All Years Seminars
[INMA] 2010-03-02 (14.00) : Numerical properties of equation-free methods for multiscale problems
At Euler building
Speaker :
Giovanni
SAMAEY
(CESAME)
Abstract :
In quite a number of problems, there exists a wide gap between the (microscopic) level at which a system is modeled and the (macroscopic)
space and time scales at which one would like to observe the system. Equation-free methods try to bridge this gap by constructing a coarse time-stepper
for the macroscopic observables as a three-step procedure: (1) lift (from macro to micro); (2) evolve (using the microscopic model); and (3) restrict (from micro to macro).
We will discuss both how to construct lifting operators to obtain a well-defined coarse time-stepper, as well as what macroscopic tasks can be performed once a coarse time-stepper is available.
Particular emphasis will be put on the numerical properties of the resulting methods (accuracy and efficiency) and on the use of approximate macroscopic models to accelerate the computations.
Collaborators that have contributed to this work will be acknowledged throughout the talk.
[INMA] 2010-02-23 (14.00) : Stability of Asynchronous Feedback-Interconnected Dissipative Systems
At Euler building
Speaker :
Manuel
MARTINEZ (University of Seville
)
Abstract :
This work presents a method to analyze the stability of the
feedback interconnection of a class of dissipative linear systems
when the sampling associated to the feedback interconnection is
asynchronous. We consider systems that are either Input/Output
Strictly passive (IOSP), or systems which have bounded L2-gains
less than one. The analysis is performed by using the concept of
MAximum Sampling time preserving Dissipation (MASD), for each
interconnected system. We investigate the impact of using the
scattering transformation in the computation of the MASD, and we
provide a numerical algorithm (based on a set of LMI's) that allows
to choose the most suitable configuration for the interconnection.
[INMA] 2010-02-18 (16.30) : Colloquium CESAME-MAPA - Random walks on graphs: what for?
At Auditoire Ch.-J. de La Vallée Poussin (cycl 01)
Speaker :
Delvenne, Jean-Charles
Abstract :
How does Google work? What is so magical about expander graph? How do we
detect communities in large social graphs? The three answers involve one
same tool: random walks on graphs.
Given a graph, one may start from a vertex and, at each time step, jump
randomly to one of its neighbours. This random walk transforms the graph
into a Markov chain. The study of this Markov chain provides a lot of
insight into the combinatorial properties of the graph. We will cover a
sample of applications of this technique.
[INMA] 2010-02-11 (16.30) : Colloquium CESAME-MAPA - Géométrie non commutative et théorie de Lie.
At Auditoire Euler
Speaker :
Pierre
BIELIAVSKY (UCL)
Abstract :
Mon but est d'exposer de manière élémentaire les concepts à la base de
la géométrie non commutative d'Alain Connes et, si le temps le permet,
de montrer certaines implications en géométrie différentielle et en
théorie de Lie.
L'ensemble C(X) des fonctions continues à valeurs complexes sur un
espace topologique X a naturellement une structure d'algèbre
associative commutative. En effet, étant données deux fonctions
f:X-->C et g:X-->C, leur produit est la fonction f.g définie par
(f.g)(x)=f(x).g(x).
Dans de nombreux cas, cette structure d'algèbre encode complètement la
structure topologique de l'espace X. Ce résultat classique,
dû à I. Guelfan'd, fournit un outil algébrique pour l'étude des
espaces topologiques.
Dans les années 80, Alain Connes introduit la géométrie différentielle
non commutative basée sur l'idée que des situations géométriques
apparemment disparates, incluant la géométrie riemannienne et la
théorie des feuilletages, peuvent elles aussi être encodées par une
notion algébrique commune: celle de triplet spectral.
Dans mon exposé, j'introduirai les paradigmes de bases de cette
théorie en me basant sur un concept émergeant de la théorie quantique:
la notion de déformation.
[INMA] 2010-02-09 (14.00) : Numerical methods for the best low multilinear rank approximation of higher-order tensors.
At Euler building
Speaker :
Ishteva, Mariya
Abstract :
Higher-order tensors are generalizations of vectors and matrices to third-
or even higher-order arrays of numbers. The multilinear rank of a tensor
is a direct generalization of column and row rank of a matrix. In this
talk we discuss the best low multilinear rank approximation. Given a
higher-order tensor, we are looking for another tensor, as close as
possible to the original one and with multilinear rank bounded by
prespecified numbers. This approximation is used for dimensionality
reduction and signal subspace estimation in higher-order statistics,
biomedical signal processing, telecommunications and many other fields. We
consider two particular applications, present several algorithms for the
computation of the approximation and discuss the issue of local minima.
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