Namur Institute For Complex Systems

Hidemasa Ishii (University of Tokyo)
2024-11-26 13:00 - 14:00 (Europe/Brussels)
Title : Employing dynamical modeling to understand complex phenomena: from interacting bistable systems to football ball possession
Abstract :
I have been motivated to employ dynamical modeling to understand complex phenomena in social sciences and biology.
In this seminar, I would like to introduce two projects I have worked on.
The first involves a system of diffusively coupled bistable elements, which have been utilized to study abrupt transitions in complex systems.
We analysed the influence of coupling strength over the mean escape time.
We numerically found that weak coupling facilitates escape on average, while strong coupling impedes escape [1].
We argue, although diffusive coupling both facilitates and impedes escape, the facilitating effect is dominant under weak coupling.
The second topic concerns dynamical modelling for three-versus-one ball possession in football.
In our model, motion of the players and ball is governed by a system of ODEs during a single pass.
The model is nevertheless stochastic, because the initial condition and parameters of the ODEs depend probabilistically on the final state of the previous pass.
We aim to utilize this approach to quantify the difference between high- and lower-level football teams.
[1] H. Ishii and H. Kori, Diffusive coupling facilitates and impedes noise-induced escape in interacting bistable elements, Sci Rep 14, 11019 (2024).
The seminar will take place in Room S07 at the Faculty of Sciences.

Yusuke Kato (University of Tokyo)
2024-11-28 13:00 - 14:00
Title : Introduction of two studies: (1) analysis of oscillation quenching by periodic perturbation and (2) parameter estimation of Kuramoto model
Abstract :
I will talk about the two different projects that I am working on in Japan. (1) In the first part, I will present the oscillation quenching (annihilation of oscillation) in a bistable oscillator model [1]. We find that multiplicative periodic forces combined with feedback induce quenching and perform the bifurcation analysis using an averaging approximation. (2) The second part would be about the parameter estimation of the Kuramoto model, which is a mathematical model that describes synchronization phenomena. We use the time series of the order parameter (a macroscopic quantity that characterizes how the whole population is synchronized) as given data and estimate the model parameters. The estimation is performed by the Bayesian approach, using the exchage Monte Carlo method for computation.

[1] Y. Kato and H. Kori. Periodic forces combined with feedback induce quenching in a bistable oscillator. Chaos 34, 103125 (2024).

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

Gauvain Leconte-Chevillard and Andrea Roselli (UNamur)
2024-12-12 13:00 - 14:00
Title : Did the laws of physics emerge at t=10-34 s?
Abstract :
According to the vast majority of the inflationary models of the universe, the exponential expansion of space ends ≃10-34 s after the Big Bang with a phase of reheating during which the inflation field decays into the particles of the Standard Model. In this talk, we do not address issues regarding the empirical adequacy of this cosmological scenario; we rather focus on the two following questions:
– According to this scenario, can we argue that the particles of the Standard Model emerged from the inflation field, and in which sense?
– If we claim that the interaction carriers of the Standard Model (e.g. gluons) emerged during the reheating, does it mean that thelaws describing or governing these interactions (e.g. the strong nuclear force) emerged at the same time?
In the first part of the talk, we give a brief overview of the physics underlying the reheating of the universe at the end of inflation. Then, in the second part, we claim that the models of reheating are sufficiently developed to answer unambiguously to the first question – if and only if we are careful to distinguish two kinds of emergence: synchronic and diachronic. However, such a precise answer might not be available for the second question, which depends on our metaphysical conception of physical laws. In the last part, we will present a metaphysical conception that could elegantly accommodate the physical claims made above, we will show how things would “hang together” in such a metaphysics, and make some more general philosophical remarks.
The seminar will take place in Room S08 at the Faculty of Sciences.

Corentin Bisot (VU Amsterdam)
2025-02-04 13:00 - 14:00
Title : Transport logistics in a growing fungal network
Abstract :
Branching forms are ubiquitous in nature and evolved repeatedly throughout evolution. An example are mycorrhizal fungi, that have, for nearly 450 million years, constructed networks to collect and trade nutrient resources with plant roots. Because of their dependence on host-derived carbon, these fungi face conflicting trade-offs in building networks that balance construction costs against geographic coverage and long-distance resource transport to and from roots. How they navigate these design challenges is unknown. Using data from fungal colonies at the scale of individual filament growth but also internal cytoplasmic flows our lab investigates intimate relationships between growth, transport and trade.

We found a consistant pattern of colony propagation that emerges through the combination of branching and fusion of individual hyphae. The exact balance of these two ingredients can be adapted depending on fungal strategy. I will show how the necessity of transport within the colony, and trade with the plant, constrain the space of possible propagation patterns. I will conclude discussing how intracellular flows could be organizing to allow coherent trade patterns explaining the stability of the symbiosis accross such a wide timescale.

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

Jeff Arnoldi (CNRS)
2025-02-06 13:00 - 14:00
Title : Linear Functional Ecology – Rethinking species contributions to ecosystem functions
Abstract :
Ecosystem functions describe processes like biomass production, respiration, or nutrient cycling, that can be key to the livelihood of humans and other life forms.
These functions are collectively performed by the many species that constitute an ecosystem; trees is a forest, plants in a grassland, or bacterial strains in a microbiome.
Here, I will use linear algebra to show that generically, there is a sense in which a rare species can have as much importance as an abundant one.

The reason why this claim is not obvious comes from the fact that in functional ecology, under the mass ratio hypothesis, species contributions should be well predicted by their traits and abundance. I will illustrate using soil-microbiome data that this is indeed true, even for complex functions related to nutrient cycling. Yet the mass-ratio hypothesis has an awkward corollary: functions are typically performed by just a few dominant species, so that most of an ecosystem diversity appears useless. This a description of the state of the system, but says nothing of its dynamics. To understand why rare species can be important, one has to take a perturbative approach, looking at the sensitivity of a function to say, added mortality (a pathogen) on a given species. In doing so, we reveal a completely different picture than the one the mass-ratio hypothesis depicts. Via direct and indirect interactions between them, species can have large impacts on a given function, even if those species are rare or if they do not possess traits that relate to the function.

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

Benoît Legat (UCLouvain)
2025-02-27 13:00 - 14:00
Title : Hidden convexity in linear neural networks
Abstract :
Training neural networks involves minimising a loss function that is nonconvex with respect to the network’s weights. Despite this nonconvexity, when the optimization converges to a local minimum, it is often close to globally optimal. This transfer from local properties to global properties is often achieved through convexity in optimization which neural networks seem to lack, or is it hidden ? There are two sources of nonconvexity in neural networks : 1) the nonlinear activation functions and 2) the multilinear product of the weight matrices.
Interestingly, recent research has demonstrated that the second source does not, on its own, lead to local minima that are not global when paired with a mean squared error loss. Although this result is promising, the complexity of the proof limits its generalization to more complex models, such as those with nonlinear activation functions or other loss structures. In this talk, we reveal the convexity hidden in the problem and show how it allows for a simpler and more insightful proof. By exposing this underlying structure, we aim to open the door to recognizing which types of models are more likely to train well and to extend this understanding to other machine learning architectures.
The seminar will take place in Room S08 at the Faculty of Sciences.

Thomas Robiglio (IT-U University, Linz)
2025-02-28 13:00 - 14:00
Title : Mechanisms and behaviors in complex systems with group interactions
Abstract :

The interplay between causal mechanisms and emerging collective behaviors is a central aspect of understanding, controlling, and predicting complex networked systems.
Existing methods to study each of the two facets mostly adopt lower-order descriptions: pairwise network representations for mechanisms, and low-order information-theoretic metrics for behaviors. Despite their success, these low-order methods often fail to fully capture the intricate nuances inherent to many complex systems, thus beyond-pairwise methods are being developed: higher-order network representations and higher-order behavorial metrics.
As both low-order and higher-order mechanisms can determine the observation of both low and higher-order behaviors, the connection between behavioral observables and microscopic mechanisms in systems with pairwise and group interactions is not trivial; a systematic investigation of this complex relationship across different orders of interactions is needed.
In this talk, I will present recent work exploring the link between higher-order mechanisms and higher-order behavioral observables in two representative models with group interactions: a simplicial Ising model and a social contagion model. Our findings reveal that group (higher-order) interactions give rise to emergent synergistic (higher-order) behaviors in both systems.
Finally, I will argue that fully disentangling behaviors and mechanisms in complex systems—especially those with group interactions—requires principled approaches based on generative models and statistical inference.
REFERENCES:

Robiglio, T., Neri, M., Coppes, D., Agostinelli, C., Battiston, F., Lucas, M., & Petri, G. (2024). Synergistic signatures of group mechanisms in higher-order systems. arXiv preprint arXiv:2401.11588.
Peel, L., Peixoto, T. P., & De Domenico, M. (2022). Statistical inference links data and theory in network science. Nature Communications, 13(1), 6794.

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

Hélène Verhaeghe (UCLouvain)
2025-03-13 13:00 - 14:00
Title : The best of both worlds: when constraint programming and machine learning help each other
Abstract : Constraint programming is well known to be performant at finding the solutions of combinatorial optimization problems and can provide guarantees about these solutions. However, as problems tend to grow in size and complexity, the limits of such methods tend to show. Machine learning, on the other hand, is very good at extracting statistical patterns and dealing with uncertainty. However, they are really bad at reasoning, even on simple tasks. This talk is about how, by combining the two families of techniques, they can benefit one another. This talk will start with an introduction to constraint programming for those not knowledgeable about the technique.
The seminar will take place in Room S08 at the Faculty of Sciences.

Jérémy Rekier (Royal Observatory of Belgium)
2025-03-20 13:00 - 14:00
Title : The Tidal Flows in Ocean Worlds
Abstract :
Planet Earth is one of a handful of ocean worlds in the solar system. Others include Jupiter’s moons Europa, Ganymede and Callisto, as well as Saturn’s moons Titan and Enceladus. Other such worlds might also exist in orbit around Uranus and Neptune. In all of these bodies, tidal heating plays a crucial role in keeping the water ocean from freezing, thereby extending the habitable zone beyond its classical circumstellar (aka Goldilocks) limit. Some authors even consider that such worlds orbiting exoplanets might be more habitable than analogues to the Earth. This makes exploration of the ocean worlds that are closer to us crucial in order to understand the conditions for extraterrestrial life, and this involves paying close attention to the process of tidal heating.
Exploration of the fluid dynamics of icy moons is a complicated matter, in large part due to the subsurface nature of their oceans. We present some of the methods developed to this end, inspired by previous work focusing on the study of the Earth’s and planet’s deep interior. We show how tidal flows can be resonantly amplified by inertial modes (restored by the Coriolis force) in liquid oceans, and how these modes are affected by density stratification.
The seminar will take place in Room S08 at the Faculty of Sciences.

Hugues Annoye (UCLouvain Saint-Louis)
2025-03-27 13:00 - 14:00
Title : BEAMM project : How do we deal with data ? Statistical matching and WGAN generation.
Abstract :

In the framework of the BEAMM project (BElgian Arithmetic Micro-simulation Model), we propose several methods to address data issues. The core of this project is to develop a tax-benefit microsimulation model for Belgium accessible online, requiring intensive data handling. Our challenges consist in creating a unified data set containing variables from different surveys and developing a completely synthetic database for the online development of the BEAMM platform.
Indeed, in the BEAMM context, we use a large number of variables available in different databases. We thus need to analyze data from different sources; the obser- vations, which only share a subset of the variables, cannot always be paired to detect common individuals. This is the case, for example, when the information required to study a certain phenomenon comes from different sample surveys. Statistical matching is a common practice to combine these data sets. In this talk, we investigate and extend to statistical matching three methods based on Kernel Canonical Correlation Analy- sis (KCCA; [2]), Super-Organizing Map (Super-OM; [1]) and Autoencoders-Canonical Correlation Analysis (A-CCA; [3]). These methods are designed to deal with various variable types, sampling weights and incompatibilities among categorical variables.
In our context, data privacy and anonymization are important. Under these cir- cumstances, the need for synthetic databases that replicate the characteristics of the population while preserving privacy is arising. In this presentation, we also investigate how we can employ a range of data generation approaches utilizing various advance- ments in the Wasserstein Generative Adversarial Network (WGAN) literature to create survey databases. WGANs were introduced by Arjovsky 2017 [4] in the context of im- age synthesis. Our algorithms have been adjusted to account for sampling weights. Moreover, survey and adminstrative data have the specificity of mixing continuous and categorical data, which should be taken into account in the architecture of the WGANs.

References :

[1] Kohonen, T. (1982), Self-organized formation of topologically correct feature map. Biological Cybernetics, 43 (1), 59–69.

[2] Lai, P. L. and Fyfe, C. (2000), Kernel and nonlinear canonical correlation analysis. International Journal of Neural Systems, 10 (05), 365–377.

[3] Rumelhart, D. E., Hinton, G. E. and Williams, R. J. (1986), Learning Internal Representations by Error Propagation in Parallel Distributed Processing: Explo- rations in the Microstructure of Cognition. Cambridge: MIT Press, 318–362.

[4] Arjovsky, M., Chintala, S., & Bottou, L. (2017, July). Wasserstein generative adversarial networks.In International conference on machine learning (pp. 214- 223). PMLR.

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

Gabriele Gionti (Vatican Observatory)
2025-04-01 13:00 - 14:00
Title : ON THE CANONICAL EQUIVALENCE OF JORDAN AND EINSTEIN FRAMES
Abstract :
Following a seminal paper by Dicke, physics should be invariant under redefinition of units of measurement, which implies invariance under conformal transformation of the metric coefficients. This implies the longstanding issue of the equivalence between the Jordan and the Einstein frames. It is believed, but not completely proved, that the cosmological physical observables are the same in the two frames. Our aim is to tackle this problem from the perspective of the Hamiltonian formalism. For this reason, we will perform the Hamiltonian analysis of the Brans-Dicke theory with Gibbons-Hawking-York boundary term both in the Jordan and the Einstein frames. The Hamiltonian Dirac’s constraint analysis will be carefully carried out in both cases and, contrary to several claims made in the literature in the past, it will be shown that the transformations from the Jordan to the Einstein frames do not strictly appear Hamiltonian canonical transformations on the extended phase space. We will show that if we perform a gauge fixing on the lapse and shift functions and implement them as secondary Dirac’s constraints in the ADM formalism, the primary first-class constraints become second class. Replacing Poisson Brackets with Dirac’s Brackets and solve “strongly” the second-class constraints, the transformation from the Jordan to the Einstein frames is Hamiltonian canonical transformation. We will briefly mention the study of the ADM formalism in the case of spherical symmetry both in the Jordan and the Einstein frames with appropriate boundary terms. We derive the equations of motion in both frames. Weintroduce the Fisher–Janis–Newman–Winicour naked singularity solution in the Einstein frame and Bocharova–Bronnikov-Melnikov-Bekenstein black hole solution in the Jordan frame. They are, respectively, solution of the equations of motion in the two frames and are connected by the Hamiltonian canonical transformation from the Jordan to the Einstein frames on the reduced phase space. This very fact highlights that the transformation from the Jordan to the Einstein frames is useful to generate solutions of the equations of motion in gravity.

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

Kovács Tamás (Eötvös Loránd University)
2025-04-15 13:00 - 14:00
Title : Spin-orbit dynamics under parameter drift
Abstract :

The dynamics of an oblate satellite in a fixed elliptical orbit, with its spin axis perpendicular to the orbital plane around a spherical planet, exhibit rich coupling between rotation and orbital motion, known as spin-orbit resonance. It is well known that the phase space of low-dimensional deterministic Hamiltonian systems contains regions of both chaotic and regular motion—namely, chaotic bands and invariant curves (KAM tori)—depending on the initial conditions and system parameters. In the steady-state case, these structures remain stationary. However, when system parameters change over time, due to various internal or external influences, the geometry of phase space evolves accordingly. This necessitates a shift from traditional single-trajectory analysis to an ensemble-based approach, enabling a consistent framework to describe phase space structures in non-autonomous Hamiltonian systems. In this context, we also generalize the classical Lyapunov exponent to account for explicit time dependence, providing a numerical stability measure for the evolving dynamics. Finally, we propose a preliminary analytical description of the time-dependent phase space structures and their evolution.

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

Judicaël Mohet (UNamur)
2025-04-17 13:00 - 14:00
Title : How to estimate the state of a nonlinear system in a linear way?
Abstract :

In this work, we present a novel approach for the observer design of nonlinear systems based on a Koopman operator framework. The (linear) dual Koopman system, associated with a nonlinear dynamic, is introduced and analyzed in an infinite dimensional context. Moreover, new concepts of observability and detectability are defined in the dual Koopman system, which are shown to be equivalent to the observability and detectability of the nonlinear system, respectively. The theoretical framework is applied to a class of holomorphic dynamics. For this class, a Luenberger-type observer is designed for the dual Koopman system via a spectral method, yielding an estimate of the state of the nonlinear system. Spectral observability and detectability conditions are derived in this setting, and the exponential convergence of the Koopman observer is shown

Authors: Judicaël Mohet, Alexandre Mauroy and Joseph Winkin

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

Ludovic Ducobu (UNamur)
2025-05-16 13:00 - 14:00
Title : Is the dynamics of the dark sector hidden in Einstein Equation ? Variational Geometry at the rescue of cosmologists.
Abstract :

In this talk, I will present recent results of my work in cosmology and variational geometry.

I will review basic notions in variational geometry and present the method of variational completion, allowing to reconstruct a Lagrangian for a theory from a guess of its field equations.

I will then show how this can be extended to theories that involve at least two sets of dynamical variables.
This new method, introduced in [1] and called variational bootstrapping, allows one to construct a Lagrangian for a physical theory depending on two sets of field variables, starting from a guess of the field equations for only one such set.

This setup is particularly appealing for the construction of modified theories of gravity, since one can take lessons from General Relativity for an “educated guess” of the metric field equations; the field equations for the other fields are then fixed by the obtained Lagrangian (up to terms that are completely independent from the metric tensor).

To conclude, following [2], I will show how one can apply variational bootstrapping to determine metric-affine models which are, in a variational sense, the closest to the ΛCDM model of cosmology : starting from an “educated guess” that formally resembles the Einstein field equations with a cosmological constant and a dark matter term, the method allows to find “corrected” metric equations and to “bootstrap” the connection field equations.

This presentation is based on the work done in [1] and [2] in collaboration with Nicoleta Voicu.

[1] – https://arxiv.org/abs/2403.15564
[2] – https://arxiv.org/abs/2406.09540
The seminar will take place in Room S08 at the Faculty of Sciences.

Jan Baetens (UGent)
2025-05-22 13:00 - 14:00
Title : Spatially explicit models: some methods of analysis and applications in biosciences
Abstract :
During this talk I’ll give a general overview of the research lines on spatially explicit models covered in my group BionamiX at Ghent University. Then I’ll go in more detail for what concerns the methodological advances we were able to achieve in the field of cellular automata, and I’ll illustrate some of the more practical studies we completed over the last few years. The latter illustrate how spatially explicit models can support decision-making and policy.
The seminar will take place in Room 266 at the Faculty of Sciences.

Arash Rahimi-Iman (Justus-Liebig-Universität Gießen)
2025-06-11 13:00 - 14:00
Title : Machine learning optimization of chiral photonic metasurface: evolution-based algorithm and deep learning approach
Abstract :

Chirality of light in the simplest case is given by the handedness of circular polarization, for which handedness preserving or nonpreserving optics can be developed [1]. Chiral photonics has been receiving increased attention due to prospects of delivering tailored metamaterials with strong ‘chiroptical’ responses at predesignated wavelengths [2], as interactions between chiral light and matter could be attractive for applications ranging from chemistry to information processing.
Circularly polarized electromagnetic waves incident on a nanopatterned metasurface can experience frequency-dependent filtering effects in reflection and/or transmission geometry. Such structures can be produced in thin-film dielectrics or metallic sheets. In a comparison of two machine learning optimization approaches, the predicted chiral patterns (here in GaP) feature an increased circular dichroism [3], which resulted in a stronger reflectivity of right circularly than left circularly polarized waves compared to a simpler human-designed pattern. Outcomes from the multi-epoch design improvement procedure – through an evolution- and neural network-based algorithm – are discussed for a frequency-range-neutral comparative consideration promising for both (nano)printing or ablating of surface patterns, and indicating further improvements potentials with more advanced algorithms.
References:
[1] E. Plum, N. I. Zheludev (2015): Chiral Mirrors, Appl.Phys.Lett. 106, 221901.
[2] W. Ma, F. Cheng, Y. Liu (2018): Deep-Learning-Enabled On-Demand Design of Chiral Metamaterials, ACS Nano 12, 6326.
[3] O. Mey, A. Rahimi-Iman (2021): Machine Learning-Based Optimization of Chiral Photonic Nanostructures: Evolution- and Neural Network-Based Designs, Phys.Stat.Sol. RRL 16, 202100571.
This seminar will take place in Room 266 at the Faculty of Sciences.

Denis Langevin (Université Clermont Auvergne)
2025-09-10 13:00 - 14:00
Title: Global optimization for inverse design in nanophotonics
Abstract:

Global optimization algorithms for the inverse design of nanophotonic structures is a tool which is providing increasingly convincing results – even though the wave nature of photonic behaviours makes them particularly complex. In the meantime, the field of global optimization itself is rapidly evolving but is prone to reproducibility problems, making it harder to identify the right algorithms to use.
In this talk, I will provide a physicist’s insight into the specificities about photonic problems, and how they help or hinder us when using optimisation tools. I will present “observables” – such as spontaneous emergence of regularity or consistency curves – which are relevant for judging the reliability of an optimization and the quality of the generated solution. To support this discussion, I will draw on benchmark examples covering a broad range of topics, including 1D, 2D, 3D problems.
References:
[1] Barry, M. A., Berthier, V., Wilts, B.. D., Cambourieux, M.-C., Bennet, P., Pollès, R., Teytaud, O., Centeno, E., Biais, N., and Moreau, A., “Evolutionary algorithms converge towards evolved biological photonic structures,” Scientific reports 10(1), 1–10 (2020).
[2] Sörensen, K., “Metaheuristics—the metaphor exposed,” International Transactions in Operational Research 22(1), 3–18 (2015).
[3] Bennet, P., Langevin, D., Essoual, C., Khaireh-Walieh, A., Teytaud, O., Wiecha, P., and Moreau, A., “An illustrated tutorial on global optimization in nanophotonics,” (2023).
This seminar will take place in Room S08 at the Faculty of Sciences.

Alessandro Moirano (LPAP, ULiège)
2025-10-16 13:00 - 14:00
Title: Io, a natural probe for Jupiter’s atmosphere and magnetosphere – (Some of) Juno’s results on the moon-magnetosphere interaction.
Abstract:

Jupiter hosts persistent and bright auroras, which are driven by the precipitation of particles into its atmosphere, mostly electrons. The presence of Io orbiting in the inner Jovian magnetosphere produce a specific auroral emission on Jupiter, called “footprint”, whose details are determined by the atmosphere and the energy of the precipitating electrons. The latter, in turn, depends on the physical magnetospheric process(es) that accelerates the particles along the magnetic field and towards the planet. Therefore, the footprint – and the aurora in general – wraps information about the atmospheric structure and the energy spectrum of the precipitation. During this talk, we will discuss the recent analyses of the observations performed by Juno, which has been orbiting Jupiter and providing an unprecedented vantage point on its polar regions since 2016. To begin with, the position of the Io footprint is affected by the Io plasma torus, a dense and variable plasma cloud around Io’s orbit that is ultimately produced by the volcanic activity on the moon. We were able to constrain the spatial distribution of plasma near Io thanks to several years of observations with Juno. Moreover, during some of the closest-approach to Jupiter, Juno was able to observe the Io footprint near the planetary limb, which provided a unique opportunity to investigate the relation between the spectrum of the precipitating electrons and the vertical profile of the auroral emission. Lastly, by looking at the absorption in the far-ultraviolet recorded with the Ultraviolet Spectrograph onboard Juno, we aim at estimating the hydrocarbon abundances in the upper layers of Jupiter’s atmosphere.

The seminar mostly presents results driven by observations, interpreted in light of current theoretical models. Infrared, ultraviolet and radio observations, as well as in-situ particle measurements, are the backbone of the talk, although the moon-magnetosphere interaction welcome all types of expertise and observations.

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

Isao Ishikawa (Kyoto University)
2025-10-21 13:00 - 14:00
Title: Finite dimensional approximations of push-forwards on analytic functionals
Abstract:
In this talk, we will introduce JetEDMD, a new theoretical framework as a finite-dimensional approximation of the push-forward on the spaces of analytic functionals.
JetEDMD provides a reasonable modification of Extended Dynamic Mode Decomposition (EDMD) and elucidates new mathematical machinery by combining the structure of reproducing kernel Hilbert spaces with the jet spaces on the state space of the dynamical system.

As applications, I will present methods for the data-driven estimation of equilibrium points and the reconstruction of vector fields from discrete trajectory data with a rigorous convergence result. Finally, I will discuss some current mathematical challenges and future research directions within this framework.

Laura Gagliardi (University of Chicago)
2025-10-22 15:00 - 16:30
Title: Reticular frameworks after the Nobel prize: their application for catalysis and CO2 capture
Abstract:

Following reflection on the 2025 Nobel Prize in Chemistry awarded for advances in metal– organic frameworks (MOFs), this lecture explores how state-of-the-art quantum chemical and classical simulations are driving the discovery of reticular materials. Computation today is not only a tool to rationalize experiments, but increasingly a predictive engine for tailoring functional frameworks.
In the first part of the presentation, I will highlight our integrated computational and experimental study of catalytic MOFs, where post-synthetic modification introduced metal– sulfur active sites [1]. Quantum chemical calculations elucidated how sulfur incorporation modulates the electronic structure and catalytic reactivity of the frameworks. These insights provided a mechanistic understanding of hydrogenation catalysis and guided the design of MOFs with tunable properties.
The second part will focus on covalent organic frameworks (COFs). Thanks to their modular architectures and tunable functionalities, COFs offer a highly versatile platform for CO2 direct air capture. We performed a multiscale investigation of COF-999 and its amine- functionalized precursor COF-999-NH₂, integrating density functional theory, molecular dynamics, and grand canonical Monte Carlo simulations with experimental validation [2]. Our findings highlight subtle energy differences in laterally shifted stackings, intrinsic stacking heterogeneity, and pronounced layer buckling. We found that extensive amine– nitrile hydrogen bonding and persistent pore water lead to undesired polymerization that undermines CO₂ uptake. The predicted presence of water is confirmed by subsequent experiments. These insights point to a single, actionable design rule: exclude retained water by introducing hydrophobic pore environments to maximize CO2 capture efficiency.
[1] H. Xie, M. A. Khoshooei, M. Mandal, S. M. Vornholt, J. Hofmann, L. M. Tufaro, K. O. Kirlikovali, D. A. Grimes, S. Lee, S. Su, S. Reischauer, D. Sengupta, K. Fahy, K. Ma, X. Wang, F. Sha, W. Gong, Y. Che, J. G. Vitillo, J. S. Anderson, J. M. Notestein, K. W. Chapman, L. Gagliardi, and O. K. Farha, Introducing Metal–Sulfur Active Sites in Metal–Organic Frameworks Via Post-Synthetic Modification for Hydrogenation Catalysis, Nature Chemistry, 2025. DOI: 10.1038/s41557-025-01876-y
[2] H. Daglar, Z. Zhou, R. Zhu, P. Parihar, J. I. Siepmann, O. M. Yaghi, L. Gagliardi Discovery of Stacking Heterogeneity, Layer Buckling, and Residual Water in COF-999-NH₂ and Implications on CO2 Capture Submitted (2025)

This seminar will take place in Room S05 at the Faculty of Sciences.

Serge Nicaise (Université de Valenciennes)
2025-11-27 13:00 - 14:00
Title: Stability results of dissipative systems via the frequency domain approach
Abstract:

The frequency domain approach goes back to J. Prüss [Trans. Amer. Math. Soc. 284 (1984), 847-857] and F. L. Huang [Ann. Differential Equations 1 (1985), 43-56] that show that a C0 semigroup (etA)t≥0 of contractions in a Hilbert space H is exponentially stable if and only if the resolvent of A is uniformly bounded on the imaginary axis. Afterwards Z. Liu and B. Rao, [Z. Angew. Math. Phys. 56 (2005), 630-644], C. J. K. Batty and T. Duyckaerts [J. Evol. Equ. 8 (2008), 765-780], and A. Bátkai, K.-J. Engel, J. Prüss and R. Schnaubelt [Math. Nachr. 279 (2006), 1425-1440] have given some sufficient conditions on the behavior of the resolvent of A on the imaginary axis that guarantee an almost polynomial decay of the semigroup. Finally an optimal result about the polynomial decay was found by A. A. Borichev and Yu. V. Tomilov [Math. Ann. 347 (2010), 455-478]. This approach is a powerful tool for the study of the decay of the semigroup associated with concrete dissipative systems since it reduces to the study of the resolvent on the imaginary axis.
In my talk, I will first recall these two results and then illustrate them on two particular dissipative systems.

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

session pitch naXys
2026-01-29 13:00 - 14:00
“Coexistence and stability in ecological systems” (Frederik De Laender)
“Measuring and implementing sustainability in finance: The need for systemic approaches” (Sophie Béreau)
“Analysis of the control system mechanisms of robots” (Elio Tuci)
“Rare trajectories in quantum field theory and classical dynamics” (Yves Caudano)
“Emergent properties of complex systems. A tale of networks and nonlinearity” (Timoteo Carletti)
This seminar will take place in Room S09 at the Faculty of Sciences.

Diego Samuel Rodrigues (University of Campinas, SP, Brazil)
2026-02-11 13:00 - 14:00
Title: Parameter Estimation for Ordinary Differential Equation Models in Cancer Research
Abstract:
Inferring parameter values from data is a fundamental problem in quantitative oncology. This seminar offers a concise, example-driven overview of parameter estimation and experimental design for ordinary differential equation systems, bridging theoretical foundations with practical applications in cancer research.

This seminar will take place in Room S0 at the Faculty of Sciences.

Federica Garin (Université Grenoble Alpes)
2026-02-26 13:00 - 14:00
Title: Graphons and opinion dynamics on large graphs.
Abstract:
Graphons are the limits of large dense graphs. Graphons can also be used to define families of random graphs (the so-called W-random graphs), similar to Erdős-Rényi random graphs but with some inhomogeneity in the edge probabilities.
Graphons and W-random graphs, together with their signed counterparts, can be useful tools to study opinion dynamics on large-scale networks.
In this talk, I will review some recent work in this direction, where we explore some infinite-dimensional dynamics associated with the graphon, and we study how well they can approximate the corresponding dynamics associated with finite but large W-random graphs.

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

Thomas Koffel (Université Claude Bernard Lyon 1)
2026-04-16 13:00 - 14:00
Title: From Metacommunities to Masting: How Spatial Structure Privatizes Environmental Benefits
Abstract:
Indirect positive interactions, mediated by environmental modification, are central to niche construction and cooperation. However, eco-evolutionary theory predicts that when modified environments are fully public, the ‘tragedy of the commons’ favors cheaters, destabilizing such processes. This talk explores how spatial structure overcomes this barrier through two complementary lines of work. First, using a minimal stochastic model, we show how competition-colonization dynamics in a patch-structured metacommunity allow cooperators to coexist with or even exclude cheaters. Second, we demonstrate how environmental privatization at the individual level enables niche construction, specifically in the context of masting (intermittent, synchronous fruiting by trees). By satiating and starving seed consumers—a ‘public bad’—masting evolves as a coordinated defense strategy that effectively privatizes the benefits of predator control. Together, these results underline how the scale of environmental modification fundamentally dictates the scale at which selection acts in ecosystems.
This seminar will take place in Room S08 at the Faculty of Sciences.

Koichiro Yawata (Institute of Science, Tokyo)
2026-05-07 13:00 - 14:00
Title: Reduced-Order Control of Coupled Phase Oscillator Populations and Hypergraph-Based Approaches to Combinatorial Optimization Problems

Abstract:
In this seminar, I will present two recent pieces of work from my research: one on reduced-order control of coupled oscillator populations and another on hypergraph-based approaches to combinatorial optimization.
1. Reduced-Order Control of Coupled Oscillator Populations via Frequency-Dependent Forcing
We propose a frequency-dependent forcing framework for heterogeneous populations of coupled phase oscillators. Using the Ott-Antonsen ansatz, we derive a low-dimensional description of the collective dynamics and study control methods for the macroscopic phase and synchronization level.
2. Hypergraph Neural Networks Accelerate MUS Enumeration
We propose a hypergraph-neural-network-based method for accelerating MUS enumeration in constraint satisfaction problems. By combining hypergraph modeling with reinforcement learning, the method reduces satisfiability checks and improves enumeration efficiency.
This seminar will take place in Room S08 at the Faculty of Sciences.

Nelly Litvak (Eindhoven University of Technology)
2026-10-01 13:00 - 2026-12-03 14:00
Title:
Abstract:
This seminar will take place in Room S08 at the Faculty of Sciences.
