All Years Seminars
[INMA] 2013-04-23 (14.00) : Resilient distributed network dynamics
At Euler building
Speaker :
Como, Giacomo
Abstract : Resilience has become a key issue in the analysis and design of network dynamics.
Applications are of wide range and include complex infrastructure systems, such as data, transportation and power networks, as well as social, economic and financial networks. Given a networked dynamical system, one is interested in how perturbations, typically localized on small parts of the network, can propagate through cascading mechanisms and possibly have a systemic impact on the global behavior.
This talk will focus on resilience properties of distributed routing in dynamical flow networks. Dynamical flow networks are modeled as ordinary differential equation describing mass conservation with routing policies regulating the way the total outflow at each non-destination node gets split among its outgoing links using local information. Disturbances are modeled as (possibly adversarial) reductions of the link flow capacities. A class of maximally robust distributed policies and show that, if the routing policies are allowed to use local information only from the links immediately downstream, then the resilience is in general strictly smaller than the min-cut capacity of the network, while there is no such a loss if the policies are allowed to use local information also from the links immediately upstream.
Time permitting, some recent results on resilience of distributed averaging dynamics on large-scale networks will be also presented.
[INMA] 2013-04-16 (14.00) : Policy Iteration has exponential complexity for solving Markov Decision Processes. But there is more to the story...
At Euler building
Speaker :
Hollanders, Romain
Abstract : Many decision problems can be formulated as Markov Decision Processes and solved efficiently using the Policy Iteration algorithm, an algorithm
that is closely related to the simplex algorithm for Linear Programming.
Efficiently... unfortunately only in practice since it has been shown, like the simplex algorithm, to require an exponential number of steps to
converge in the worst case. However, the story does not end here. In this talk, we highlight particular problem structures for which Policy
Iteration is guaranteed to converge in polynomial time. We also propose an elegant combinatorial problem whose solution would provide tighter upper
bounds on the real worst case complexity of Policy Iteration.
[INMA] 2013-04-03 (14.00) : Model Reduction by Moment Matching
At Euler building
Speaker :
Alessandro Astolfi (Imperial College London)
Abstract : The model reduction problem for (single-input, single-output) linear and nonlinear systems is addressed using the notion of moment. A re-visitation of the linear theory allows to obtain novel results for linear systems and to develop a nonlinear enhancement of the notion of moment. This, in turn, is used to pose and solve the model reduction problem by moment matching for nonlinear systems, to develop a notion of frequency response for nonlinear systems, and to solve model reduction problems in the presence of constraints on the reduced model. Connections between the proposed results and projection methods are presented.
[INMA] 2013-03-19 (14.00) : On solving KKT linear systems arising in Model Predictive Control
At Euler building
Speaker :
Nicola Mastronardi (Istituto per le Applicazioni del Calcolo M. Picone, Consiglio Nazionale delle Ricerche, sede di Bari )
Abstract : The approximate solution of Model Predictive Control problems [2,3,4] is often computed in an iterative fashion, requiring to compute, at each iteration, the solution of a quadratic optimization problem. The most expensive part of the latter problem is the solution of symmetric indefinite KKT systems, where the involved matrices are highly structured.
Recently, an algorithm for computing a block anti--triangular factorization of symmetric indefinite matrices, based on orthogonal transformations, has been proposed[1].
The aim of this talk is to show that such a factorization, implemented in a suitable way, can be efficiently used for solving the mentioned KKT linear systems. This talk is based on a joint work with Paul Van Dooren.
References : [1] N. Mastronardi, P. Van Dooren, The anti-triangular factorization of symmetric matrices, SIAM Journal on Matrix Analysis and Applications, to appear.--
[2] C. Kirches, H. Bock, J.P. Schloder, S. Sager, A factorization with update procedures for a KKT matrix arising in direct optimal control, Mathematical Programming Computation, 3(4), pp.319348, 2011 --
[3] Y. Wang, S. Boyd, Fast Model Predictive Control Using Online Optimization, IEEE Transactions on Control Systems Technology, 18(2), pp.267-278, 2010 -- [4] V.M. Zavala, C.D. Laird, L,T. Biegler,
A fast moving horizon estimation algorithm based on nonlinear programming sensitivity, Journal of Process Control, 18(9), pp.876-884, 2008.
[INMA] 2013-02-26 (14.00) : Model-assisted integration of genetic and environmental constraints for the improvement of crop water use
At Euler building
Speaker :
Xavier Draye (UCL, ELI)
Abstract : Most crops are heavily dependent on water capture to produce new biomass. In a context of water scarcity, improving water capture has become an obligate step towards meeting an increased demand for agricultural products. Unfortunately, water capture has proven to be amongst the most difficult traits to improve, despite major technological advances in plant biology. Integrating processes operating at different scales while taking into account the spatial and temporal dynamics of water fluxes in the soil-plant-atmosphere system appears to be a major challenge to dissect the components of water capture and, hopefully, develop novel and achievable ideotypes. To address this challenge, a small community of plant scientists has started to develop functional structural plant models that provide an mathematical framework to integrate the incomplete set of experimental data, test hypotheses in silico and, in some cases, to predict crop behavior in complex environments.
The objective of this seminar is to attract the INMA community to these scientific challenges and try to establish new collaborations between our institutes. Similar initiatives have been made in many prestigious universities and the payback is generally clear.
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