UCL

PhD Students

BOUMAL Nicolas (Starting date : September 2010)
Design and analysis of algorithms for smooth curve fitting on Riemannian manifolds
Thesis advisors : V. Blondel, P.-A. Absil
We focus on designing and analyzing practical algorithms to fit smooth curves to data on manifolds. The regression tools we intend to develop will be useful in a number of applications, providing the means for data resampling and denoising. We specifically aim to provide efficient software to achieve these tasks on tractable and practically important Riemannian manifolds.

BROWET Arnaud (Starting date : 15 September 2008)
Image segmentation and Video tracking
Thesis advisors : P. Van Dooren, P.-A. Absil
Computer vision has been a very active topic in the last few decades but there is still progress needed in robust, reliable and stable algorithms (for handling problems such as noise, occlusion, deformable objects, etc..). We will focus on the real-time video tracking problem and explore different image segmentation techniques from a wide variety of domains such as graph cuts, community detection in graphs, snakes or active contours, partial differential equation solutions for energy minimization. We plan to adapt those techniques and develop new ones to address real-time video tracking in various fields such as remote sensing for collision detection and video surveillance.

CHEVALIER Pierre-Yves (Starting date : September 2012)
Decentralized computation and information fusion in a dynamical environment
Thesis advisors : J. Hendrickx, R. Jungers, R. Sepulchre (ULg)
We consider a set of agents having each a certain information or measure. The goal is to compute a function of all the available information in a decentralized way, giving a priori similar computing roles to all the agents.
We will consider this problem in the context of multiple dynamics: (i) The network of connections between the agents evolves, while possibly temporarily preserving certain local structure. (ii) The information accessible to the agents may change with time. (iii) The very composition of the system may also by dynamics, with agents entering and leaving the system.

DECUYPER Adeline (Starting date : September 2011)
Dynamics and mathematics of time-evolving social networks
Thesis advisor : V. Blondel
The project focuses on the study of the dynamical aspects of large time-evolving graphs as representations of underlying social networks. The goal is then to develop mathematical models that reproduce properties observed on real social networks. Among other applications, this project will investigate the spreading dynamics of e.g. information or infectious diseases in social networks through numerical simulations and analysis of empirical data of real social networks.

DELHAYE Benoît (Starting date : September 2009)
Dexterous Manipulation in Microgravity: Mechanical properties of the fingertip-object contact
Thesis advisors : Ph. Lefèvre, J-L Thonnard
The aim of the project is to study the mechanical properties of the fingertip and their influence on the control of grip force when manipulating objects. More specifically, we will study how information from skin afferent s allows characterising the properties of an object when grasping it. This information is of prime importance to prevent the object from slipping with a minimal grip force as a function of the friction properties of the fingertip-object interface. Moreover, we will focus on the influence of a change in gravity on these mechanisms.

DERAVET Nicolas (Starting date : September 2012)
Study of the oculomotor memory and application to the diagnosis of frontotemporal degeneration (FTLD
Thesis advisor : P. Lefèvre
This thesis investigates the oculomotor memory, which plays a decisive role in the visual tracking system. The oculomotor memory takes part in the anticipation and the prediction of the displacement of moving objects in our surrounding environment. A recent study, performed in the laboratory of my promotor, has shown that a better understanding of this memory can be a very useful tool for the early diagnosis of some diseases. Indeed, the study of the eye movements of patients has allowed to clearly discriminate between patients with fronto-temporal lobar degeneration (FTLD) and those with Alzheimer disease (AD) or healthy subjects. During this thesis, I will study the oculomotor memory following several approaches, with healthy subjects and different categories of patients (FTLD, AD, MCI: Minor Cognitive Impairment). Finally, I will develop a specific toolkit, in two complementary stages. The first will aim at improving and optimizing our experimental setup to allow for faster and easier operation, whereas the second will aim at simplifying this setup to open new prospects of use of our tools in a "standard" clinical environment (able to measure the eye movements of patients). This interdisciplinary project combines an experimental approach based on fundamental research in neuroscience with a clinical study and the use of engineering tools.

DEVILLE Pierre (Starting date : October 2011)
Detection and Stability Analysis of Communities in Large Social Networks
Thesis advisor : V. Blondel
Social, technological and information systems can often be described in terms of complex networks. The analysis of the structural and statistical properties of such networks is one of the major foci of complex systems science at the moment. The main objective of this research is to design methods that aim to quantify and visualize the stability of communities in large social networks. Communities in a network are almost connected subnetworks. Their analysis may have several applications as well as impact at different levels of our society (economical, sociological and political) and their interest is numerous.

DEVOLDER Olivier (Starting date : October 2009)
Infinite-dimensional structured convex optimization
Thesis advisors : F. Glineur, Y. Nesterov
The goal of this thesis is to generalize results and algorithms developed for finite-dimensional structured convex optimization to the infinite-dimensional case, i.e. to problems with an infinite number of variables and/or constraints.

EGO Caroline (Starting date : September 2009)
Visual tracking in cerebral-palsied children: influence on learning and exploration of new avenues for rehabilitation
Thesis advisor : Ph. Lefèvre
Cerebral palsy (CP) is a group of permanent disorders of movement and posture due to a non-progressive lesion of the fetal or infant brain. The motor disorders in cerebral-palsied children are often accompanied by several disabilities including sensation, perception or cognition impairments. In particular, previous studies showed a high prevalence of learning disorders in cerebral-palsied children. The goal of the present study is to point out subclinical deficits in oculomotor performances of children with low level of severity of CP by comparing their performances to those recorded in age-matched control children between 6 and 14 years. Indeed, subtle oculomotor deficits such as impaired saccade pursuit interaction or poor anticipation and prediction skills in oculomotor strategies may be an important cause of learning disabilities. The underlying purpose of this study is to investigate new paths for rehabilitation in cerebral-palsied children.

GIARD Thibault (Starting date : September 2010)
Dextrous Manipulation in Microgravity: Motor Planning and Interaction with Robots
Thesis advisors : P. Lefèvre, J-L Thonnard
With the thumb opposing the index, humans have the ability to grasp and manipulate objects. More specifically, this grip permits to modulate the force developed to hold objects stable in our hands. Indeed, the force required to hold an object depends on many factors, including the object weight, the mechanical and frictional properties of the skin/object interface and the inertial constraints induced when the object is accelerated. In most situations encountered on daily bases, manipulating an object also implies the anticipation of static and inertial torques. In this project, two tools are used in order to study those aspects : parabolic flights and robotic arms (the Phantom by SensAble and the Kinarm by b-kin).

HOLLANDERS Romain (Starting date : September 2010)
On the Policy Iteration Algorithm for PageRank Optimization
Thesis advisors : J-C Delvenne, R. Jungers
In search engines, it is critical to be able to sort web pages according to their relative importance (or PageRank). Here, we study the specific problem of optimizing the PageRank of a node (i.e. a web page) in a directed graph (i.e. the web graph) when the presence of some edges (i.e. hyperlinks) is uncertain. We explain how this problem can be formulated as a Markov Decision Process. Classical algorithms can then be used to solve this new formulation of the problem. Among these, one of them, built following the principle of Policy Iteration, seems to perform very well in practice. Yet, its theoretical behavior is unknown. In the thesis, we study the complexity of this algorithm and we show that it converges in polynomial time under the natural assumption that the random walk restarts with a fixed probability called damping. We also try to extend our result to the case in which there is no damping and we give some clues towards new results in this direction.

KLEPPER Audrey (Starting date : October 2011)
Modelling and control of chemical processes
Thesis advisor : D. Dochain

LATIERS Arnaud (Starting date : October 2012)
Dynamic demand response as provider of ancillary services
Thesis advisors : F. Glineur, E. De Jaeger
Describe, model and understand dynamic demand response and assess its potential as a provider of ancillary services (flexibility) in a high-voltage grid.

LECLERCQ Guillaume (Starting date : September 2007)
Visuomotor velocity transformation for visually guided manual tracking
Thesis advisor : Ph. Lefèvre
Visually guided reaching movements are common tasks in our everyday life. To achieve such tasks, the brain needs to transform the retinal information into an appropriate motor command for the muscles. The goal of this research is to investigate if the brain accounts for the complex non-linear 3-dimensional eye-head-shoulder geometry when performing visually guided manual tracking tasks. A next step is to understand how it can be achieved in the brain.

ORBAN DE XIVRY François-Xavier (Starting date : 16 February 2009)
Nearest stable system
Thesis advisors : P. Van Dooren, Y. Nesterov
Stability is a crucial property in dynamical systems. Here, we are interested in finding a practical algorithm that would find the nearest stable system to an unstable one. We approach this problem from an optimization point of view using convex approximations and non-linear optimization techniques. In that framework, theoretical guarantees of the approximation and complexity bounds are important questions that require an answer. Associated problems such as the problem of finding the nearest stable polynomial to an unstable one will also be investigated.

RENARD Emilie (Starting date : January 2013)
Dimensionality reduction using matrix factorization
Thesis advisors : P-A Absil, V. Blondel
More information soon.

RENTMEESTERS Quentin (Starting date : 15 September 2008)
Geometric data fitting and subspace tracking
Thesis advisors : Paul Van Dooren, P.-A. Absil
Several problems in signal processing and image processing deal with corrupted data which belong to a nonlinear space. In this research, we investigate filtering and approximation techniques on nonlinear spaces to reduce the influence of the noise on the data. We are particularly interested in applying these techniques to subspace tracking problems.

TAYLOR Adrien (Starting date : September 2012)
Control and optimization of networked systems with poorly understood couplings; with application to wind farms
Thesis advisors : J. Hendrickx, F. Glineur
Classical control and optimization techniques rely either on the fact we know how the studied system behaves (i.e. we have a reliable model of it) or on black-box models. Our aim is to develop methods standing in-between those two approaches: using partial informations about the system (e.g. bounds on the function or the derivative). This is in particular of interest in the case of wind farms, where the interactions between the different wind turbines - through wind field - cannot be computed accurately in a reasonable time.

TRAAG Vincent (Starting date : May 2009)
Social networks with positive and negative weights
Thesis advisor : P. Van Dooren
Negative links, such as animosity, bad references or inhibiting effects are not often considered in network analysis. The goal of this research is to understand the dynamics of such links. More specifically, we will analyze a model for reputation dynamics in the context of indirect reciprocity game theory.

TREFOIS Maguy (Starting date : September 2010)
Algorithmics of structured matrices and algebraic graph theory
Thesis advisor : J.-C. Delvenne
We investigate the minimum rank problem of a graph, that is the minimum possible rank of a real matrix whose zero-nonzero pattern is described by the graph. The project consists in developing efficient methods allowing to compute the minimum rank of particular graphs, namely the loop directed trees. Moreover, we work on complex networks and more precisely we develop methods in order to reduce the size of a network by preserving its dynamics.

VANKEERBERGHEN Guillaume (Starting date : September 2012)
Control theoretic techniques for multi-agent systems and application to decentralized Big Data analysis
Thesis advisors : J.Hendrickx, R. Jungers, A Sarlette (UGent)
The goal of my thesis is to push the boundaries of the theory for what is locally achievable by smart devices connected in an open network (where agents can enter and leave) and apply this to realistic problems of the type of those encountered in smart cities. This implies modeling and analyzing how and to what extent one can decentrally interpret and control entities based on the locally collected data.

Post-Docs and Research Associates

COPPE Sébastien (Starting date : October 2012)
Modeling the oculomotor system with a patient study
Host : P. Lefèvre
We analyze the eye movements of patients suffering from a kind of neurodegenerative dementia in order to correlate their oculomotor impairment with their neurological damages. Through the evolution of the disease, the frontal and temporal lobes of the brain, which are crucial for correct eye movements, are more and more impaired. We therefore assess the oculomotor performances regularly in the course of the disease in order to link these oculomotor performances with the brain impairment of these patients.

DAVID Robert (Starting date : March 2008)
Identification of a model for the real-time application for riverflow forecasting
Host : G. Bastin
HYDROMAX is a real-time application for riverflow forecasting which is developed by CESAME and is operational at SETHY (Service d'études hydrologiques, Walloon Ministry of Public Works - Belgium) for the management of flood alarms and the on-line information of the rescue services in the Meuse river basin. HYDROMAX is connected with the telemetering network of SETHY. This network is essentially made up of water level chart recorders and rain gauges.
HYDROMAX provides in real-time:

short term predictions of riverflows based on rainfal and past riverflow measurements
long term flood forecastings based on weather forecasts

For each river basin, the predictions are produced by a mathematical model. HYDROMAX has been developed to be user friendly and to fulfill the real time forecasting requirements. HYDROMAX is successfully in operation since 1995 on the main Meuse tributaries (Semois, Ourthe, Lesse, Viroin, etc.).

DE KERCHOVE Cristobald (Starting date : September 2009)
Ranking Large Networks
Diffusion of information through large graphs. I generally work on real datasets - for example mobile phone networks - and I identify leaders who are efficient to spread some information or behavior in the network. Another trendy topic is the votes on the Web. I consider graphs of votes where users evaluate items as we can find in many web sites. A third point includes all sorts of algorithms based on some random walk on the graph. Community detection and visualization of large graphs are certainly two problems that I like looking at.

GILLIS Nicolas (Starting date : October 2012)
Constrained low-rank matrix and tensor approximations: complexity, algorithms, and applications
Host : F. Glineur
Low-rank approximation of a given matrix or tensor is a fundamental problem in numerical linear algebra, related to many application domains, e.g. machine learning, graph theory and model reduction. This linear dimensionality reduction technique is an indispensable tool for the analysis of high-dimensional data. Moreover, in many settings, it is beneficial or necessary to incorporate constraints on the factors of the decomposition (such as sparsity or nonnegativity), or to use different objective functions (e.g., to enhance robustness), leading to a large class of Constrained Low-Rank Approximation problems (CLRA).

Some of these problems are suspected to be difficult to solve (i.e. NP-hard), other are much easier, but most have not been investigated thoroughly. Therefore, practitioners rely on the use of ad hoc nonlinear optimization schemes to solve them, which typically only guarantee convergence to first-order stationary points. The aim of this project is to characterize and understand CLRA problems better, in order to reduce the gap between theory and practice.

HERNANDEZ Hector (Starting date : October 2012)
Discrete Control of BioReactors
Host : D. Dochain
In this research stay, the problem of controlling continuous stirred tank fermenters of discrete-delayed measurements is studied. The work begins with a reactor to produce ethanol (as a biofuel), and it is intended to extend it to a certain class of bioreactors. In practical scenarios, these reactors are operated on the basis of discrete-delayed measurements, provided by laboratory analysis of reacting mixture samples, taken out at certain instants. On the design of control systems, it will be followed a feedback linearization approach, including the following aspects: (i) characteristics of the dynamic behavior of the reactor, (ii) feasible control configurations since instrumentation infrastructure, (iii) construction of control systems, (iv) a systematic scheme of tuning from a pole-assignment approach, and (v) a discussion about the trade-off between sampling interval and convergence rate, since an analytical point of view provided by a convergence analysis.

HOANG Ngoc-Ha (Starting date : May 2012)
Thermodynamics and systems theory
Host : D. Dochain
The objective of this research is to further investigate the link between irreversible thermodynamics and system theory. More precisely, we propose some thermodynamic foundations usable both for the dynamical analysis and control design of reaction systems. Typical examples (which cover a large class of reaction systems ranging in particular from CSTR networks to tubular reactors) are given to illustrate the application of the theoretical results.

HUDON Nicolas (Starting date : September 2012)
Robust dissipative control of nonlinear systems interconnected in networks
Host : D. Dochain This research focuses on nonlinear analysis and distributed feedback control design of interconnected systems in the context of chemical processes. At a plant-wide level, extensions of classical results on the stability of large-scale interconnected systems lead to input-output dissipative constraints for each subsystems, encoded as supply rates from input to output interconnecting ports. Then, for each subsystem, a parameterized nonlinear feedback controller is designed using nonlinear dissipative inequalities to ensure that the aforementioned dissipative constraints are met in closed-loop. This distributed approach is considered for uncertain systems and systems affected by unknown disturbances using robust adaptive feedback controllers.

KRINGS Gautier (Starting date : March 2012)
Analysis of mobile phone data records
Host : V. Blondel
Mobile operators gather daily massive datasets composed of records of calls placed by their customers. These records represent detailed information about the daily activity of large populations, enabling to study the behaviour of unprecedently large populations. In this research, we conduct several studies of such datasets in collaboration with European and African operators, with questions related to social dynamics and personal mobility.

KUMAR Alok (Starting date : February 2012)
Experimental updateless evolutive routing
Host : V. Blondel
The main objective of exploratory research is to investigate new routing paradigms so as to design, develop, and validate experimentally a distributed and dynamic routing scheme suitable for the future Internet and its evolution. We are investigating trade-offs between routing table size (to enhance scalability), routing scheme stretch (to ensure routing quality) and communication cost (to efficiently and timely react to various failures). The driving idea of this research project is to make use of the structural and statistical properties of the Internet topology as well as the stability and convergence properties of the Internet policy in order to specialize the design of a distributed routing scheme known to perform efficiently under dynamic network and policy conditions when these properties are met.

LEFEVRE Augustin (Starting date : October 2012)
Nonnegative matrix factorization
Host : P.-A. Absil
My main topic of research is matrix factorization for signal processing applications (blind sensor calibration in compressed sensing, blind deconvolution, sound source separation, hyperspectral imaging, graph clustering, etc.). I am also interested in (non-)smooth optimization on differential manifolds.

MARTIN Samuel (Starting date : January 2013)
Algebraic graph theory
Host : V. Blondel
Formal consensus theory aims at finding under which conditions a group can reach an agreement. The consensus theory applies to the control of fleets of vehicles, e.g. to reach velocity alignment (flocking). To analyze such system, I use graph robustness analysis for connectivity preservation of the interaction network. I also work with sociologists to analyze the social network linked to the controversy concerning off-road motorized leisure in France. We study links between different scenes using large graph visualization tools and actor centrality measures. Another interest of mine is the understanding of emergence of cooperation in networks using evolutionary game theory.

MOENS Luc (Starting date : October 1994)
Identification of a model for the real-time application for riverflow forecasting
HYDROMAX is a real-time application for riverflow forecasting which is developed by CESAME and is operational at SETHY (Service d'études hydrologiques, Walloon Ministry of Public Works - Belgium) for the management of flood alarms and the on-line information of the rescue services in the Meuse river basin. HYDROMAX is connected with the telemetering network of SETHY. This network is essentially made up of water level chart recorders and rain gauges.
HYDROMAX provides in real-time:

short term predictions of riverflows based on rainfal and past riverflow measurements
long term flood forecastings based on weather forecasts

ORBAN DE XIVRY Jean-Jacques (Starting date : December 2010)
Adapting to a changing environment
Host : Philippe Lefèvre
Our environment is very dynamic and we need to adapt our motor behavior continuously in order to perform everyday life tasks. Indeed, every task contains specific demands (writing vs. clapping) and our brain needs to take them into account. In addition, changes in our body due to age or disease also affect our movements and need to be considered. In this research, how the brain adapts to this changing environment is investigated by means of mathematical modeling, behavioral studies, patient studies and neurophysiological techniques.

ROCHA Luis (Starting date : October 2011)
Temporal structures in empirical social networks
Host : Vincent Blondel
The interactions between people create contact networks in which infections and information can potentially spread. Within this context, my research is mostly focused on creating new methods to characterize temporal structures of empirical networks (e.g. sexual and communication networks), and on studying how the temporal and topological structures affect dynamical processes taking place on these evolving networks (e.g. epidemics or opinion formation).

SUVISESHAMUTHU Easter Selvan (Starting date : September 2010)
Optimization algorithms : A statistical signal processing perspective
Host : P.-A. Absil
We consider foundational aspects of statistical signal processing pertaining to optimization techniques, with a particular interest for Riemannian optimization. On the application side, the focus is on blind source separation techniques in various medical domains.