Namur Institute For Complex Systems

Workshop Dynamical processes on networks
2019-09-05 14:00 - 18:00 (Europe/Paris) | 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.

Justin Buhendwa Nyenyezi (ISP-Bukavu, DRC) - POSTPONED
2020-03-26 13:00 - 14:00 | E25
Title: A Low-rank tensor based preconditioner for accelerating deformable 3D medical image registration
Abstract: Modeled as a variational problem, the deformable 3D image registration problem needs to solve a sequence of linear systems during the optimization process. Although these linear systems are sparse and structured, they are very large and ill conditioned. This leads to low convergence rate of the algorithms used to solve the problem or to inaccurate solutions.
Since it is observed that much of the time is spent in the solution of these linear systems, there is a need to provide efficient system solvers. Let N be the number of voxels in the image. Algorithms with linear complexity O(N ) based on fast Discrete Cosine Transforms or additive spliting operator are yet available. However, this linear complexity may be far too large for large 3D medical images. In our current study, we examine the contribution of a specific Low-rank preconditioner in a tensor-train format. We argue that, this preconditioner offers the most compromise between complexity and precision, since it allows to replace this linear
complexity by a logarithmic complexity O(log N ).
For this purpose, we first propose to use a compressed representation of data with a given accuracy ε using tensor train format. Then, within this tensor train format, we propose this low-rank preconditioner build with spectral information to speedup and stabilize the system solver. A benchmark of some registration algorithms on a large set of 3D medical images and different from their linear system solvers is provided. The benchmark relies on the perfomance profile based on the earlier decrease of the function value.

Keven Bluteau (UGhent & HEC Montréal) - POSTPONED
2020-04-02 13:00 - 14:00 | E25
Title: The Concern in Climate Change News: A Driver of Flight-to-Green Investment Episodes?
Abstract: Climate change can have a negative economic, social, and environmental impact. Investors can help in fighting climate change is buying “Green” firms and selling “Brown” firms. Several studies, however, indicate that green assets underperform brown assets, therefore incurring a cost for being a green investor. The recent asset pricing model of Pastor et al. (2020), however, suggests that green assets can outperform brown assets when concern about climate change is unusually large among investors. We empirically study this result by deriving climate change concerns indices using news articles from highly read US newspapers. We evaluate if a high value of climate change concern is associated with a higher abnormal return of a green minus brown portfolio from the S&P 500 universe between 2003 to 2017. Overall, our results indicate that high (low) climate change concern implies higher (lower) abnormal returns in a green minus brown portfolio. Furthermore, we demonstrate that not all topics surrounding the climate change discussion in the media drive concern among investors.

Philippe Naveau (LSCE, Saclay) - POSTPONED
2020-04-23 13:00 - 14:00 | E25

Carlos Ordas (Université Laval, Canada) - POSTPONED
2020-04-30 13:00 - 14:00 | E25

Principiae - POSTPONED
2020-05-14 14:00 - 16:00

Joseph O'Brien (MACSI, University of Limerick, Ireland)
2020-11-05 13:00 - 14:00 | Online Event
Title: Branching Processes and their Application to Popularity Dynamics
Abstract: Arising from a desire to understand the likelihood of a family name becoming extinct, branching processes have a rich history and have been shown to be applicable to a wide range of domains. The simplest description of these processes is the case where a parent has a random number of children during their lifetime who themselves proceed to have a similarly distributed number of children. In this talk I demonstrate how the processes may be used to describe social spreading phenomena whereby the parents are now online pieces of information (tweets, online threads, …) while their children correspond to interactions with these pieces (retweets, comments,…) and as such may represent social contagion dynamics. Two specific examples will be considered in detail, the first being a mathematical model of online spreading across multiple social media platforms and the effect said network structure has upon the dynamics [2]. Second, I will demonstrate how a branching process approach can allow a unified understanding of the well-studied Hawkes process (which has itself been shown to accurately describe social spreading features [3]) and may in fact allow one to make predictions regarding future dynamics of such processes [4].
[1] K. B. Athreya and P. E. Ney, Branching Processes (Springer Science + Business Media, New York, 2013).
[2] J. D. O’Brien, I. K. Dassios, and J. P. Gleeson. Spreading of memes on multiplex networks. New Journal of Physics, 21(2):025001, 2019.
[3] A. N. Medvedev, R. Lambiotte, and J.-C. Delvenne. The anatomy of Reddit: An overview of academic research. In Dynamics on and of Complex Networks, pages 183–204. Springer, 2017.
[4] J. D. O’Brien, A. Aleta, Y. Moreno, and J. P. Gleeson. Quantifying uncertainty in a predictive model for popularity dynamics. Physical Review E, 101(6):62311, 2020.

Link to the teams group “naXys Seminars” (unamur members) here
Link to the seminar here
For any problem with the links, write to riccardo.muolo@unamur.be

Delivering your presentation remotely by Principiae
2020-11-13 14:00 - 16:00 | Online Event
Abstract: When circumstances oblige you to deliver your presentation remotely from your office, your home, or even a hotel room, you face additional challenges, technological and otherwise. This remote lecture discusses and demonstrates the necessary adaptations to your tools, your environment, and your delivery so as to focus the attention of the audience, minimize distractions, and get your message across optimally.
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.
Registration
Registration is free but mandatory through the following web form.
PhD students will receive a certificate with ECTS mention for their doctoral training.

Benedetta Franceschiello (University of Lausanne, Switzerland)
2020-11-19 13:00 - 14:00 | Online Event
Title: MIME – Magnetic resonance Imaging of the Moving Eye
Abstract: Vision is arguably the most important of our senses and it relies on the synchronous functioning of the eyes and the brain. These organs are highly interdependent: pathologies of the eyes can impact brain functionality [1], and brain impairments affect how the visual information is encoded at the eye-level [2], [3]. While nowadays ophthalmic biomedical devices are able to extract high-resolution anatomical measurements and behavioral measurements of the eyes, no technology is able to perform anatomical assessments of the eye while it moves, yet eye-movements are a behavioral readout encompassing valuable biomarkers in brain disorders [4], [5]. Magnetic Resonance Imaging (MRI) is a particularly promising non-invasive and versatile technique because it provides measurements related both to the tissue/organ structure and to the regional neural activity. However, the image artefacts arising from eye motion prevent the applicability of MR techniques to eye imaging, therefore impeding the investigation of the interplay between anatomical structures and their motion.
In this talk I will present our patented structural MRI protocol [6], [7] that allows dynamic acquisitions of the eye while it moves during quasi-naturalistic vision. To test the efficacy of this method, eye-movements and eye-axial lengths – as extracted from the MR images – were compared with eye-tracker measurements and optical biometry, respectively. This new non-invasive technology can estimate the rotation axes from the MR images with up to 97% accuracy with respect to the eye-tracker hardware. The high-resolution MRI scans of the human eye (1 mm3) – acquired during natural movement – permit to quantify the optical axial length with an accuracy having the same order of magnitude of the one obtained with ocular biometry. Finally, I will discuss the possible applications of this technique and the new frontiers it opens both in the field of ophthalmic MRI and vision neuroscience.
[1] Zheng, D. D. et al. Longitudinal Associations Between Visual Impairment and Cognitive Functioning. JAMA Ophthalmol. 136, 989 (2018).
[2] Ciuffreda, K. J. et al. Occurrence of oculomotor dysfunctions in acquired brain injury: A retrospective analysis. Optom. – J. Am. Optom. Assoc. 78, 155–161 (2007).
[3] DeBuc, D. C. et al. Seeing the Brain Through the Eye: What Is Next for Neuroimaging and Neurology Applications. in OCT and Imaging in Central Nervous System Diseases 55–82 (Springer International Publishing, 2020). doi: 10.1007/978-3-030-26269-3_5
[4] MacAskill, M. R. & Anderson, T. J. Eye movements in neurodegenerative diseases. Curr. Opin. Neurol. 29, 61–68 (2016).
[5] Anderson, T. J. & MacAskill, M. R. Eye movements in patients with neurodegenerative disorders. Nat. Rev. Neurol. 9, 74–85 (2013).
[6] Franceschiello, B. et al. 3-Dimensional magnetic resonance imaging of the freely moving human eye. Prog. Neurobiol. 101885 (2020). doi:10.1016/j.pneurobio.2020.101885
[7] Franceschiello, B. et al. WO2020178397 – Magnetic Resonance Imaging Methods and device. (2020). Available at: https://patentscope.wipo.int/search/en/detail.jsf?docId=WO2020178397&tab=PCTBIBLIO.
Benedetta Franceschiello
Laboratory for Investigative Neurophysiology,
Ophthalmology Department, Fondation Asile des aveugles, Lausanne, Switzerland
Radiology Department, University Hospital Center and University of Lausanne, Switzerland
Link to the teams group “naXys Seminars” (unamur members) here
Link to the seminar here

Greta Malaspina (University of Novi Sad, Serbia)
2020-12-03 13:00
Title: A modified Levenberg-Maquardt method for large scale network adjustment
We present a numerical optimization approach for the solution of large scale Network Adjustment Problems that arise in localization problems such as GPS positioning, surveying and large scale Wireless Sensors Networks localization. We consider a modification of Levenberg Marquardt method that attempts to deal with the non-convex nature of the objective function and the large number of variables.
At each iteration of the classical method the search direction is computed by solving a linear system of equations, which is an expensive procedure when the number of unknowns in the problem that we consider is large, and represents the major obstacle to the solution of realistic, large scale, problems. We develop a scheme for the decomposition of the linear system, which consist in computing an approximation of the Levenberg Marquardt direction by solving a number of independent linear systems of smaller size, and we propose a correction strategy of the right sides that improves the quality of the approximated direction while retaining separability of the linear systems. The convergence analysis of the resulting method is studied under standard regularity assumptions of the objective function. Moreover, the algorithm we propose is tested on realistic adjustment problems and compared with Levenberg Marquardt in terms of both accuracy and computational cost.
Link to the teams group “naXys Seminars” (unamur members) here
Link to the seminar here

Jean Teyssandier (Université de Namur)
2020-12-17 13:00 - 14:00
Title: An overview of astrophysical disc dynamics
Astrophysical discs appear at all scales in the Universe, from rings around planets to galactic discs. Despite the differences in scales and building blocks, they all share similar underlying physical processes. In this talk I will review some of these processes and explain the origin of certain features, such as the spiral arms of galaxies, the fine structures in the rings of Saturn, or the recent observations of gaseous discs where planetary systems are born.
Link to the teams group “naXys Seminars” (unamur members) here
Link to the seminar here

Rodrigo Martínez (University of the Balearic Islands, Spain)
2021-02-04 13:00 - 14:00
Title: Dynamical Phase Transitions in Quantum Reservoir Computing
Closed quantum systems may exhibit different dynamical regimes, such as Many-Body Localization or thermalization, that can affect their ability to process information. Specifically, we establish the role of dynamical phases of Ising spin networks in the field of quantum reservoir computing.
Reservoir computing is an unconventional computing paradigm that consists in exploiting classical or quantum dynamical systems to solve nonlinear and temporal tasks. We observe that the thermal phase of the spin model is naturally adapted to the requirements of reservoir computing while the localized phase is detrimental for the purposes of this computational approach, finding an improved performance for linear and mildly nonlinear tasks in the transition regime. We uncover the physical mechanisms behind optimal information processing capabilities of the spin networks, essential for future experimental implementations.

Link to the teams group “naXys Seminars” (unamur members) here
Link to the seminar here

International Day of Women and Girls in Science at UNamur
2021-02-11 14:00 - 17:00
Join to the online scientific & outreach conferences held at UNamur in the framework of the International Day of Women and Girls in Science to attend the scientific presentations of two naXys members, Morgane Dumont and Anne-Sophie Libert.
The International Day of Women and Girls in Science takes place every 11th of February, following the declaration by the General Assembly of the United Nations on 22nd of December 2015. It aims at promoting the access of women and girls to science and technology as well as their full and fair participation. This event reminds the important role of women in the scientific community. It constitutes a great opportunity to encourage girls and young women to participate in scientific developments. This year, the University of Namur takes fully part in this day. In this context, a scientific conference [in English] will highlight the research contributions of women from UNamur and promote the interaction among them and our fellow colleagues.
Check out our invited speakers. The programme is available online: https://cds.unamur.be/activites/femmesenscience/university
Please take a minute to register (it’s free!) in order to help us in terms of logistics: https://www.billetweb.fr/womeninscience
In the evening, an outreach conference [in French] is organised in collaboration with Confluent des Savoirs: https://cds.unamur.be/activites/femmesenscience/public. Do not hesitate to spread the word, specifically to non-scientists and younger generations!
Looking forward to welcoming you to our conferences!
An initiative by Lorena Ballesteros Ferraz, Aishwarya Saxena, and Sébastien R. Mouchet from the Faculty of Sciences with the support of Confluent des Savoirs (CDS) and Unité d’Appui à la Recherche (UAR).

Maxime Lucas (Aix-Marseille University, France)
2021-02-18 13:00 - 14:00
Title: The cell cycle as a temporal network of protein interactions
Biological systems often operate over multiple timescales and their temporal organisation is often crucial to their functioning. The cell cycle illustrates this well: before dividing into two cells, the cell must go through 4 consecutive phases. Each of these phases corresponds to specific and well-timed physiological processes, driven by protein-protein interactions (PPIs), necessary to the cycle progression. Any deviation from this temporal organisation can indicate a change in cell fate, behaviour, or a malfunction which can lead to diseases. Identifying, predicting, and understanding this temporal organisation can thus yield important insight into their behaviour.
Here, we infer phases of the cell cycle by modelling it as a temporal network of protein-protein interactions and clustering the network’s snapshots. First, we build our temporal network by integrating time series of protein concentrations to a static PPI network. Second, we group snapshots at different times that have a similar structure by clustering them. We show that the obtained clusters recover the 4 known phases of the cell cycle. Moreover, we identify the temporal organisation across a range of timescales by clustering the snapshots for different number of clusters. Results are in agreement with biological knowledge, and we show how robust they are against method changes. Finally, we explore how the input time series affect the results. For example, we investigate the effect of having only partial temporal information in the networks, i.e. not for all edges, which is a common situation in biology. Finally, we show that gene expression data from RNA-seq, widely available for many biological systems, can be used too. This method can be used to further our understanding of the multiscale temporal organisation of many biological networks.
Link to the seminar here
People outside Université de Namur with a Microsoft Teams account may join. For any questions, write to riccardo.muolo@unamur.be

François Lamoline (University of Luxembourg, Luxembourg)
2021-03-04 13:00 - 14:00
Title: Gene expression modelling from cell population snapshot data using optimal mass transport
Modelling gene expression is a central problem in systems biology. Accurate predictive models provide powerful tools for understanding cellular mechanisms and exploring the regulatory relations between genes. Perturbations of these regulatory structures affect the cellular functions. The ability to predict the effects of these perturbations is critical for finding sources of complex diseases and developing new treatments. Recently, single-cell techniques have enabled to perform sequencing at the level of individual cells for a large number of cells at a time. Unfortunately, the cells are destroyed in the measurement process, and so the data consist of population snapshots at different times. Traditional methods aim at modelling from time series data and cannot utilise the full information in the richer single-cell data. Therefore, these new sequencing techniques have raised the need of tailored computational methods for modelling the gene expression from single-cell data. In this talk we introduce new methods based on the 18th century problem of optimal mass transport. The idea consists in tracking the evolution of the distribution of cells over time and finding the dynamical system that minimises the transport cost between consecutive time points. The performance of the methods is compared in numerical experiments.
Link to the seminar here
People outside Université de Namur with a Microsoft Teams account may join. For any questions, write to riccardo.muolo@unamur.be
