All Years Seminars
[INMA] 2009-12-16 (16.15) : Colloquium CESAME-MAPA - Surfaces, espaces de Teichmüller et groupes de Lie
At Auditoire Ch.-J. de La Vallée Poussin (cycl 01)
Speaker :
Marc
BURGER (ULg)
Abstract :
Il est bien connu depuis Nielsen que les surfaces (compactes connexes sans bord) sont classifiées, à homéomorphisme près, par leur groupe fondamental.
Cette situation change radicalement lorsqu'on adopte le point de vue de la géométrie différentielle: une surface S donnée porte en effet de nombreuses structures Riemanniennes différentes.
En fait, à toute métrique hyperbolique sur S on associe une représentation linéaire de son groupe fondamental $\pi_1(S)$ à valeurs dans SL(2,R).
On obtient ainsi un ensemble de représentations, appelé "espace de Teichmüller", qui porte une structure très riche. L'objet de cet exposé est de décrire
ce qui se produit lorsqu'on remplace SL(2,R) par un groupe de Lie de dimension supérieure.
[INMA] 2009-12-15 (14.00) : Filtering on manifolds
At Euler building
Speaker :
Rentmeesters, Quentin
Abstract :
In many applications related to signal processing and image processing, a
filtering technique is required to reduce the influence of perturbations
on the measurements. In this talk, we will see different approaches to
implement such a filtering technique when the measurements belong to a
nonlinear space. We will illustrate these techniques on the sphere and
show some concrete applications related to the Grassmann manifold.
[INMA] 2009-12-08 (14.00) : Modelling Competitive Coexistence in a Chemostat by LQ Type State Feedback
At Euler building
Speaker :
Charlotte Beauthier (FUNDP)
Abstract :
The chemostat model describes the interaction of microbial species which
are competing for a single nutrient. It has been used for different systems such
as lakes, waste-water treatment processes and biological reactors producing ge-
netically altered organisms. The competitive exclusion principle is a theoretical
result which states that the competition process yields at best a single winning
species in the long run. However, in nature, many species seem to coexist. This
contradiction between the theory and the real world is our main motivation for
modifying the model in order to try to bring them in better accordance.
In this talk, the problem is stated in terms of an LQ-optimal control problem
for a linearized system in order to garantee the coexistence of species for the
(nonlinear) chemostat model. Conditions are reported such that the computed
linear state feedback can be applied to the nonlinear system and garantee the
well-posedness of the model and in particular the coexistence of the species.
The results are illustrated by some numerical experiments.
This contribution is a joint work in progress with Joseph Winkin (FUNDP)
and Denis Dochain (UCL).
[INMA] 2009-12-01 (14.00) : Learning about near-optimal policies from a sample of trajectories
At Euler building
Speaker :
Damien Ernst (ULg)
Abstract :
In many areas of the medical, social and engineering
sciences the following planning problem arises. A training set of
trajectories representing the system state, the actions and the
instantaneous rewards at successive discrete instants is known and a
decision policy leading to high cumulative rewards has to be found. In
the first part of this talk, I will present some batch mode
reinforcement learning algorithms for solving these types of problems.
Afterwards, I will report on some recent work to build lower bounds on
the performance of a policy from trajectories. Finally, I will show
that this work has opened a frontier in batch mode reinforcement
learning whose final implications we cannot yet guess.
[INMA] 2009-11-24 (14.00) : Rate-Optimized Power Allocation for Relayed OFDM transmission in different scenarii
At Euler building
Speaker :
Vandendorpe, Luc
Abstract :
We consider an OFDM (orthogonal frequency division multiplexing) point-to-point transmission scheme which is enhanced by
means of a relay. Symbols sent by the source during a first time slot may be (but are not necessarily) retransmitted by the relay
during a second time slot. The relay is assumed to be of the DF (decode-and-forward) type. For each relayed carrier, the destination
implements maximum ratio combining. Two protocols are considered. Assuming perfect CSI (channel state information), the
paper investigates the power allocation problem so as to maximize the rate offered by the scheme for two types of power constraints.
Both cases of sum power constraint and individual power constraints at the source and at the relay are addressed. The theoretical
analysis is illustrated through numerical results for the two protocols and both types of constraints.
Then the case where multiple relays are active will be considered. Finally a multiple access scheme with 2 users will be discussed.
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