Martin Gueuning (UNamur)
2019-02-14 13:00 - 14:00 (Europe/Paris) | E25
Title: Information Diffusion on Twitter: exploiting the timings of Retweets
Abstract: Online social networks such as Twitter have emerged as influential platforms for real-time information diffusion. Due to various factors, a tweet may become viral and initiate a cascade of retweets. In this work, we first show how the timings of the retweets reveal information about how the tweet propagated on the underlying network. In particular, we show under which circumstances the transition of the tweet from one group to another on Twitter leaves fingerprints on its retweets timings [1]. Then, we leverage these findings to address the question of multi-targets selection for maximal information diffusion. We present SmartInf, an algorithm providing a list of influential users of the network only based on an historic of popular cascades, or refining the list when the ego networks of the users may be extracted. We illustrate the performance of SmartInf against the baselines on both real-world and simulated data [2] .
[1] Temporal patterns of (re)tweets reveal cascade migration. AK Bhowmick, M Gueuning, JC Delvenne, R Lambiotte, B Mitra. Proceedings of the 2017 IEEE/ACM International Conference on Advances in in Social Networks Analysis and Mining.
[2] Temporal patterns of (re)tweets help to detect influential nodes for information diffusion in social networks. AK Bhowmick, M Gueuning, JC Delvenne, R Lambiotte, B Mitra. Submitted.

naXys board meeting
2019-02-15 12:00 - 14:00

Ines Wilms (Maastricht University, Netherlands)
2019-03-07 13:00 - 14:00 | E25
Title: Multi-class Vector AutoRegressive Models for Multi-Market Commodity Data
Abstract: Vector AutoRegressive (VAR) models form a special case of multivariate regression models in that the response variables are observed over time and modeled as a function of their own past values. We use VAR models to study dynamics among agricultural, metal and energy commodity returns. As the increasing integration of the world economy suggests commodity dynamics to be comparable for different markets, we aim to jointly analyze these dynamics across markets. To this end, we introduce a sparse estimator of the Multi-Class Vector AutoRegressive model. We jointly estimate several VAR models, one for each market (“class”), to borrow strength across markets. Our methodology encourages effects to be similar across markets, while still allowing for small differences between them. Moreover, we focus on multi-class estimation of high-dimensional VAR models, i.e. models with a large number of time series relative to the time series length. Therefore, our estimate is sparse: unimportant effects are estimated as exactly zero, which facilitates the interpretation of the results.
This is joint work with Luca Barbaglia and Christophe Croux

Anna Kiriliouk (UNamur)
2019-03-12 13:00 - 14:00 | E25
Title: Climate event attribution using multivariate peaks-over-thresholds modelling
Abstract: Quantifying the human influence on climate change and identifying potential causes is a highly relevant research area which is often referred to as detection and attribution. A common approach is to compare the probability of an extreme event in the factual world to the probability of an extreme event in a counterfactual world, i.e., a world that might have been if no humans would have existed. The event probabilities can be calculated using large scale climate model runs that simulate the evolution of the climate with and without anthropogenic forcings. The Fraction of Attributable Risk (FAR) is defined as the relative ratio of event probabilities in the factual and in the counterfactual world. Estimating the FAR will allow us to quantify the extent to which human activities have increased the risk of occurrence of an extreme event. We propose a model for the FAR based on the multivariate generalized Pareto distribution, i.e., the asymptotic distribution of suitably normalized exceedances over a high threshold. The model is used to quantify the increased risk of an extreme rainfall event in central Europe.

Making the most of your presentation by Principiae
2019-03-22 14:00 - 16:30 | E13, 8 Rempart de la Vierge, Namur, Belgium
Abstract: Strong presentation skills are a key to success for researchers, yet many speakers are at a loss to tackle the task. Systematic as they otherwise can be in their work, they go at it intuitively or haphazardly, with much good will but seldom good results. In this talk, Dr Doumont proposes a systematic way to prepare and deliver an oral presentation: he covers structure, slides, and delivery, as well as stage fright.

Speaker: An engineer (UCLouvain) and PhD in applied physics (Stanford), Jean-luc Doumont is acclaimed worldwide for his no-nonsense approach, his highly applicable, often life-changing recommendations on a wide range of topics, and Trees, maps, and theorems, his book about “effective communication for rational minds.”
For additional information, visit www.principiae.be.
A drink will follow the conference.

Registration
Registration is free but mandatory through the following web form.
PhD students will receive a certificate with ECTS mention for their doctoral training.

Gyuri Barabas (University of Linköping, Sweden)
2019-04-04 13:00 - 14:00 | E25
Title: The evolution of trait variance creates a tension between species and functional diversity

Abstract: It seems intuitively obvious that species diversity begets functional (trait) diversity: the more plant species there are, the more varied their leaf chemistry will be; more species of crops provide more kinds of food; etc. In this talk I argue that the evolution of trait variance challenges this view. In a trait-based eco-evolutionary model, I show that when species richness is low, individual species evolve large trait variation, while in species-rich communities species avoid competition by evolving narrow trait breadths. This effect can be so strong that those communities with more but narrower species will cover a smaller fraction of the available trait space than those with fewer but intraspecifically more variable ones. The expected positive relationship between species and functional diversity is thus overhauled, a result which proved extremely robust to changes in model setup and parameterization. I finish by presenting and discussing empirical data consistent with this claim.

Liubov Tupikina (CRI, Paris Descartes, France & MIPT, Moscow, Russia)
2019-04-09 13:00 - 14:00 | E25
Title: Heterogeneous spreading in network models
Abstract: I will present some of our recent results on the random walks on networks with spatial and temporal heterogeneity. In particular, I will discuss how the influence of heterogeneities can be encoded into the dynamics of continuous time random walk model. Continuous limits of heterogeneous continuous time random walks can lead to various diffusion equations, which opens new questions about general description of spreading in heterogeneous media. Then I will talk about some applications of the heterogeneous random walk model to the epidemiological models on static or temporal networks and some potential applications to the real data.

Emmanuel Flachaire (Aix-Marseille University, France)
2019-04-23 13:00 - 14:00 | E25
Title: Pareto Models for Top Incomes
Authors: Arthur Charpentier (UQAM) and Emmanuel Flachaire (Aix-Marseille University)
Abstract: Top incomes are often related to Pareto distribution. To date, economists have mostly used Pareto Type I distribution to model the upper tail of income and wealth distribution. It is a parametric distribution, with an attractive property, that can be easily linked to economic theory. In this paper, we first show that modelling top incomes with Pareto Type I distribution can lead to severe over-estimation of inequality, even with millions of observations. To overcome this problem, we then show that other Pareto distributions should be used. Two applications illustrate these characteristics, with the distribution of income in South Africa in 2012 and the distribution of wealth in the United States in 2013.

Oscar Godoy (University of Cádiz, Spain)
2019-04-25 13:00 - 14:00 | E25
Title: Functional and multitrophic mechanisms of species coexistence
Abstract: Ecologists have been investigating for decades the mechanisms that maintain the diversity of species within ecological communities. In particular, several functional traits and interactions with mutualists have been thought to promote the stabilizing effect of niche differences on population dynamics of competing species. These effects occur because higher intraspecific compared to interspecific competition limits species abundance when they are common yet buffer them again extinction when they are rare. Recent advances in coexistence theory have now the opportunity to rigorously evaluate such relationships by coupling experimental and observational approaches to species population models, and surprisingly, they are showing a much more complex mapping that previously thought. As an example, most of the plant functional traits widely measured in the literature does not provide a demographic signature of niche differences, instead, they promote competitive imbalances between species that lead to competitive exclusion. Only traits and phenotypic plasticity related to light and water use efficiency promote niche partitioning between species. In a similar fashion, multitrophic interactions that plant species establish with floral visitors are an equalising rather than a stabilizing, which promote species coexistence by decreasing the competitive ability of superior competitors and increase that of inferior competitors. Although the examples I will provide are mostly for plant species, the underlying ecological mechanisms are common to any taxa type, so I expect a fruitful discussion of how these ideas and experiments can be extended to other ecological systems.

Joni Virta (Aalto University, Helsinki, Finland)
2019-04-30 14:00 - 15:00 | E25
Title: Statistical properties of second-order tensor decompositions
Authors: Joni Virta, Christoph Kösner, Niko Lietzén, Klaus Nordhausen
Abstract: Two classical tensor decompositions are considered from a statistical viewpoint: the Tucker decomposition and the higher order singular value decomposition (HOSVD). Both decompositions are shown to be consistent estimators of the parameters of a certain noisy latent variable model. The decompositions’ asymptotic properties allow comparisons between them. Also inference for the true latent dimension is discussed. The theory is illustrated with examples.

Nathanaël Kasoro Mulenda (University of Kinshasa, DRC)
2019-05-02 13:00 - 14:00 | E25
Title: PercoMCV: A hybrid approach of community detection in social networks
Abstract: Knowledge extraction in social networks is a needful tool as it touches every aspect of our lives such as politic, socio-economic, scientific, etc. Community detection is one of the objectives of this specific tool used for knowledge extraction in social networks.
Many algorithms of knowledge extraction from social networks have been developed these last years. However, many of them are not constant, effective and accurate when facing these social networks with many edges.
In this paper, we propose a new approach of community detection in social networks with many links between communities. The proposed approach has two steps. In the first step, the algorithm attempts to determine all communities that the clique percolation algorithm may find. In the second step, the algorithm computes the Eigenvector Centrality method on the output of the first step in order to measure the influence of network nodes and reduce the rate of the unclassified nodes.
To assess this new approach, we test it on different types of networks. Relevant communities that have been detected testifies effectiveness and performance of the approach over other community detection algorithms.

Debmalya Das (Harish-Chandra Research Institute, Allahabad, India)
2019-05-07 13:00 - 14:00 | E25
Title: The Quantum Chesire Cat: some new aspects and possibilities
Abstract: Quantum Chesire Cat is a counterintuitive phenomenon that provides a new window into the nature of the quantum systems in relation to multiple degrees of freedom associated with a single physical entity. Under suitable pre and postselections, a photon (the cat) can be decoupled from its circular polarization (its grin). In this talk, we will explore whether the grin without the cat can be teleported to a distant location. This will be a totally disembodied teleportation protocol. Based on the original Quantum Chesire Cat setup, we design a protocol where the circular polarization is successfully teleported between two spatially separated parties even though the photon is not physically present with them. The process raises questions in our understanding about properties of quantum system. We further demonstrate this by swapping the grins of two Quantum Chesire Cats. We also briefly discuss about a few interesting new aspects of the original Quantum Chesire Cat protocol.

Maxime Lucas (Lancaster University, UK & University of Florence, Italy)
2019-05-16 13:00 - 14:00 | E25
Title: Network synchronisation with time-varying parameters

Abstract: Many systems in nature, especially living systems, can be modelled as a collection of interacting oscillators. Such systems can exhibit a plethora of behaviours, amongst which synchronisation, a state of maximal coherence, is of crucial importance. Indeed, the synchronisation of the beat of heart cells is as necessary as the synchronisation of neurons can be a bad pre-seizure sign. So far, ensembles of oscillators have been studied mostly as closed systems so that all parameters such as coupling or frequencies are constant. Very often, however, living systems are open and under the non-negligible influence of their environment, which is in general varying with time. This in turn influence internal parameters of the systems: the frequency of our heartbeat is far from constant, for example. Here, we include time-variability in our mode, and see the effect it has on the dynamics and its stability.

Here, we will see how time-variability can benefit or reduce stability in two scenarios: a temporal network (time-varying topology), and a network driven by an external oscillator with a time-varying frequency. We will discuss how, in the first case, synchronization is negatively affected, but that the region of synchronization is enlarged in the second case. Moreover, we will discuss the importance of looking into the dynamics of such systems at both short and long timescales. Time-varying features in models of living systems can bring us closer to realistic models, and allow us to reveal richer dynamics resulting from it.

Pauline Thémans (UNamur)
2019-05-23 13:00 - 14:00 | E25
Title: Pharmacokinetic modeling and strategies for drug dosing recommendation: application to meropenem
Abstract: Pharmacokinetics (PK) studies the link between the dose and the drug exposure in patients. Thanks to mathematical modeling, clinical pharmacology is an interesting and promising field of application of control and system theory. We focus on time-dependent antibiotic given by constant intravenous (IV) infusion at regular dosing interval. Meropenem is used as a particular case for which numerical results are reported.
We report on the study of two kinds of models: a population pharmacokinetic (popPK) model (empirical approach, mixed effects modeling) and a physiologically-based pharmacokinetic (PBPK) model (mechanistic approach, human physiology and drug’s properties considerations). We will describe the results of internal and external validations that were performed. Both models are described by linear time-invariant state-space models. They are shown to be nonnegative and stable, as expected.
From these models, the aim is to provide guidelines (decision-making aid) for drug dosing in patients based on relevant patient’s characteristics (covariates) and on other practical conditions (target exposure for efficacy, dosing interval, time of infusion, etc.). The studied approaches are based on open-loop methods. (1) A standard input-output analysis yields the analytical expression of the output trajectories and the asymptotic response. A formula is then derived to compute the dose needed to maintain the (steady) concentration trajectory above a given lower bound. (2) We also describe an optimal control approach on the discrete-time model (minimum principle).

Workshop Dynamical processes on networks
2019-09-05 14:00 - 18:00 | E22
14h00-15h00: Hiroya Nakao, Tokyo Institute of Technology (Japan)
Collective oscillations and synchronization in networks of coupled dynamical elements
15h00-15h40: Duccio Fanelli, Università di Firenze (Italy)
Turing-like patterns from purely reactive systems
15h40-16h20: Julien Petit, UNamur
Diffusion-based processes: from graphs to graphons
16h20-16h50: Coffee break
16h50-17h40: Alexandre Mauroy, UNamur
Operator theoretic methods for network identification
17h40-18h20: Malbor Asllani, MACSI University of Limerick (Ireland)
Emergence of non-normality in real life networked systems

Hamza Bennani (University Paris Nanterre)
2019-09-19 13:00 - 14:00 | E25
Titre: Does a Sentiment Shock Help to Predict Monetary Policy: Evidence from the ECB
Abstract: This paper aims to assess whether sentiment shock is helpful in predicting ECB monetary policy decisions. We use a bag-of-words approach and several dictionar- ies on ECB’s President introductory statements to derive a measure of sentiment. Next, we orthogonalize the sentiment measure on a set of macroeconomic and finan- cial variables to compute sentiment shock. Finally, we test whether sentiment shock is useful to predict ECB monetary policy decisions. We find that sentiment shock is significantly and positively related to future ECB policy decisions, even when controlling for future economic conditions and market expectations about mone- tary policy. Further extensions show that the predictive power of sentiment shock is robust to (i) the scoring method chosen to compute sentiment, (ii) alternative expectations about monetary policy and (iii) the macroeconomic forecasts used in the monetary policy Taylor rule. However, we find that the predictive power of sentiment shock is sensitive to the dictionaries used to compute the measure of sentiment. These findings highlight an additional channel by which ECB communication improves monetary policy predictability.

Anthony Hastir (UNamur)
2019-10-03 13:00 - 14:00 | E25
Title: On stability and control of nonlinear infinite-dimensional systems
Abstract: We consider nonlinear dynamical systems whose state evolves in an infinite-dimensional space, typically a Hilbert space. Stability properties of an equilibrium of such systems can be deduced by looking at a Gâteaux/Fréchet linearization of the nonlinear dynamics around that equilibrium. From « Al Jamal, R., Morris, K., 2018, Linearized stability of partial differential equations with application to stabilization of the Kuramoto-Sivashinsky equation, SIAM Journal on Control and Optimization 56 (1), 120–147 », it is known that, if the Gâteaux/Fréchet linearization generates an exponentially stable C 0 -semigroup that is Fréchet differentiable at the equilibrium, the nonlinear dynamics will generate a locally exponentially stable C 0 – semigroup of nonlinear operators. However, checking the Fréchet differentiability of nonlinear operators is really challenging when they are unbounded. This observation leads us to propose a weakenend definition of Fréchet differentiability by considering different norms. This allows more flexibility in the manipulation of norm inequalities, providing Fréchet differentiability conditions that can be verified more easily. Under some appropriate assumptions, we show how to get Fréchet differentiability of the nonlinear semigroup in a weaker sense in order to establish appropriate local exponential stability of the nonlinear system.
We show also that every linear and bounded perturbation added to the nonlinear dynamics preserves the Fréchet differentiability property of the nonlinear C 0 -semigroup generated by these perturbed dynamics. For instance, this property is of primary importance when the system is controlled by means of a feedback law. Indeed, it implies that every stabilizing feedback for the linearized dynamics will locally exponentially stabilize the nonlinear dynamics around an equilibrium. This allows to design a stabilizing controller on a linearized model instead of a nonlinear one.
Our results are applied to a nonisothermal axial dispersion tubular reactor model for which we consider distributed LQ-optimal regulation of the temperature.

Riccardo Muolo (UNamur)
2019-10-17 13:00 - 14:00 | E25
Title: Effects of Non-normality on Turing Instability
Abstract: Turing mechanism describes the emergence of spatial patterns in a reaction-diffusion system of two or more species: when certain conditions are matched, a perturbation starting from a homogeneous stable state guides the system towards a nonhomogeneous one, the celebrated Turing patterns, following a diffusion-driven instability [1]. The classical linear stability analysis that describes such phenomenon is based on the spectra of involved the operators. However, such analysis may fail when the involved linear operators are non-normal, due to a transient growth [2]. Such effect is even stronger when the system is studied on a non-normal network, i.e., a network whose adjacency matrix is non-normal [3]. In a recently published work [4] we have made use of such theoretical background to extend the original theory by obtaining non-normality patterns when Turing contitions are not satisfied.
In this seminar I will go through the main steps of this itinerary, from classical Turing Instability to patterns of non-normality. Firstly, I will present qualitatively the idea of Turing, then, after having introduced the notion of network, I will talk about processes of diffusion on discrete support, I will explain a recent extention of Turing Theory on symmetric network [5] and how an asymmetric topology can affect the mechanism of pattern formation [6]. Before moving to the last part, I will introduce the concept of non-normality and its effects on the dynamics [7]. Finally, I will present our extension of Turing Theory on non-normal networks [4] and discuss some open problems and possible future developments.
References
[1] Turing, A.M., 1952. The chemical basis of morphogenesis. Phil. Trans. R. Soc. B 237, 37–72;
[2] Trefethen, L.N., Embree, M., 2005. Spectra and Pseudospectra: The Behavior of Nonnormal Matrices and Operators. Princeton University Press, Princeton, NJ;
[3] Asllani, M., Lambiotte, R., Carletti, T., 2018. Structure and dynamics of non-normal networks, Sci. Adv. 4, 1–8. Eaau9403;
[4] Muolo, R., Asllani, M., Fanelli, D., Maini, P.K., Carletti, T., 2019. Patterns of non-normality in networked systems. Journal of Theoretical Biology 480 81–91;
[5] Nakao, H., Mikhailov, A.S., 2010. Turing patterns in network-organized activator-inhibitor systems. Nat. Phys. 6, 544;
[6] Asllani, M., Challenger, J.D., Pavone, F.S., Sacconi, L., Fanelli, D., 2014. The theory of pattern formation on directed networks. Nat. Comm. 5, 4517;
[7] Asllani, M., Carletti, T., 2018. Topological resilience in non-normal networked systems. Phys. Rev. E 97, 042302.

naXys Research Day 2019
2019-10-24 10:00 - 17:00 | Salle Lucien Wargnies, 41B rue du Belvédère, 5000 Namur (Salzinnes), Belgium
Annual naXys Research Day 2019
Who are we? What are our specificities?
October 24th 2019
Salle Lucien Wargnies, 41B rue du Belvédère, 5000 Namur (Salzinnes)
Program
Welcome & Introduction: 10h00-10h30: Pr. Anne-Sophie Libert
Morning session: Interactions between Institutes of Complex Systems
Invited speakers:

10h30-11h15: Pr. Diaz-Guilera, Complex Systems of Barcelona UBICS
11h15-12h00: Pr. Paul Bourgine, Complex Systems Institute of Paris IDF

Lunch: 12h00-13h00
Afternoon session: Our research in naXys
Contributed Talk of Pole ECO: 13h00-13h30: Pr. Jean-Yves Gnabo
Contributed Talk of Pole BIO: 13h30-14h00: Pr. Frederik De Laender
Flash-talks (3 minutes each): 14h00-15h00
Coffee Break: 15h00-15h30
Contributed Talk of Pole SPACE: 15h30-16h00: Pr. Anne Lemaître
Contributed Talk of Pole ROBUST: 16h00-16h30: Julien Petit
Contributed Talk of Pole OPTI: 16h30-17h00: Dr. Yves Caudano
Conclusions
Registration
Registration is free but mandatory, before Wednesday, October 15th, through the following web form.
We strongly encourage submissions of flash talks for all PhD students to present their research in 3 slides, 3 minutes. PhD students will receive certificate with ECTS mention for their participation and/or presentation.

Elias Fernández (VUB)
2019-11-21 13:00 - 14:00 | E25
Title: Uncertain times promote polarisation and reciprocation in a public goods game with risky commons
Abstract: Anthropogenic climate change, public health measures or even group hunting, are some of the many collective endeavors characterized by uncertain, long-term and non-linear returns. We operationalize these scenarios in a collective-risk dilemma, where players can invest into a public good over a number of rounds, and will only observe their payoff when the game ends. The risk of crossing a dangerous threshold is able to transform a traditional public goods game, where players incur in the well-known tragedy of commons, into a coordination game, where success depends on surpassing a coordination barrier. Behavioral experiments indicate that, when the risk of collective loss is high, slightly more than half of the experimental groups are able to coordinate and avoid the dangerous threshold. However, uncertainties over environmental variables, such as the placement of the threshold, revert the game back into a prisoner’s dilemma, decreasing group success. Here we show experimentally the effect of uncertainty about the number of rounds the game will take, i.e., how much time the players have to avoid the consequences of surpassing a dangerous threshold. Surprisingly, our results indicate that, for low levels of this timing uncertainty, not only collective success does not decrease significantly, but we observe a behavioral shift. Contrarily to what happens when there is no uncertainty, participants invest earlier and in a more polarized manner. Also, a behavioural analysis of the experimental data reveals that, under timing uncertainty, participants of successful groups tend to reciprocate in a similar fashion to the group analogous of the Tit-for-Tat strategy, where players only increase their investments if the group does the same. Such a result indicate that certain behavioral ecosystems are more successful than others in achieving the equilibrium that is socially optimum.

Yoshihiko Susuki (Osaka Prefecture University)
2019-12-05 13:00 - 14:00 | E25
Title: Koopman resolvent and Laplace-domain analysis of nonlinear autonomous dynamical systems
Abstract: The motivation of the research is to establish a Laplace-domain theory that provides principles and methodology to analyze and synthesize systems with nonlinear dynamics. A class of composition operators defined for nonlinear dynamical systems, called the Koopman operator, plays a central role in this study. In my talk, based on the characterization of such systems via spectral properties of the Koopman operator in literature, I will consider the resolvent of the Koopman operator, called Koopman resolvent, for continuous-time autonomous dynamical systems, and will discuss its expansion formulae for systems with three types of nonlinear dynamics: on-attractor evolution possibly exhibiting aperiodicity and off-attractor evolutions to stable equilibrium point and limit cycle. The Koopman resolvent is utilized for structural analysis of the systems such as location of modes (poles), which mirror the classic approach to linear autonomous systems. A computational aspect of the Laplace-domain representation will be also discussed with emphasis on non-stationary Koopman modes. The contents of my talk are joint work with Professor Alexandre Mauroy (University of Namur) and Professor Igor Mezic (University of California, Santa Barbara).

EDT FNRS Doctoral School
2020-02-04 13:00 - 16:00 | E25

INAUGURAL FRANCQUI CHAIR LECTURE. Complex systems: physics beyond physics
2020-02-04 17:00 - 18:00 | E13, 8 Rempart de la Vierge, Namur, Belgium
Guest lecturer: Pr. Duccio Fanelli
Macroscopic order from microscopic disorder:
from endogenous noise to topology driven instability
More info: https://www.unamur.be/sciences/mathematique/chairfrancqui2020

2020 international FRANCQUI Chair - Stochastic processes
2020-02-05 16:30 - 18:30 | E25
Guest lecturer: Pr. Duccio Fanelli

2020 international FRANCQUI Chair - Master equation
2020-02-06 16:30 - 18:30 | E25

William Polycarpe (IMCCE, France)
2020-02-17 11:00 - 12:00 | E25
Title: Iapetus, the key to the whole system ?
Abstract : Iapetus possesses all the attributes and characteristics of the whole system of Saturn. It is mysterious and very complex. In my doctoral study I have been runing numerical and semi-analytical simulations of a past mean motion resonance between Titan and Iapetus. The goal was to expose the consequences of a fast tidal migration for Titan and find an explanation for the eccentricity and the inclination of Iapetus’ orbit. The outcomes of simulations depend on the rate of migration and, consequently, there are constraints on the quality factor of Saturn at Titan’s frequency. The resonance allows Iapetus to pass from a circular orbit to an eccentric orbit and from its local Laplace plane to a tilted orbit. A few simulations show consistency in the orbital elements of Iapetus today.
A long semi-analytic expansion was performed to run fast simulations but alternative approaches can be taken. Inverting the order of Taylor and, eventually Fourier, expansions of the direct part of the disturbing function yields a series which is more suited to the resonance with Titan and the inclined orbit of Iapetus. More work is waiting to be done and the goal would be to obtain a universal formalism for the expansion.
The migration of Titan is becoming a reality today and, here again, further studies need to be carried out concerning, for example, the rotation of Iapetus.

Giovanni Reina (University of Sheffield, UK)
2020-02-18 13:00 - 14:00 | E25
Title: Collective Decision Making: From Bees to Robots via Multiscale Modelling
Abstract: I will give an overview of my studies on collective decision making in distributed systems of the last seven years. Such systems, found in biology, sociology, and engineering, are composed of a large number of interacting individuals that coordinate in order to reach a consensus. The main phases of the collective decision making process consist of identifying the available options, estimating their quality, and selecting the best option or any of them. I will present the main results of my research in understanding and designing each of these phases. Collective systems are inherently difficult to analyse as the stochastic nonlinear interactions between individuals can give rise to complex emergent dynamics. Therefore, I employ a collection of advanced techniques, commonly defined as multiscale modelling. Relying on a set of methods, rather than a single one, gives the benefit of having complementary techniques addressing one another’s limitations. In fact, through multiscale modelling, it is possible to analyse the systems at various levels of complexity and detail, from macroscopic group-level dynamics to microscopic individual-level behaviour, and from noise-free deterministic models to stochastic spatial descriptions. I finally shed a light on the recently developed opensource software for automated multiscale modelling.

2020 international FRANCQUI Chair – Impact of the noise
2020-02-18 16:30 - 18:30 | E25

2020 international FRANCQUI Chair – Spatially extended systems
2020-02-19 16:30 - 18:30 | E25

Jérôme Daquin (UNamur)
2020-03-12 13:00 - 14:00 | E22
Title: Aspects of the secular dynamics of terrestrial orbits
Abstract: The threat raised by space debris has vivified long-term studies of terrestrial orbits. To apprehend the motion on long time scales, the Astrodynamics community has translated, deployed and adapted many tools coming from Dynamical System theory and Celestial Mechanics: from the construction of effective averaged models more amenable to analytical and/or numerical investigations to the construction of resonant normal forms, including modern dynamical chaos indicator to visualise the template of the phase-space. Those efforts contributed to foster the `passive debris removal ideology’, where natural perturbations are ‘surfed’ rather than `counteracted’. In this talk, I will review and discuss some aspects of the long-term dynamics of terrestrial orbits, especially in the range of medium altitude. I will then present recent results obtained by the author and collaborators on the 2g+h lunisolar resonance where a (normally hyperbolic) manifold structure has been identified.
