All Years Seminars
[INMA] 2011-02-22 (15.00) : Eigenvalue based techniques for the stability analysis and robust control of linear systems with time-delay
At Euler building
Speaker :
Wim MICHIELS (KUL)
Abstract : Time-delay systems are a subclass of infinite-dimensional system. They have a variety of applications in engineering,
economics and the bio-sciences. Two mainstream control design approaches for linear time-delay systems can be distinguished in the literature.
The first type is based on applying a generalization of the classical systems and control theory to infinite-dimensional systems.
It mostly results in infinite-dimensional or distributed controllers which may be hard to implement in practical applications where
the controller structure is fixed or restricted, or in controllers that include observers, requiring an on-line numerical simulation
of the systems' equations. The second approach consists of identifying approximate finite-dimensional models of low order, and applying
the existing design methods that typically yield controllers whose dimensions are larger or equal than the dimension of the plant model.
As a drawback the properties of the resulting closed-loop system may heavily depend on the accuracy of the approximation, and the design
involves a trade-off between accuracy and reliability on the one hand and the feasibility of the controller implementation on the other hand.
In my presentation I will give an overview of recent work in my group on control design methods that aim at bridging the gap between the two
types of approaches described above, by designing directly controllers for a broad class of linear time-delay systems (without starting
from a low-order approximation), where the controller structure or order is a-priori specified (e.g. imposed from practical considerations).
These methods are based on a direct optimization of appropriately defined cost functions and inspired by recent work on low-order control
design for finite-dimensional systems within an eigenvalue optimization framework. The analysis and design problems under consideration include
the stabilization problem and the computation and optimization of H2 and H-infinity type cost functions.
Many of the methods discussed heavily rely on the property that eigenvalue problems associated with linear time-delay systems, which
are usually finite-dimensional and nonlinear, can be equivalently expressed as infinite-dimensional linear eigenvalue problems.
I will point out how the underlying linearization approach can be generalized, resulting in generic computational tools for nonlinear eigenvalue
problems.
[INMA] 2011-02-15 (15.00) : The role of second trials in cascades of information over networks
At Euler building
Speaker :
Krings, Gautier
Abstract : We study the propagation of information in social networks. To do so, we focus on a cascade model where nodes are infected with
probability p_1 after their first contact with the information and with probability p_2 at all subsequent contacts. The diffusion starts
from one random node and leads to a cascade of infection. It is shown that first and subsequent trials play different roles in the propagation
and that the size of the cascade depends in a nontrivial way on p_1, p_2, and on the network structure. Second trials are shown to amplify
the propagation in dense parts of the network while first trials are dominant for the exploration of new parts of the network and launching
new seeds of infection.
[INMA] 2011-02-10 (16.30) : COLLOQUIUM CESAME/MAPA: Public Goods, Free Riders and the Emergence of Sanctions
At CYCL01
Speaker :
Karl SIGMUND (University of Vienna, Austria)
Abstract : Punishment of free-riders is generally viewed as an important factor in promoting cooperation. But since it is often costly to sanction
exploiters, the emergence of such a behavior and its stability raise interesting problems. Players who do not contribute to the sanctions, but profit from
the increased level of cooperation caused by them, act as 'second-order exploiters' and threaten the joint enterprise. In this paper, we review the role
of voluntary participation in establishing and upholding cooperation with or without punishment. In particular, we deal with two distinct forms of
punishment, namely peer punishment and pool punishment, and compare their stability and their efficiency. The emergence and upkeep of collaborative
undertakings can strongly depend on whether participation is voluntary or mandatory. The possibility to opt out of a joint enterprise often helps
in curbing exploiters and boosting pro-social behavior.
[INMA] 2011-02-01 (15.00) : Derivative-Free Optimization on Matrix Manifolds: Introduction and applications
At Euler building
Speaker :
Borckmans, Pierre
Abstract : Optimization on manifolds is a recent field that is gaining more and more interest. Many applications can naturally be expressed on manifolds,
in order to tackle structure constraints (orthogonality, ...) or invariance properties (rotation, scaling, ...). However, some of these problems may be
non-smooth or non-differentiable, therefore preventing the legit/efficient use of classical optimization algorithms on manifolds (Descent, Newton, ...).
In this talk, we focus on derivative-free optimization on Riemannian manifolds. We briefly present the concept of optimization on manifold, then we
focus on a specific algorithm, namely Particle Swarm Optimization (PSO). We show how PSO can be adapted to matrix manifolds, before applying it
to two practical applications: (1) Oriented Bounding Box Computation (P.B.Borckmans and P.-A. Absil), (2) Low Multilinear-Rank Tensor Approximation
(P.B.Borckmans, M. Isheva and P.-A.Absil). These two problems illustrate the feasibility of the proposed method.
[INMA] 2010-12-21 (15.00) : Some basics of quantum control: introduction and application
At Euler building
Speaker :
Sarlette, Alain
Abstract : The goal of this talk is to give a simple introduction to some specificities of "quantum control", that is the design and analysis of systems
evolving according to the laws of quantum dynamics. The essence of this subject is in fact relatively simple to convey to applied mathematicians: a major
success of quantum mechanics is its elegant mathematical formulation. Recently, experimental systems with simple enough quantum dynamics have been built,
allowing control engineers to join the research domain. After introducing basic aspects of quantum dynamics in detail, a few of our research results
on actual systems will be used for illustration.
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