Namur Institute For Complex Systems

Nicolas Franco (Université de Namur & Hasselt Universiteit)
2022-03-31 13:00 - 14:00 (Europe/Brussels)
Title: “Using social contact data to inform covid-19 epidemiological models and parameters”
Concerning SARS-CoV-2 transmission as well as many other infectious diseases, heterogeneity and age-related characteristics are crucial in order to correctly capture the dynamics. The use of social contact data is a modern method which has been widely used during the covid-19 pandemic in Belgium and other European countries thanks to the CoMix survey. In this talk, we present why and how those social contact data should be used. We develop the next generation principle based on Perron-Frobenius theory and derive a new numerical tool to estimate the time-evolution of heterogeneous susceptibility regarding SARS-CoV-2 infection and transmission.

The seminar will take place in room S08 at the Faculty of Sciences

Alessio Franci (Universidad Nacional Autónoma de México, México)
2022-04-07 13:00 - 14:00
Title: DECISION FROM INDECISION IN MULTI-AGENT MULTI-OPTION DYNAMICS
How does a group of agents break indecision about a set of options? Many biological and artificial multi-agent systems, from honeybees and bird flocks to bacteria and humans, face indecision when choosing between options in situations in which the fitness or even the survival of the group is at stake. Here, we develop a mathematical theory to study decision from indecision. Our approach is grounded in network bifurcation theory. We model decision from indecision as synchrony-breaking in influence networks in which each node is the value assigned by an agent to an option. We show that there are three generic types of value patterns emerging at synchrony-breaking bifurcations for influence networks: deadlock, consensus, dissensus. Deadlock and consensus value patterns are predicted by the symmetry of the influence networks. Conversely, we show that there are many “exotic” dissensus value patterns; that is, patterns that are not predicted by network symmetries, but are predicted by network architecture. Numerical simulations of a novel influence network model illustrate our theoretical results.

This is a joint work with Martin Golubitsky, Ian Stewart, Anastasia Bizyaeva, and Naomi E. Leonard.

The seminar will take place in room S08 at the Faculty of Sciences

Matteo Romano (Université de Namur)
2022-04-21 13:00 - 14:00
Title:
Studying the topology of the space population using networks
Abstract:
With more than 23.000 tracked objects orbiting Earth, satellite operators find themselves dealing with hundreds of conjunctions at risk of collision per week, situation only getting worse due to the increasingly frequent satellite launches in recent years. Within the WALSAT project, in collaboration with the industrial partner Aerospacelab, we aim to represent the space population as a network in order to exploit its properties to study the topology of the population as a whole. The catalogue objects are represented as nodes and links between them are established whenever a close encounter occurs over a given time window. The resulting network is then analysed in terms of properties such as degree, centrality, and clustering to identify the most dangerous objects. Moreover, by weighting the links using the collision probability or the associated risk, we can identify the most sensitive orbital regions and the objects contributing the most to the overall collisional risk.

The seminar will take place in room S08 at the Faculty of Sciences

Ahmed Almansoori (Université de Namur)
2022-05-05 13:00 - 14:00
Title: On the evolution of mechanisms for collective decision making in a swarm of robots
Abstract – A swarm of robots can collectively select an option among the available alternatives offered by the environment through a process known as collective decision-making. This process is characterised by the fact that once the group makes a decision, it can not be attributed to any of its group members. In the large majority of the swarm robotics literature, only a few types of mechanisms have been used to allow the robots of a swarm to make collective decisions. Namely, the mechanisms referred to as the Voter and the Majority model or variations of these two models. In this talk, we present an alternative solution based on the use of dynamical neural networks as individual decision-making mechanisms. This alternative solution proved effective in a perceptual discrimination task. We will show the results of extensive comparative tests that quantitatively evaluate the Voter model and the dynamic neural network model on a variety of operating conditions and for swarms that differ in their size on a perceptual discrimination task. The results of our study clearly indicate that the performances of a swarm employing dynamical neural networks as the decision-making mechanism are more robust, more adaptable to a dynamic environment, and more scalable to a larger swarm size than the performances of a swarm employing the Voter model as the decision-making mechanism.
The seminar will take place in room S08 at the Faculty of Sciences

naXys Research Day
2022-05-13 09:30 - 16:00
CH01 (Rue Grafé 2, 5000 Namur)
Program
MORNING SESSION (9.30-12.30)

Registration
9.40-9.55: Welcome
9.55-10.40: Flash talks (Part I)
10.40-11.10: Coffee break
11.10-11.30: Discover AI Research Pole (E. Tuci)
11.30-11.40: Meet Jérôme Mallargé, ADRE ICT scientific advisor
11.40-12.30: Brainstorming “The post-it game”

LUNCH
AFTERNOON SESSION (14.00-16.00)

14.00-14.45: Flash talks (Part II)
14.45-15.30: Brainstorming “The puzzle game”
Conclusions

Registration
Registration is free but mandatory, before Friday, May 6th, through the following web form.
We strongly encourage submissions of flash talks for all PhD students. PhD students will receive a certificate with ECTS mention for their participation and/or presentation.

Lluc Bono Rossello (Université Libre de Bruxelles)
2022-06-02 13:00 - 14:00
Title: Modelling Music with Multilayer Networks
In this talk I will introduce our Multilayer Network Model for musical pieces. The goal of this model is to describe the dynamics and interactions occurring during the generative process of music: how each voice or instrument creates a new melody while interacting with other voices. In such a way, we establish a link between the properties of these networks and the music that is encoded in such networks. Then, I will present some of the applications of this model such as the generation of music via synchronized random walks.

The seminar will take place in room S08 at the Faculty of Sciences

Corbinian Schlosser (LAAS-CNRS, Toulouse, France)
2022-06-16 13:00 - 14:00
Title: Koopman and Perron-Frobenius operators on reproducing kernel Hilbert spaces
Abstract: Koopman operators and reproducing kernel Hilbert spaces (RKHS) share a common idea: Giving linear representations of non-linear tasks or objects. We follow the recent path of combining both concepts. The Koopman operator is a composition operator and therefore acts on a function space – in this talk we consider the underlying function space to be an RKHS. We give an introduction to the Koopman operator on an RKHS as well as to its adjoint, the so-called Perron-Frobenius operator. Both operators are linear but not necessarily bounded. Thus we focus on elementary properties of these operators including closedness and boundedness. We present a collection of known and new results on Koopman operators on RKHS. These results, with no further specifications on the RKHS, are restrained and we emphasize that the RKHS should be chosen accordingly to the problem at hand in order to flourish the interplay between the Koopman operator and the RKHS.
The seminar will take place in room CH12 at the Faculty of Sciences

Aaron Bryan Rosengren (University of California San Diego, USA)
2022-07-12 13:00 - 14:00
Title: Space-Manifold Dynamics for Small Solar-System Bodies
Abstract: Centaurs are a prominent group of small Solar-System bodies in highly dispersed orbits between the two main belts. They are a short-lived transient phase connecting their source reservoir in the Kuiper Belt to the Jupiter-family comets (JFCs) and Halley-type comets (HTCs), but the precise dynamical hand-off process between the Centaurs and these short-period comet (SPC) populations remains unclear. The recent finding of an orbital gateway between Jupiter and Saturn, funneling Centaurs into the inner Solar System, has cast some light on the orbital migration of these enigmatic bodies. Yet, no dynamical mechanism to explain the existence of this apparent conduit has hitherto been offered. The talk will link seminal research conducted over two decades ago on Jovian-induced space manifolds with our recent understanding of the orbital architecture of these gravitational structures to elucidate the nature of this and other dynamical channels for the rapid transport of planet-crossing small bodies of the outer Solar System. This work combines the well-developed techniques of astrodynamics for the study of space manifolds with the modern analytical and numerical tools of planetary dynamics, harnessing these in unique ways to probe for the first time the high time-resolution details of such strongly chaotic orbital evolution. Building from the notion and success of the interplanetary transport network, studying and understanding space-manifold dynamics in the planetary-science context may also give rise to new and unexpected results that could uncover new Centaur-mission concepts, motivate entirely new classes of missions, and amplify scientific return.
The seminar will take place in room S08 at the Faculty of Sciences

Pascal Jerono (University of Namur)
2022-09-22 13:00 - 14:00
Title: Observer and control design for cell population models of yeast fermentation processes
Abstract: In bioreactors, the lack of available online process information is a common issue, due to physical limitations of the reactor or a lack of available sensors. Especially online measurements of the cell distribution with respect to process relevant properties remain to be a challenging task. From a control theory perspective, the lack of online measurements restricts the control strategy to feed forward control, which is sensitive to process disturbances or model inaccuracies, which are typically present in biological systems. This motivates the design of reliable observer schemes for the online estimation of process-relevant state variables based on classically available measurements during cultivation. This talk addresses the observer and control design for cell population models of yeast fermentation processes. Cell population models describe the time evolution of the cell distribution with respect to specific properties like cell mass, size, age, or chemical composition. Modeling of the cell division cycle by means of the population balance equation leads to a partial integro-differential equation, which is coupled to a set of ordinary differential equations due to the interaction with nutrient dynamics. Different methods to analyze the observability, detectability, and controllability of the considered class of systems are discussed and applied to the discretized model equations. Based on the results, observers and control schemes are designed and validated for the estimation and control of the cell mass distribution in a lab-scale reactor.
The seminar will take place in room S08 at the Faculty of Sciences

Jeongjin Lee (University of Namur)
2022-10-13 13:00 - 14:00
Title: Partial Tail Correlation for Extremes
Abstract: We develop a method for investigating conditional extremal relationships between variables. We consider an inner product space constructed from transformed-linear combinations of independent regularly varying random variables. By developing theprojection theorem for the inner product space, we derive the concept of partial tailcorrelation via projection theorem. We show that the partial tail correlation can beunderstood as the inner product of the prediction errors associated with transformed-linear prediction. Similar to Gaussian cases, we connect partial tail correlation tothe inverse of the inner product matrix and show that a zero in this inverse impliesa partial tail correlation of zero. We develop a hypothesis test for the partial tailcorrelation of zero and demonstrate the performance in a simulation study as well asin two applications: extreme river discharges in the upper Danube basin and highnitrogen dioxide levels in Washington DC.
The seminar will take place in room S08 at the Faculty of Sciences

Hiroya Nakao (Tokyo Institute of Technology, Japan)
2022-10-24 13:00 - 14:00
Authors: Yuzuru Kato and Hiroya Nakao*
Title: Turing instability in quantum activator–inhibitor systems
Abstract: We show that Turing instability, a fundamental mechanism of nonequilibrium self-organization in classical systems, can also occur in a quantum dissipative system. We propose a quantum-optical parametric oscillator with nonlinear damping as a quantum activator–inhibitor unit, and numerically demonstrate that a system of two such units can undergo Turing instability when diffusively coupled with each other. The Turing instability induces a pair of nonuniform states that are mixed due to quantum noise. Further performing continuous measurement on the coupled system reveals the nonuniformity caused by the Turing instability.
Reference: Y. Kato and H. Nakao, “Turing instability in quantum activator-inhibitor systems”, Sci. Rep. (2022) 12:15573
The seminar will take place in room S07 at the Faculty of Sciences

Manu Stalport (Geneva University, Switzerland)
2022-10-26 10:30 - 11:30
Title: A calibration of chaos for the refinement of planetary systems
Abstract: The observed diversity of planetary systems raises questions that challenge the formation and evolution models. A precise knowledge of the systems architectures is key in order to identify the leading processes shaping them. In this presentation, I will discuss on how the study of chaos can provide a more precise view of the planetary systems. A particular interest is drawn on a numerical chaos indicator, namely the Numerical Analysis of Fundamental Frequencies (NAFF, J. Laskar), and its link with orbital instability. Combining the lessons learned together with a Bayesian exploration of the parameter space, we design a technique used to refine the planet parameters. I will illustrate this approach on a few notable planetary systems.
The seminar will take place in room S09 at the Faculty of Sciences

Philippe Toint (University of Namur)
2022-10-27 13:00 - 14:00
Title: Recent topics in complexity for nonconvex optimization problems
Abstract: The talk will review results on the evaluation complexity of algorithms for nonlinear (nonconvex) optimization problems. The focus is on unconstrained and “simply constrained” cases and will consider obtaining high-order optimality conditions. Algorithms using inexact information will also be covered.
The seminar will take place in room S08 at the Faculty of Sciences

Christian Mugisho Zagabe (University of Namur)
2022-11-10 13:00 - 14:00
Title: Lie-algebraic condition for uniform stability of switched nonlinear systems: Construction of a common Lyapunov function by the Koopman operator approach.
Abstract: Switched systems consist of a finite set of dynamical systems and a switching signal indicating which system is activated. Their interest is due to the fact that many phenomena in nature, engineering,… are represented by different models depending on the change of some parameters. Their stability properties are not intuitive. Indeed, for two individually stable (or unstable) systems one can construct a signal that makes the whole system unstable (or stable). In the uniform stability theory, the main goal is to find sufficient (and/or necessary) conditions that make the whole system stable for all possible switching laws.
In 1999, Daniel Liberzon and co-workers proved that for a switched linear system, the solvability property of the Lie algebra generated by Hurwitz matrices is a sufficient condition for uniform stability.
In 2004, Daniel Liberzon formulated an open problem to find which condition of the Lie algebra (of vector fields) can be used to guarantee the global uniform asymptotic stability (GUAS) of switched nonlinear systems.
Using the Koopman operator approach, we propose an answer to the open Liberzon problem. Our result is related to the dynamics on the polydisc and shows that a sufficient condition for GUAS follows from the solvability of the Lie algebra generated by the Hurwitz Jacobian matrices of the vector fields.
More precisely, we construct a common Lyapunov function for switched nonlinear systems, which is convergent in a specific region of the state space. This is done by defining the Koopman operator on the Hardy space on the polydisc where the reproducing kernel property allows us to obtain a Lyapunov function via the evolution of the evaluation functional. We then infer the GUAS property of switched nonlinear systems on a specific invariant set.
The seminar will take place in room S08 at the Faculty of Sciences

Ariane Courtot (IMCCE, France)
2022-11-17 13:00 - 14:00
Title: Chaos in the Geminid Meteoroid stream
Abstract: Meteor showers originate from a parent body (an asteroid or a comet). This parent body ejects several meteroids, forming a meteoroid stream, which will meet the Earth. From Earth, several meteors can then be observed. To prove they actually form a shower, we need to show whether they come from the same source.
But dynamically linking a meteor shower with its parent body can be challenging. This is in part due to the complex dynamics of meteoroid streams and the influence of non-gravitational forces (FNG).
We develop a method to study chaos in meteoroid streams and apply it to the Geminid meteoroid stream. We choose to draw chaos maps, which relies on chaos indicator. We show that the best one is the Orthogonal Fast Lyapunov Indicator, after comparison with the Fast Lyapunov Indicator, the FLI for close encounters and the Mean Exponential Growth factor for Nearby Orbits.
We apply this method on the Geminids, a well-known meteoroid stream. We show the influence of mean-motion resonances with the Earth and with Venus, which tend to trap the largest particles. The chaos maps present 3 distinct regimes in eccentricity, reflecting close encounters with the planets. We also study the effect of NGFs and we determine a first approximation of the particle size needed to counterbalance the resonances with the diffusion created by NGFs.
The seminar will take place in room S08 at the Faculty of Sciences

Johan SEGERS (UCLouvain)
2022-12-01 13:00 - 14:00
Title: Graphical and uniform consistency of estimated optimal transport plans
Abstract: A general theory is provided delivering convergence of maximal cyclically monotone mappings containing the supports of coupling measures of sequences of pairs of possibly random probability measures on Euclidean space. The theory is based on the identification of such a mapping with a closed subset of a Cartesian product of Euclidean spaces and leveraging tools from random set theory. Weak convergence in the appropriate Fell space together with the maximal cyclical monotonicity then automatically yields local uniform convergence of the associated mappings. Viewing such mappings as optimal transport plans between probability measures with respect to the squared Euclidean distance as cost function yields consistency results for notions of multivariate ranks and quantiles based on optimal transport, notably the empirical center-outward distribution and quantile functions.
The seminar will take place in room S08 at the Faculty of Sciences

Victor Pessanha (Mathematical Institute of the Federal University of Rio de Janeiro, Brazil)
2022-12-15 13:00 - 14:00
Title: A quick introduction to knots and braids
Abstract: In this talk I will do an overview on the theory of knots and braids. The idea is to define what these objects are and to present two major theorems relating them by just giving an intuition behind their proof. If possible, I will also talk about how these objects can naturally appear in the dynamics of point masses.
The seminar will take place in Room S08 at the Faculty of Sciences.

Stefanella Boatto (Department of Applied mathematics, Institute of Mathematics, Federal University of Rio de Janeiro, Brazil)
2023-01-24 10:30 - 11:30
Title: A journey into the mathematical universe: Topological Data Analysis. A briefintroduction and some applications.
Abstract: Data Science is a highly evolving discipline that presents many challenges. Forexample, in the case of an epidemic, how can we identify which socio–economic and socialparameters, tourist mobility, prevention and control measures have the greatest impact onthe dynamics of an epidemic? In recent years Topological Data Analysis has proved to be apowerful tool, a lens to highlight some dominant features of collective phenomena andexplore large and complex data sets. In this lecture we will see a brief introduction and howthis new discipline is highly multidisciplinary within mathematics itself, bringing togetherareas such as Space–Time Analysis, Algebraic Topology, Complex Networks, Alg. Linear andComputation.
The seminar will take place in Room S08 at the Faculty of Sciences.

FNRS - EDT Stat - PhD School Nonparametric classification and machine learning - introduction, comparison and applications
2023-01-25 - 2023-01-27
Lecturers: Professor Christophe Ley (University of Luxembourg) and Professor Germain Van Bever (University of Namur)
In this short course, we shall make an introduction to classical tools in nonparametric classification. We shall also take a stroll along Machine Learning from a statistical perspective (discussing inter alia Random Forests), including a comparison to statistical learning. We shall illustrate the working of Machine Learning via some concrete applications, including a prediction of international football tournaments.
The school will take place at University of Namur (Room PA02, Faculty of Sciences) in the mornings (9h30-12h45) of the days 25-26-27 of January. The lectures will be only in person.
Website: https://www.unamur.be/sciences/mathematique/EDStat
The school is organized by the department of Mathematics of Université de Namur and naXys, the Namur Institute for Complex Systems and funded by the FNRS and Université de Namur.

Women and Girls in Science Workshop
2023-02-09 08:30 - 18:00
The event will take place in the Auditorium Rosalind Franklin (S01) of the Faculty of Sciences
More information on the event, including the program, can be found here

Sylvério Pool Marquez (University of Namur)
2023-02-16 13:00 - 14:00
Title: Impact of mixed-variable management by probability features in an Evolutionary Algorithm
Abstract: The management of mixed variables, i.e., of different natures (continuous, integer, discrete, categorical) in optimization problems is a challenge, especially in the presence of categorical variables since there is no concept of order between their possible “values”. In a black-box optimization context, a way to handle categorical variables has been proposed within a particle swarm algorithm to improve its performance. It consists in defining a probability feature for each “value” of each categorical variable which will evolve during the optimization process. These probabilities are then used to update the population of the algorithm.In this talk, we present an adaptation of this method to be used in the evolutionary algorithm (EA) implemented in Minamo, the in-house design space exploration and multi-disciplinary optimization platform developed in the applied research center Cenaero. More specifically, we propose several ways to use these probabilities inside the genetic operators of the EA and we compare them in order to get a modified version of the EA which is more efficient in the presence of categorical variables.
The seminar will take place in Room S08 at the Faculty of Sciences.

Ramachandran CHITTUR ANANTHARAMAN (University of Namur)
2023-03-09 13:00 - 14:00
Title: Inverses of polynomials over finite fields – A Koopman operator based approach
Abstract: Any map (linear or non-linear) from a finite field to itself can be represented as a polynomial with coefficients over the finite field. When the map is invertible (under composition), the inverse map will also have a polynomial representation. Verifying invertibility and computing the inverse of the non-linear maps over finite fields have significant applications in cryptography and coding theory. In this talk, I discuss a Koopman operator-based approach to test invertibility, compute the inverse of polynomials, and define a linear representation for non-linear maps. This approach can be extended to a parametric family of maps over finite fields (such as Dickson polynomials), and the parametric inverse is computed (when the parametric map is invertible). Further, I shall discuss how this linear representation defined through the Koopman operator leads to a group representation for a group generated by multiple invertible maps under composition.
The seminar will take place in Room S08 at the Faculty of Sciences.

Lorena Ballesteros Ferraz (University of Namur)
2023-03-16 13:00 - 14:00
Title: Geometrical and non–normal properties of weak values
Authors: Lorena Ballesteros Ferraz, Dominique Lambert, Timoteo Carletti, Riccardo Muolo,and Yves Caudano
Abstract: Quantum measurements are essential to all quantum experiments and are key to developing new quantum technologies. In particular, weak measurements have attracted much interest, for both theoretical and experimental reasons. When performing a weak measurement with preselection (which consists of choosing the initial state) and post–selection (which requires a projective measurement after the weak measurement and corresponds to choosing the final state), weak values appear. In that case, the ancilla’s wavefunction is shifted in position by a quantity that is proportional to the real part of the weak value.
Its wavefunction is also shifted in momentum by a quantity that is proportional to the imaginary part of the weak value. Even though weak values are regularly studied in terms of their real and imaginary parts, we focus our analysis in terms of their modulus and argument. In this work, we have studied the geometrical interpretation of the argument of weak values of general observables in N–level systems. This argument corresponds to a geometric phase (Berry phase) that is associated to the symplectic area generated by the geodesic triangle spanned by the three normalized vectors representing the initial state, the application of the observable over the initial state and the postselected state. These vectors represent quantum pure states in complex projective space CP^(N–1) and they can be mapped to a subspace of the (𝑁² − 2)–sphere, a generalization of the Bloch sphere (the usual S² sphere, used to describe qubits).
The argument of the weak value of an N–level system observable can also be mapped to the sum of N–1 arguments of weak values of two–level projectors up to a phase that specifies the quadrant. Each of these arguments is associated to the solid angle on the Bloch sphere spanned by three vectors representing the pre–selected state, the observable and the post–selected state. To obtain this decomposition, we employ the Majorana representation to factor the weak value of the N–level system observable into the product of N–1 weak values of qubit projectors and a real constant.
Studying the modulus of the weak value provides information on the ability of weak values toamplify minute phenomena as well as their role in improving the estimation of small physicalparameters. Weak values can be written as the expectation value of a non–normal operator.Investigating the non–normal properties of the operator, we found a direct link with theamplitude of the modulus of the weak value. This paves the way for a deeper understanding of these values from an energetics point of view and links the fields of weak measurements and non–normality.
The seminar will take place in Room S08 at the Faculty of Sciences.

Fabian Bastin (University of Montreal (Canada)) - Cinzia Cirillo (University of Maryland (USA))
2023-04-06 13:00 - 14:00
Title: Large-scale transport model systems: from synthetic population generation to agent-based simulation
Abstract:
1. Generation of synthetic populations exploiting dependency structure transfer (Fabian Bastin)
The generation of synthetic populations consists of producing a realistic representation of a target population for modeling and behavioral simulation purposes, a task that can be tricky when the available multivariate samples are small. Starting from the idea that combining various sources of information makes it possible to design more realistic population generators, we introduce a new framework based on copulas to generate synthetic data for a population whose only empirical marginal distributions are known, using a sample from another population sharing a similar dependency structure. We compare the proposed scheme to common methods such as Iterative Proportional Fitting and to modern probabilistic approaches such as Bayesian networks, variational auto-encoders and generative adversarial networks. We illustrate on American Community Survey data that the method proposed allows to study the structure of the data at different geographical levels in a way that is robust to the peculiarities of the marginal distributions.
2. Large scale transportation model systems: an agent-based simulator for the State of Maryland (Cinzia Cirillo)
The talk will introduce the architecture of a large-scale microsimulation model that combines population synthesis, land use models, transportation choices, and agent-based transport simulation. This modeling suite overcomes limitations of aggregate four step models and is completely open source. Application of this type of tools will be illustrated in the context of transportation and equity; the first considers the problem of evacuating low-income carless populations during an emergency, the second evaluates the effects of complete streets on the willingness to walk and bike under different levels of traffic stress.
The seminar will take place in Room S08 at the Faculty of Sciences.

Hilda A. CERDEIRA (Instituto de Física Teórica, UNESP, São Paulo, Brazil)
2023-04-13 13:00 - 14:00
Title: Phase transition to Synchronization in a system of Swarmalators
Abstract: Systems of oscillators called Swarmalators,whose phase and spatial dynamics are coupled, have been used to describe the dynamics of some living systems. Their collective behavior presents simultaneous aggregation in space and synchronization in phase which in turn leads in some cases to explosive synchronization in a finite population as a function of the coupling parameter between the phases of the internal dynamics. This phenomenon is described using the order parameter and the Hamiltonian formalism. Near the synchronization transition the phase energy of the particles are represented by the XY model, and they undergo a transition which can be of the first order or second depending on the distribution of natural frequencies of the internal dynamics of the swarmalators.
The seminar will take place in Room S08 at the Faculty of Sciences.

Francqui Chair 2022-2023
2023-04-17 - 2023-04-19
On topology and higher–order dynamics of networks and simplicial complexes
Guest lecturer: Prof. Ginestra Bianconi, Queen Mary University London and the Alan Turing Institute (UK)
Program:
* Inaugural lecture, “Topology shapes higher–order network dynamics”, Monday, April 17th 2023, at 17:00, Auditorium PA02
Next lectures, room S09
* Tuesday 18/4/23, 10:40 – 12:40 Introduction to simplicial complexes and algebraic topology, 14:00 – 16:00 Topological Synchronization
* Wednesday 19/04/23, 10:40 – 12:40 The Dirac operator on networks and simplicial complexes, 14:00 – 16:00 Higher–order network dynamics driven by the Dirac operator
We invite you to register by March the 31st 2023 following this link.
The lectures address Master students, PhD and researchers interested in dynamical systems, synchronisation, network science and high-order models and their interactions.
PhD students can receive credits for their attendance upon request (please check the box in the registration form).

Philippe Naveau (Laboratoire des Sciences du Climat et de l'Environnement (LSCE) CNRS, France)
2023-04-20 13:00 - 14:00
Title: Records Analysis in Climate Attribution
Abstract: Numerical climate models are complex and combine a large number of physical processes. They are key tools in quantifying the relative contribution of potential anthropogenic causes (e.g., the current increase in greenhouse gases) on high-impact atmospheric variables like heavy rainfall or temperatures. These so-called climate extreme event attribution problems are particularly challenging in a non-stationary context. In addition, global climate models like any in sillico numerical experiments are affected by different types of bias. In this talk, I will discuss about how to combine to different statistical concepts to assess records changes in the context of extreme event attribution. In addition, the question of uncertainties quantification that remains a challenge in any climate attribution analysis will be explored from various directions. In particular, a simple model bias correction step for records will described in detail. To illustrate our approach, we infer emergence times in precipitation from the CMIP5 and CMIP6 archives. This is joint work with Paula Gonzalez, Soulivanh Thao and Julien Worms.
The seminar will take place in Room S08 at the Faculty of Sciences.

Daniele Avitabile (Vrije Universiteit Amsterdam, The Netherlands)
2023-04-25 13:00 - 14:00
Title: Bump Attractors and Waves in Networks of Integrate-and-Fire Neurons

Abstract: Bump attractors are localised patterns observed in in vivo experiments of neurobiological networks. They are important for the brain’s navigational system and specific memory tasks. A bump attractor is characterised by a core in which neurons fire frequently, while those away from the core do not fire. We uncover a relationship between bump attractors and travelling waves in a classical network of excitable, leaky integrate-and-fire neurons. This relationship bears strong similarities to the one between complex spatiotemporal patterns and waves at the onset of pipe turbulence. We define and study analytical properties of the voltage mapping, an operator transforming a solution’s firing set into its spatiotemporal profile. This operator allows us to construct localised travelling waves with an arbitrary number of spikes at the core, and to study their linear stability. A homogeneous “laminar” state exists in the network, and it is linearly stable for all values of the principal control parameter. We show that one can construct waves with a seemingly arbitrary number of spikes at the core; the higher the number of spikes, the slower the wave, and the more its profile resembles a stationary bump. As in the fluid-dynamical analogy, such waves coexist with the homogeneous state, are unstable, and the solution branches to which they belong are disconnected from the laminar state. We provide evidence that the dynamics of the bump attractor displays echoes of the unstable waves.

The seminar will take place in Room S08 at the Faculty of Sciences.

Ludovic Ducobu (Transilvania University of Brasov, Romania)
2023-06-01 13:00 - 14:00
Title: Compact objects and scalar fields in theories of gravity with torsion, ep. 1 : modelisation
Abstract:
General Relativity [GR] offers an extremely successful framework to describe the gravitational interaction. That being said, the necessity to question the framework of GR is clear at both the experimental (origin and composition of dark matter and dark energy) and theoretical (search for quantum gravity) levels.
Since not all these puzzles can be purely reduced to quantum correction problems, this motivates the study of alternative theories of gravitation already at the classical level.

To this purpose, one can follow two distinct but complementary routes. On the one hand, one can try to modify Einstein’s equations by introducing new fields while maintaining the assumptions related to the geometrical structure of spacetime. On the other hand, one can precisely question the framework of pseudo-Riemannian geometry as being adapted to the full description of spacetime (typically, at cosmological scales).
In the first case, scalar-tensor theories of gravity (and especially Horndeski gravity), where the new degrees of freedom are encoded by means of a scalar field, have been extensively investigated in the litterature.
In the later case, a minimal generalisation of GR’s framework consist in relaxing the hypothesis linking the connection to the metric, leading to the framework of metric-affine gravity. This reveals new starting points to generalise GR through, for instance, an examination of the so-called geometrical trinity of gravity.

The aim of this talk is to present the main motivations and features of modified theories of gravity involving scalar fields in both the usual (GR) and metric-affine approaches.

I will first propose a quick overview of Horndeski gravity and of the metric-affine approach of GR; focussing on the so-called Teleparallel theories of gravity and on the Teleparallel equivalent to GR.
Then, “unifying” the two approaches, I will advocate for the interest of modified theories of gravity involving scalar fields in the metric-affine framework.
I will then present recent results from my own research and discuss the first study of assymptotically flat black holes endowed with scalar hair in a teleparallel generalisation of Horndeski gravity.
Our study provides an explicit demonstration of how scalar-torsion gravity (and possibly other non-Riemaniann models) provides a richer canvas for the study of phenomenology beyond GR and open the way for novel studies regarding scalarized black hole solutions in non-Riemannian theories of gravity.

This presentation is based on the work done in [arXiv:2201.11445 [gr-qc]] (https://arxiv.org/abs/2201.11445) and [arXiv:2212.07653 [gr-qc]] (https://arxiv.org/abs/2212.07653) in collaboration with Sebastian Bahamonde, Daniela Doneva, Christian Pfeifer and Stoytcho Yazadjiev.

The seminar will take place in Room S08 at the Faculty of Sciences.

Enoch Yeung (UC Santa Barbara (UCSB), USA)
2023-06-30 13:00 - 14:00
Title: Data-Driven State Criticality and Observability with Koopman
Operator Methods
Abstract: I will present two major results to show the use of data-
driven Koopman methods to identify critical states and observable
subspaces to solve problems in synthetic biology. In the first, I
present the use of dynamic mode decomposition (DMD) to model the
transcriptome-wide response of a root-isolate bacterium to a novel
chemical compound. By solving an observability maximization problem
for the DMD model, we find a panel of biomarkers that can act as
effective biosensors for the compound, even in field studies with the
bacterium. This study establishes a new precedent for reasoning about
state criticality and system observability, even without prior
knowledge of a network model. Second, I present new theoretical
results that show how Koopman methods can be used to evaluate
criticality of states to optimize performance of a nonlinear system.
Historically, this problem is solved using either direct sensitivity
analysis on a known model or by generating local function
distributions that span the nonlinear observable subspace of a system.
In the absence of a known model, I present a new Koopman-based method
for estimating the observable subspace of a nonlinear system purely
from data. Our results provide a route for data-driven discovery of
critical states that affect an output-based performance measure.
The seminar will take place in Room S08 at the Faculty of Sciences.
