a reduced
expenditure of energy, for example, during migration, and to extend
their flying range. Flow in the thermals is highly turbulent, which
poses the challenge of the orientation in strongly fluctuating
environments. The second one is gravitaxis by smart
microswimmers. These are self-propelled particles that can sense basic
mechanical cues from their surroundings and respond to them in order to
reach the highest altitude within some time horizon. We
 combine numerical simulations of fluid flow with reinforcement learning
 methods to identify strategies of navigation that can cope with
potentially dangerous flow configurations and exploit favorable ones.

Jeudi 11 mai 2017 en Salle de Conférences du LJAD à 11 heures

Stijn de Baerdemacker (Université de Gent)

"Richardson-Gaudin integrability for strongly correlated quantum systems"
One of the most intriguing problems in quantum many-particle physics
 is the emergence of strong quantum correlations and complexity.
Theoretical physics is tasked with explaining, understanding, and even
predicting these complex phenomena starting from fundamental premises.
Unfortunately, this is an unsurmountable task due to the size-explosion of the
Hilbert space, limiting exact computations to few-particle systems only.
 Interestingly, for the class of integrable systems, an exact solution can be
 obtained in the form of a Bethe Ansatz wavefunction, even when the
correlations are strong.  In this presentation, I will discuss and present the class
 of Richardson-Gaudin integrable systems, and show how they can be used as an
 optimal starting point for realistic quantum many-particle systems.

Vendredi 12 mai 2017 en Salle de Conférences du LJAD à 11 heures

Boris Nectoux (Cermics and INRIA)

"Metastability and the Eyring Kramer’s law"

In materials science, biology and chemistry, atomistic models are now used
in order to predict the macroscopic properties from a microscopic description of
matter. The basic ingredient is a potential energy function f : R d → R which
associates to a set of coordinates of particles the energy of the system. Using
this function f , two types of models are built: 1. continuous state space Markov models such as the Langevin dynamics or
the overdamped Langevin dynamics and, 2. discrete state space Markov models (jump Markov processes). One major issue for the sampling of the trajectories of the Langevin dynamics
is the possible metastable behaviour of this dynamics. Metastability refers to
the case when the typical time to observe transitions between some regions of
the configuration space is much longer than the typical timestep of vibrations
of the system. To deal with this issue, kinetic Monte Carlo methods are used
to design an underlying jump Markov process between metastable states. The
ingredient in kinetic Monte Carlo algorithms is the collection of transition rates
which are computed in practize with the help of the Eyring-Kramers law.
I will present recent results which aim at justifying rigorously the use of the
Eyring Kramers law in the small temperature regime. The proofs are based on
tools from semi-classical analysis.

Vendredi 19 mai 2017 en Salle de Conférences du LJAD de 11 à 15 heures

Suivi des thèses de l'équipe IMSC

Mercredi 14 juin 2017 à 16h (salle de conférences du LJAD)

Roberto Benzi (Università di Roma "Tor Vergata") https://scholar.google.com/citations?user=QJeFmVEAAAAJ&hl=en

"Earthquake statistics and plastic events in soft-glassy materials"

We propose a new approach for generating synthetic earthquakes based on the physics of soft
glasses. The continuum approach produces yield-stress materials based on Lattice–Boltzmann
simulations. We show that if the material is stimulated below yield stress, plastic events occur,
which have strong similarities to seismic events. Based on a suitable definition of displacement
in the continuum, we show that the plastic events obey a Gutenberg–Richter law with exponents
similar to those for real earthquakes. We also find that the average acceleration, energy release,
stress drop and interoccurrence times scale with the same exponent. Furthermore, choosing
a suitable definition for aftershocks, we show that they follow Omori’s law. Finally, the far
field power spectra of elastic waves generated by these plastic events decay as ω −2 similar to
those observed for seismic waves. Our approach is fully self-consistent and all quantities can
be calculated at all scales without the need of ad hoc friction or statistical assumptions. We
therefore suggest that our approach may lead to new insights into the physics connecting the
micro- and macroscales of earthquakes.   

Vendredi 23 juin 2017 à 11h (salle 2 du LJAD)

Mathias Legrand (McGill)

"Equation des ondes en une dimension et contraintes unilatérales"

Ce
travail concerne le calcul des modes de vibration non-linéaires d'une
structure mécanique simplifiée, une barre, encastrée à une extrémité et
sujette à des conditions de contact unilatéral, à l'autre.
Mathématiquement, il s'agit de chercher des familles continues de
trajectoires périodiques satisfaisant l'équation d'onde en une dimension
d'espace sur le domaine de la barre ainsi que la condition de
complémentarité traduisant le contact unilatéral. Ces familles forment
alors l'extension de la notion de "modes de vibration" bien connus dans
le cadre de la dynamique des structures linéaires. La méthode des
éléments finis introduit des difficultés numériques qui peuvent être
levées grâce à une variante d'un schéma de type "volumes finis". La
résolution numérique permet de dégager des solutions exactes. Le lien
entre solutions forcées périodiquement et solutions modales sera
illustré à l'aide de diagrammes "Energie-Fréquence".

Vendredi 30 juin 2017 à 11h (salle de conférences du LJAD)

Séminaire commun ATG / Interfaces

Elena Magliaro (Mines - Paritech)

"Overview of Loop Quantum Gravity"

La relativité générale et la mécanique quantique sont les deux théories
révolutionnaires qui ont changé la physique au siècle dernier. La
recherche d’une théorie qui fonde ceux deux théories dans un cadre
cohérent est la recherche d’une théorie de la gravité quantique, ou de
l’espace-temps quantique.

Aujourd’hui, un programme de recherche prometteur est la gravité quantique à boucles (Loop Quantum Gravity), développée désormais depuis 30 ans. Dans ce séminaire je présenterai les bases de cette théorie, dans sa version hamiltonienne et covariante (mousse de spin) en mentionnant les résultats les plus significatifs.

Jeudi 21 septembre 2017 à 11 h (salle de séminaires du LJAD)

P.J. Morrison (Austin)

"GEMPIC: An exact Poisson integrator for the full Vlasov-Maxwell System"

The Vlasov-Maxwell (VM) system, which couples the evolution of the phase space probability
density with the full system of Maxwell’s equations, was shown in the 1980s to be an infinite-
dimensional noncanonical Hamiltonian system. 1 Noncanonical means the Poisson operator does
not have the standard canonical form in terms of conjugate coordinates and momenta and is de-
generate, giving rise to Casimir invariants. This talk will summarize recent work 2 on a novel frame-
work for Finite Element Particle-in-Cell computation developed by discretizing the VM Hamilto-
nian structure. A semi-discrete (finite-dimensional) Poisson bracket that retains the properties of
anti-symmetry and the Jacobi-identity, as well as conservation of discrete versions of its Casimir
invariants, implying that the semi-discrete system retains the parent Hamiltonian structure, was
obtained. In order to obtain a fully discrete Poisson integrator, the semi-discrete bracket is used
in conjunction with Hamiltonian splitting methods for integration in time. Techniques from Finite
Element Exterior Calculus ensure conservation of the divergence of the magnetic field and Poisson’s
equation as well as stability of the field solver. The resulting methods are gauge-invariant, feature
exact charge conservation and show excellent long-time energy and momentum behavior. Due to
the generality of the framework, these conservation properties are guaranteed independently of any
particular choice of the Finite Element basis, as long as the corresponding Finite Element spaces
satisfy compatibility. Plasma physical examples using the GEMPIC code 3 will be described.

Mardi 26 septembre 2017 à 11 h (salle de séminaires du LJAD)

Juan Pablo Di Bella ( University of Buenos aires )

"Analysis of transient properties in Dose-Response functions"

Abstract: 

Sensing extracellular changes initiates signal transduction and is the first
stage of cellular decision-making. Ligand binding to cell membrane
receptors is a key event in those sensing stages. It is rarely certain
whether cellular responses are related to initial changes in receptor
binding or to the level of receptor binding achieved at some later time,
but it is likely that the dynamics of receptor/ligand binding
contributes significantly to the dynamics of the response. Particularly,
certain properties of the sensing steps are usually characterized in
equilibrium, like the value of half-maximal effective concentration, the
dynamic range, and the Hill coefficient. However, if the time constant
of downstream signal transduction steps is shorter than that of
ligand-receptor binding, those properties should be evaluated in
pre-equilibrium. We explored how the main features of these properties,
are being limited when considering different signaling topologies. Two
models of receptor activation, a covalent modification cycle, and a
transcriptional model are considered here, we explored their specific
features and also the effect when we couple some of these modules.

Our results imply that pre-equilibrium sensing is possible depending on the
relation of binding and activation rates. When binding rates are slower
than activation rates, the system can sense high dose concentrations on
pre-equilibrium. Conversely, when binding is faster than activation,
pre-equilibrium sensing properties remains similar than steady state
properties. Moreover, when the time scales are similar, pre-equilibrium
sensing is possible but with certain limitations, depending on the time
constant and the ligand concentration involved on the downstream
process.

Vendredi 13 octobre 2017 à 11 h (salle 1 du LJAD)

M. Cencini (ISC-CNR, Rome)

"Facilitated Diffusion of Transcription Factors: role of the genetic background

assessed by stochastic simulations over real energy landscapes"

Transcription factors are proteins able to bind specific sites of DNA

such as to promote or inhibit DNA transcription.  Such proteins are

able to associate to their target sites faster than the physical limit

posed by diffusion. Such high association rates can be achieved by

alternating between three-dimensional diffusion and one- dimensional

sliding along the DNA chain, a mechanism dubbed Facilitated Diffusion

and proposed in the ’80s by  Berg and von Hippel.

In this talk I will discuss the role of the genetic background around the 

target sequences of transcription factors on the one dimensional diffusion

of the proteins along the DNA. In particular, I will

show that the binding energy landscape around the

target sequences of Escherichia coli  and of Bacillus subtilis  is

organized in a funnel-like structure. By means of an extensive computational study

of  a stochastic model for  the  sliding process along the energetic landscapes obtained

from the database I will show that the funnel can significantly

enhance the probability of transcription factors to find their target

sequences when sliding in their proximity. Such enhancement leads to a

speed-up of the association process.

Vendredi 27 octobre 2017 à 11 h (salle de séminaires du LJAD)

Jean-Baptiste Fouvry (IAS, Princeton)

"Finite-N effects and the secular evolution of self-gravitating systems"

The
dynamics of long-range interacting systems generically comprise two
phases: first a phase of (violent) collisionless relaxation, followed by
a slower phase of collisional relaxation driven by the fluctuations
remaining in the system. These fluctuations may either originate from
external disturbances, or from the intrinsic graininess of the system.
When sourced by finite-N effects, the associated long-term relaxation is
captured by the inhomogeneous Balescu-Lenard equation. I will present
this kinetic formalism, and emphasise the key mechanisms at play in this
context. In particular, I will show how one can account for the
constituents' intricate individual trajectories (inhomogeneity), as well
as the system's ability to amplify perturbations (self-gravity). I will
also review the different approaches that may be used to obtain this
kinetic equation, which all offer different insights on these
mechanisms. Finally, I will briefly present recent applications of this
new framework to investigate the long-term orbital restructuration of
astrophysical self-gravitating systems, such as stellar discs and
galactic nuclei.

Vendredi 1er décembre 2017 à 11 h (salle de réunion de Fizeau)

Harold Berjamin (Aix Marseille)

"Modélisation de la propagation d’ondes non linéaire dans les solides à dynamique lente" Le comportement mécanique de solides hétérogènes tels le grès ou le béton est fortement non linéaire. En effet, la vitesse du son mesurée dans un barreau sous sollicitation dynamique chute au court du temps, puis retrouve sa valeur initiale à l’arrêt de l’excitation. Un modèle de milieu continu a été développé afin de reproduire ces phénomènes. Il comporte une variable d’état scalaire supplémentaire, qui traduit l’amollissement du matériau. L’évolution de cette variable d’état est régie par une équation thermodynamiquement admissible. Une méthode numérique de type volumes finis a été construite pour résoudre les équations du mouvement. Qualitativement, les résultats numériques sont en accord avec des résultats expérimentaux d’acousto-élasticité dynamique et de résonance non linéaire.

Vendredi 8 décembre 2017 à 11 h (salle de réunion de Fizeau)

Simon Thalabard (Oca Nice)

"Turbulent dispersion from a point-source: Insights from Markovian models."

Modeling how particles separate in a turbulent environment is a long-standing problem, that can be traced back to Richardson’s seminal description of the dispersion of particles clusters in terms of a scale-dependent diffusivity. One example of a practical outcome is to determine how localized injection statistics translate into large scale inhomogeneities for the concentration field, whose spatial patterns stem from an intricate interplay between the source statistics and the Lagrangian statistics of the turbulent field. In this talk, I will highlight this interplay in the idealized ``point-source’’ setting, using simple Markovian models for the turbulent velocity field, such as the Kraichnan model and variations thereof. While unrealistic, those models provide an insightful framework to analyze genuine turbulent datasets, as obtained from direct numerical simulations. If time permits, I will then discuss various ideas that can be used to incorporate Non-Markovian effects, in order to better capture the statistics of extreme Lagrangian events.

Vendredi 15 décembre 2017

Relâche pour cause de la conférence en honneur à Pierre Coullet, les 14 et 15 décembre.

Mardi 23 janvier  2018 à 14 heures en salle I

(seminaire commun avec l'équipe GAD)

Martin Krupa (LJAD)

"The blow-up method in singular perturbation theory and secondary canard solutions"

In
this talk, I will describe how the blow-up method can be applied to
desingularize the dynamics near non-hyperbolic points in singular
perturbation problems. Subsequently, I will briefly review the classical
canard phenomenon and define canard solutions in systems with two or
more slow dimensions, by generalizing canard cycles. Such generalized
canards can be loosely classified into primary and secondary type and
primary canard solutions are typically much easier to find. I will focus
on secondary canard solutions and discuss how they can be found using
the blow-up approach.

Lundi 5 février  2018 à 14 heures en salle de conférences

(seminaire commun avec l'équipe ATG)

Edgardo Brigatti (Rio de Janeiro)

"The advantages of a limited attention for the cohesion and order of swarms"

We
introduce a multi-agent model for exploring how selection of neighbours
determines some aspects of order and cohesion in swarms. The model
algorithm states that every agents’ motion seeks for an optimal distance
from the nearest topological neighbour encompassed in a limited
attention field. Despite the great simplicity of the implementation,
varying the amplitude of the attention landscape, swarms pass from
cohesive and regular structures towards fragmented and irregular
configurations. Interestingly, this movement rule is an ideal candidate
for implementing the selfish herd hypothesis which explains aggregation
of alarmed group of social animals.

Jeudi 8 février  2018 à 14 heures en salle I

(seminaire commun avec l'équipe ATG)

Clément Tauber (ETH Zürich)

"Bulk-edge correspondence for Floquet topological insulators"

Floquet topological insulators describe independent electrons on a lattice driven out of
equilibrium by a time-periodic Hamiltonian, beyond the usual adiabatic approximation.
In dimension two such systems are characterized by integer-valued topological indices
associated to the unitary propagator, alternatively in the bulk or at the edge of a sample.
In this talk I will give new definitions of the two indices, relying neither on translation
invariance nor on averaging, and show that they are equal. In particular weak disorder
and defects are intrinsically taken into account. Finally indices can be defined when two
driven sample are placed next to one another either in space or in time, and then shown to
be equal. The edge index is interpreted as a quantized pumping occurring at the interface
with an effective vacuum.

Vendredi  30  mars 2018 à 11 h (salle de séminaires du LJAD)

Jéremie Cabessa (Paris 2)

"Finite state machine computation with bio-inspired neural networks "

First, we present important theoretical results concerning the
computational and expressive power of recurrent neural networks. We
show that analog and evolving networks constitute a natural model for
oracle-based computation, beyond the Turing limits. Then, we present a
 bio-inspired neural architecture based on the concept of "synfire
rings". We show that finite state automata can be simulated by various
 biological models of neural networks composed of synfire rings.
Finally, we discuss the extension of these results to the case of
Turing machines. Overall, these considerations show that a
bio-inspired neural paradigm of abstract computation is possible, and
potentially harnessable. They might constitute a very first step
towards the implementation of biological neural computers.

Vendredi  18  mai 2018 à 15 h (salle de conf du LJAD)

Jason Picardo (ICTS, Bangalore)

 "Mixing in microchannels via chaotic advection and interfacial instability"

Microchannels offer several advantages, such as portability, control and safety, for bio-
chemical processing, waste treatment and medical diagnostics. On the other hand, mixing
and mass transfer is relatively slow because the the flow is almost always laminar in these
channels. One particularly effective strategy for improving mixing in laminar flows is to
introduce variations in the flow field along the channel that result in lagrangian chaos.
While several designs have been proposed to achieve this, they almost always require
complex, out of plane modifications to the channel geometry. In this talk, I will present
two strategies for inducing chaotic advection in planar channels. The first technique
utilizes the effect that wall hydrophobicity can have on microflows [1], while the second
technique makes use of electro-osmotic flows [2]. The extent of mixing is characterized
via different lagrangian measures, which are shown to be in reasonable correspondence
with recently proposed eulerian indicators of mixing. I will then shift focus to layered
two-phase flows, and show how a surfactant driven interfacial instability may hold the key
to improving inter-phase mass transfer, without disrupting the integrity of the inter-fluid
interface [3]. This newly identified instability is based on the nonlinear coupling between
advection-diffusion of a soluble surfactant and surface tension driven flows.
[1] P Garg, JR Picardo, S Pushpavanam, 2015, Phys. Fluids 27 (3), 032004.
[2] T Krishnaveni, T Renganathan, JR Picardo, S Pushpavanam, 2017, Phys. Rev. E 96 (3),
033117.
[3] JR Picardo, TG Radhakrishna, S Pushpavanam, 2016, J. Fluid Mech. 793, 280-315.

Vendredi  25  mai 2018 à 11 h (salle de conf du LJAD)

Thibaud Taillefumier (Austin)

In principle, microbes can divide tasks to reap the benefits of
specialization, as in human economies. However, the benefits and
stability of an economy of microbial specialists are far from
obvious. Here, we mathematically model the population dynamics of
bacteria that compete for steadily supplied resources. We find that
population dynamics generally leads to the coexistence of different
metabolic types, which satisfy an extended competitive exclusion
principle (even allowing for adaptive mutation). We establish that
these consortia act as cartels, whereby population dynamics pins
down resource concentrations at values for which no other strategy
can invade.
 

Mercredi  6  Juin 2018 à 11 h (salle de conf du LJAD)

 Mihir Durve (Department of Physics of the University of Trieste) "Collective Behavior in Active Systems and the Vicsek Model"

Collective behavior is seen in many natural and artificial systems. The
flock of birds, the school of fish, growth of the bacterial colony on a
petri dish, human crowd are examples of natural systems. Agents in
such systems have ability to show persistent motion. The class of
such system is also called `active systems' and the agents in such
systems are called `Self-propelled particles'[1]. Particularly in
biological systems the collective behavior is essential for survival of
the species. For example, the study shows that a group of fish can
coordinate and find food more efficiently than an individual
fish.

In this talk we shall discuss the examples of active
systems and the computational approach to study the collective
behavior in such active systems. There are various models that are
proposed to understand the mechanism for the onset of the
observed collective motion. We shall discuss one such model, now known
as the `Vicsek model'. The model assumes that the agent in such
systems try to follow it's neighbors[2]. With this simple but yet
effective assumption this model showed some of the prominent
features of the collective behavior. We shall also discuss the
effect of anisotropic, non-reciprocal interactions, delay in response to
change in the neighbourhood among the agents in such systems
within the scope of the Vicsek model and its effects on the collective behavior.

Ref.:
[1] T. Vicsek and A. Zafeiris, Phys. Rep. 517, 71 (2012).
[2] T. Vicsek, A. Czir ok, E. Ben-Jacob, I. Cohen, and O. Shochet, Phys. Rev. Lett. 75, 1226 (1995).

Lundi  5  Novembre 2018 à 11 h (salle de conf du LJAD)

Boris Gutkin (Paris/Moscou)

'Intrinsic and Synaptic Properties Determine Macroscopic Phase Response
Curves and Coherence States of Inter-communicating Gamma Oscillatory
Neural Circuits'

Abstract: Macroscopic oscillations of different brain regions show
multiple phase relations that are persistent across time [5]. Such phase
locking is believed to be implicated in a number of cognitive functions
and is key to the so-called Communication Through Coherence (CTC) theory
for neural information transfer [7]. The question is then to identify
the biophysical neuronal and synaptic properties that permit such motifs
to arise.

We investigate the dynamical emergence of phase locking within two
bidirectionally delayed-coupled spiking circuits with emergent gamma
band oscillations. Internally the circuits consist of excitatory and
inhibitory cells coupled synaptically in an all-to-all fashion and
parameterized to give either the pyramidal-interneuron (PING) or
interneuron gamma (ING) rhythm activity to emerge. Multiple circuits can
also be intercoupled together with reciprocal synaptic connections with
variable delays and targeting excitatory and/or inhibitory neurons.

Using mean-field approach combined with an exact reduction method [3,8],
we break down the description of each spiking network into a low
dimensional nonlinear system. The adjoint method can be applied to
derive semi-analytical expressions for the macroscopic infinitesimal
phase resetting-curves (mPRCs) [2,4]. Using the mPRCs we determine how
the phase of the global oscillation responds to incoming perturbations.
In fact, we find that depending on wether PING or ING is expressed and
wether the excitatory of inhibitory neurons are perturbed, the mPRC can
be either class I (purely positive) or class II (by-phasic). Hence we
show analytically how incoming excitation can either promote spiking
(advancing the phase) or retard the oscillation.

We further study the dynamical emergence of macroscopic phase locking of
two bidirectionally delayed-coupled spiking networks within the
framework of weakly coupled oscillators [1,9]. Using the  weak coupling
ansatz we  abbreviate the bidirectionally coupled circuits description
into a phase equation [1,9]. An analysis of the phase equation shows
that the delay is a necessary condition to get a symmetry breaking. We
find that a whole host of phase-locking relationships can exist,
depending on the coupling strength and delay. Our analysis further
allows us to track how the perturbation of on the interacting circuits
is transferred to its coupled partner. We find that such transfer is
phase-locking state dependent. We show that this transfer of signals can
be directional in the symmetrically coupled network.

[1] Ashwin, P., Coombes, S., Nicks, R.: Mathematical frameworks for
oscillatory network dynamics in neuroscience. The Journal of
Mathematical Neuroscience 6(1), 2 (2016).

[2] Brown, E., Moehlis, J., Holmes, P.: On the phase reduction and
response dynamics of neural oscillator populations. Neural Computation
16(4), 673{715 (2004).

[3] Deco, G., Jirsa, V.K., Robinson, P.A., Breakspear, M., Friston, K.:
The dynamic brain: From spiking neurons to neural masses and cortical
fields. PLoS Comput Biol 4(8), 1{35 (2008). DOI
10.1371/journal.pcbi.1000092. Neural Computation 25(12), 3207{3234
(2013).

[4] Dumont, G., Ermentrout, G.B., Gutkin, B.: Macroscopic
phase-resetting curves for spiking neural networks. Phys. Rev. E 96,
042,311 (2017).

[5] Fries, P.: Neuronal gamma-band synchronization as a fundamental
process in cortical computation. Annual review of neuroscience 32,
209–224 (2009)

[7] Maris, E., Fries, P., van Ede, F.: Diverse phase relations among
neuronal rhythms and their potential function. Trends in Neurosciences
39(2), 86{99. DOI 10.1016/j.tins.2015.12.004.

[8] Montbrio, E., Pazo, D., Roxin, A.: Macroscopic description for
networks of spiking neurons. Phys. Rev. X 5, 021,028 (2015).

[9] Nakao, H.: Phase reduction approach to synchronisation of nonlinear
oscillators. Contemporary Physics 57(2), 188{214 (2016).

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Mercredi  7  Novembre 2018 à 11 h (salle III du LJAD)

Yohann Scribano  (Montpellier)

"Quantum trajectories approach for low-temperature molecular reactive processes"

Based on the de Broglie-Bohm formulation of quantum mechanics [1], quantum trajectories have seen a growing
interest in the chemical dynamics community over the past ten years or so [2]. As the name suggests, quantum
trajectories (QTs),are able to capture quantum dynamical effects—tunneling, in particular—which are known or
suspected to be important for many processes of current interest: tunneling effect in reactive processes (in gas or
condensed phases), non-adiabatic effects, quantum coherence, proton transfers in biochemistry, . . . .
Originally considered an interpretative tool for quantum mechanics, QTs have been recently rediscovered as a
computational method for doing quantum reaction dynamics [2]. Traditionally, analytic quantum trajectories, often
used as an interpretative tool, were extracted from conventional wave packets. In the more recent synthetic
quantum trajectory methods, both the trajectories and the wave function are computed on the fly, each affecting the
propagation of the other [2].
We recently investigate the most recent formulation of QTs proposed by Bill Poirier and coworkers [3, 4],
which regards the trajectory ensemble itself as the fundamental quantum state entity, rather than the wavefunction.
Similarly to classical trajectories, QTs obey Hamiltonian equations of motion, albeit special ones. The resultant
quantum trajectory simulation scheme so obtained is identical to a classical trajectory simulation, apart from the
addition of one extra “quantum” coordinate. Using standard techniques to integrate the equations of motion,
quantum trajectory simulations is indeed capable of providing accurate quantum dynamical information, but with
the same ease-use and computational effort as classical trajectory simulation. In this QTs formulation, a 4th-order
Newtonian-like ordinary differential equation (ODE) was derived that describes 1D stationary scattering states
exactly, solely in terms of quantum trajectories. The concept of those QTs will be presented and illustrated by our
application for a 1D Eckart barrier system [5] as well as its application in a capture model of the cold and ultra-cold
Li + CaH reaction [6]. Some perpectives will also presented on the way to perform quantum-classical trajectories
simulations for chemical reaction involving many degree of freedom (high dimensional reaction dynamics).
References
[1] D. Bohm, Phys.Rev. 85, 85.
[2] R. E. Wyatt, Quantum Dynamics with Trajectories (Springer, New York, 2005).
[3] B. Poirier, Chem. Phys. 370, 4 (2010).
[4] J. Schiff and B. Poirier. J. Chem. Phys., 136, 031102 (2012).
[5] G. Parlant, Y.-C. Ou, K. Park, and B. Poirier, Comp. Theor. Chem. 990, 3 (2012).
[6] Y. Scribano, G. Parlant, and B. Poirier, J. Chem. Phys. 149, 021101 (2018).

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Vendredi 30 novembre 2018 à 11 heures en salle de Conférences du LJAD

Daniele Avitabile (Nottingham)

"Analysing waves and bumps with interfacial dynamics: spatio-temporal canards and
localised spatiotemporal chaos in neural networks"

We will discuss level-set based approaches to study the existence and
bifurcation structure of spatio-temporal patterns in biological neural
networks. Using this framework, which extends previous ideas in the
study of neural field models, we study the first example of canards in
an infinite-dimensional dynamical system, and we give a novel
characterisation of localised structures, informally called “bumps”,
supported by spiking neural networks.

Vendredi 11 janvier 2019 à 11 heures en salle 2 du LJAD

Giovanni Manfredi (Strasbourg)

"Self-gravitating systems: quantum and classical"

In recent years, there has been a renewal of interest in the set of nonlinear equations known as the
Schrödinger–Newton equations (SNEs). These consist of the ordinary Schrödinger equation endowed with
a gravitational potential that is obtained self-consistently from Poisson’s equation. The source term in
Poisson’s equation is provided by a matter density
ρ ( r , t )= | ψ |^2 that is proportional to the probability
density as given by the wave function ψ (r , t) .
The SNEs were originally put forward by Diosi [1] and Penrose [2] as a fundamental modification of
quantum mechanics for massy objects. The SNE equations may also represent the weak-field limit of
semiclassical gravity [3]. Finally, in an astrophysical context, they were used to study self-gravitating
objects (boson stars) [4] or to describe dark matter by means of a scalar field [5].
Here, I will first revisit the SNEs using a variational method [6]. With this approach, one can arrive at a
single ODE that describes the evolution of the width of the mass density. This method reproduces all the
results for the ground state and linear dynamics derived previously. In addition, it is not restricted to linear
theory and can be used to investigate nonlinear oscillations or the long-time dynamics.
Second, I will present a general-relativistic extension of the SNEs. The strategy adopted here is to first
replace Poisson’s equation with the equations of gravitoelectromagnetism (GEM). These are a linearized
approximation of the Einstein equations, which is formally almost identical to Maxwell’s equations of
ordinary electromagnetism. Next, we derive the nonrelativistic limit of the GEM equations using the
methods developed in [6]. The resulting equations can be coupled to the Schrödinger equation to obtain a
new set of SNEs augmented by a gravitomagnetic field. Such modified SNEs, though still Galilei covariant,
incorporate all three fundamental constants (G, h, c) and can be conveniently expressed in Planck’s units.
Finally, I will discuss some recent results on a class of idealized 1D models for cosmological applications.
Most existing results rely on N-body simulations, which solve the equations of motion of many particles
interacting via Newton’s force. N-body simulations show the formation of a hierarchical structure [7] that
suggests a fractal distribution of mass. The analyses are robust for high-density regions, but noisy in the
low-density ones. To gain a better insight, we propose to use a continuous probability distribution in the
phase space, which evolves according to the Vlasov-Poisson equations. As the entire phase space is
covered with a uniform mesh, regions of high and low density are sampled with equal precision and the
numerical noise remains low. Here, I will present the results of these Vlasov-Poisson simulations [8].
[1] Diosi, L.: Phys. Lett. A 105, 199 (1984)
[2] Penrose, R.: Gen. Relativ. Gravit. 28, 581 (1996)
[3] Carlip, S.: Class. Quantum Gravit. 25, 154010 (2008)
[4] Schunck, F.E., Mielke, E.W.: Class. Quantum Gravit. 20, R301 (2003)
[5] Guzman, F.S., Urena-Lopez, L.A.: Phys. Rev. D 68, 024023 (2003)
[6] Manfredi, G.: Eur. J. Phys. 34, 859 (2013)
[7] B. N. Miller and J.-L. Rouet. J. Stat. Mech. P12028 (2010); Phys. Rev. E 82, 066203 (2010).
[8] G. Manfredi, J.-L. Rouet, B. Miller, and Y. Shiozawa, Phys. Rev. E 93, 042211 (2016).

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Vendredi 25 janvier 2019 à 14 heures en salle 2 du LJAD

Marco Martins Afonso (Centro de Matemática da Universidade do Porto)

"EFFECTIVE TRANSPORT OF INERTIAL PARTICLES IN FLUID FLOWS"

We study how an imposed fluid flow - laminar or turbulent - modifies
the transport properties of inertial particles (e.g. aerosols,
droplets or bubbles), namely their terminal velocity, effective
diffusivity, and concentration following a point-source emission.
Such quantities are investigated by means of analytical and
numerical computations, as functions of the control parameters of
both flow and particle; i.e., density ratio, inertia, Brownian
diffusivity, gravity (or other external forces), turbulence
intensity, compressibility degree, space dimension, and
geometric\temporal properties.

The complex interplay between these parameters leads to the
following conclusion of interest in the realm of applications: any
attempt to model dispersion and sedimentation processes - or
equivalently the wind-driven surface transport of floaters -
cannot avoid taking into account the full details of the flow
field and of the inertial particle.

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Mardi 26 mars 2019 à 14 heures en salle de conférence du LJAD

Prof. Katsuhiko Sato (Hokkaido Univ.)

"Left-right asymmetric cell intercalation drives directional collective cell movement in epithelial morphogenesis"

"Multiplexing information flow through dynamic signalling systems"

Recent work has suggested that in dynamic cellular signalling systems
like NF-κB and ERK the mutual information between input and response is
relatively low and less than 2 bits. On the other hand, it is generally
believed that the NF-κB system uses the information from a large number
of input signals to regulate gene transcription of more than 500 genes
in a highly versatile way. It regulates cell fate and inflammatory
signalling in response to diverse stimuli, including viral and bacterial
pathogens, free radicals, cytokines and growth factors. I will propose a
resolution of this potential paradox by studying signal multiplexing
and its relation to decision-making. I will describe some experiments
that support this idea, discuss the underlying mathematical approach,
explain when systems can and can't multiplex, and model some aspects of
the multiplexing seen in NF-κB. As part of this I will introduce a new
mathematical method for the analysis and simulation of large stochastic
non-linear oscillating systems. This allows an analytic analysis of the
stochastic relationship between input and response and shows that for
tightly-coupled systems like those based on current models for
signalling systems, clocks, and the cell cycle this relationship is
highly constrained and non-generic.

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Mardi 7 Mai 2019 à 15 heures en salle de conf. du LJAD

Antonio Celani (ICTP, Trieste, Italie)

"Cooperative search games"

Résumé:

Cooperative search games are collective tasks where all agents share
the same goal of reaching a target in the shortest time while limiting
energy expenditure and avoiding collisions. Here we show that the
equations that characterize the optimal strategy are identical to a
long-known phenomenological model of chemotaxis, the directed motion of
microorganisms guided by chemical cues. Within this analogy, the
substance to which searchers respond acts as the memory over which
agents share information about the environment. The actions of writing,
erasing, and forgetting are equivalent to production, consumption, and
degradation of chemoattractant. The rates at which these biochemical
processes take place are tightly related to the parameters that
characterize the decision-making problem, such as learning rate, costs
for time, control, collisions and their tradeoffs, as well as the
attitude of agents toward risk. We establish a dictionary that maps
notions from decision-making theory to biophysical observables in
chemotaxis, and vice versa. Our results offer a fundamental explanation
of why search algorithms that mimic microbial chemotaxis can be very
effective and suggest how to optimize their performance.

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Mardi 11 Juin 2019 à 11 heures en salle 1 du LJAD

Rahul Pandit (Indian Institute of Science, Bangalore)

"The formation of compact objects at finite temperatures in a self-gravitating bosonic system"

Résumé:

Self-gravitating bosonic systems are considered to be strong candidates for
dark-matter halos. We study the self-gravitating Bose-Einstein condensate
in the three-dimensional (3D) Gross-Pitaevskii-Poisson Equations (GPPE)
by using a pseudospectral method. We carry out extensive direct numerical
simulations (DNSs), with conserved mass, energy, and momentum (and
2/3 dealiasing), of the Fourier-truncated 3D GPPE to study the formation
of finite-temperature compact objects in this system. In particular, the
Fourier truncation allows us to explore the properties of compact object
here at a finite temperature. We can also get the final states of the GPPE, in
the large-time limit, by fine-tuning the temperature in the Stochastic
Ginzburg-Landau-Poisson equation (SGLPE). We calculate the radius of
the asymptotic compact object, for different values of short-range
interaction between bosons, and its mass at both zero temperature and
finite temperature. We show that suitable initial conditions can also lead to
the formation of a rotating binary star in the GPPE.

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Vendredi 14 Juin 2019 à 11 heures en salle de séminaire Fizeau

Pascal Viot (Sorbonne Université)

"Multi-channel particulate transport with blockage: Non-Markovian versus Markovian models"

Résumé:

We consider particulate flows through confined geometries, ranging from a single
channel to a bundle of $N_c$ identical coupled channels, under conditions of reversible blockage:

Particles enter randomly the channel, but the channel capacity is
limited: when $N$ particles are simultaneously in the channel, the
particulate transport is interrupted and the channel becomes blocked .
After a finite duration $\tau_b$, a reset of the channel is possible and
all particles are released.

When the particle velocity is constant, we build non Markovian
models, where analytic solutions for the stationary properties of a
single channel with capacity $N\le 3$ and for a bundle of channels each of capacity $N = 1$. For
larger values of $N$ and $N_c$, the system's steady state behavior is explored by numerical
simulation. Depending on the deblocking time, the exiting flux has a non trivial dependence on the intensity.

Inspired by the Queuning, we then consider Markovian models which give similar behaviors and enhights

the numerical results for the non Markovian models where analytical solution cannot be obtained.

We finally compare the relative efficiency of coupled and uncoupled bundles.

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Vendredi 21 Juin 2019 à 11 heures en salle de Conférences du LJAD

Isabelle Cossette (Centre interdisciplinaire de recherche en musique, médias et technologie, École de musique Schulich, Université McGill, Montréal, Québec, Canada).

" Recherche sans frontière @ CIRMMT "

Résumé: