Mardi 10 Mars
Heure: 
12:30  13:30 
Lieu: 
Salle B107, bâtiment B, Université de Villetaneuse 
Résumé: 
Multiband Robust Optimization: theory and applications 
Description: 
Fabio D'Andreagiovanni Over the last years, Robust Optimization (RO) has emerged as an effective and efficient methodology to tackle data uncertainty in realworld optimization problems. RO takes into account data uncertainty in the shape of hard constraints that restrict the feasible set and maintain only robust solutions, i.e. solutions that remain feasible even when the values of the input data change. In this talk, we provide an overview of our research about theory and applications of RO. Specifically, we present Multiband Robustness, a new model for RO that we recently proposed to generalize and refine the classical Gammarobustness model by Bertsimas and Sim. The main aim of our new model is to provide a refined representation of arbitrary non symmetric distributions of the uncertainty, that are commonly present in realworld applications. Such refined representation grants a reduction in conservatism of robust solutions, while maintaining the accessibility and computational tractability that have been a key factor of success of Gammarobustness. We also provide an overview of applications of the Multiband model to realworld problems that we have considered in past and ongoing research and industrial projects. 
Heure: 
14:00  17:00 
Lieu: 
Salle B107, bâtiment B, Université de Villetaneuse 
Résumé: 
Une généralisation des mots de Christoffel en dimension d. 
Description: 
Sébastien Labbé In this work, we extend the definition of Christoffel words todirected subgraphs of the hypercubic lattice in arbitrary dimensionthat wecall Christoffel graphs. Christoffel graphs when $d=2$ correspond towellknown Christoffel words.We show that Christoffel graphs have similar properties to those ofChristoffel words: symmetry of their central part and conjugation withtheirreversal. Our main result extends Pirillo's theorem (characterization ofChristoffel words which asserts that a word $amb$ is a Christoffelword if andonly if it is conjugate to $bma$) in arbitrary dimension.In the generalization, the map $ambmapsto bma$ is seen as a flipoperation ongraphs embedded in $mathbb{Z}^d$ and the conjugation is a translation.We show that a fully periodic subgraph of the hypercubic lattice is atranslate of its flip if and only if it is a Christoffel graph.This is joint work with Christophe Reutenauer.Preprint is available at http://arxiv.org/abs/1404.4021. 

