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4 événements

  • Algèbre Géométrie

    Mardi 10 avril 2018 10:00-11:00 - Nicolas Perrin - UVSQ

    Nicolas Perrin : la représentation de Steinberg

    Lieu : Fermat 2205

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  • Probabilités Statistiques

    Mardi 10 avril 2018 11:30-12:30 - Erwan Scornet - Ecole Polytechnique

    Consistency and minimax rates of random forests

    Résumé : The recent and ongoing digital world expansion now allows anyone to have access to a tremendous amount of information. However collecting data is not an end in itself and thus techniques must be designed to gain in-depth knowledge from these large data bases. This has led to a growing interest for statistics, as a tool to find patterns in complex data structures, and particularly for turnkey algorithms which do not require specific skills from the user.
    Such algorithms are quite often designed based on a hunch without any theoretical guarantee. Indeed, the overlay of several simple steps (as in random forests or neural networks) makes the analysis more arduous. Nonetheless, the theory is vital to give assurance on how algorithms operate thus preventing their outputs to be misunderstood.
    Among the most basic statistical properties is the consistency which states that predictions are asymptotically accurate when the number of observations increases. In this talk, I will present a first result on Breiman’s forests consistency and show how it sheds some lights on its good performance in a sparse regression setting. I will also present new results on minimax rates of Mondrian forests which highlight the benefits of forests compared to individual regression trees.

    Lieu : bâtiment Fermat, en salle 2102

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  • Algèbre Géométrie

    Mardi 10 avril 2018 11:30-12:30 - Mattia Cafasso - Université d'Angers

    Mattia Cafasso : Le modèle de Kontsevich : équations de Painlevé et universalité

    Résumé : Le modèle de matrices de Kontsevich est un des outils essentiels de la première preuve (celle de Kontsevich) de la conjecture de Witten. Pendant mon exposé, je discuterai la relation entre ce modèle et une série d’équations aux dérivées partielles connues sous le nom de "hiérarchie de la première équation de Painlevé". Ensuite, j’exposerai quelques résultats sur l’universalité (dans le sens des matrices aléatoires) du même modèle. Si le temps le permets, je parlerai aussi de possibles généralisations aux modèles de Kontsevich dites ’’généralisés".

    Lieu : Fermat 2205

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  • Cryptographie

    Mardi 10 avril 2018 14:00-15:00 - Ferdinand Sibleyras - Inria Paris

    The Missing Difference Problem, and its Applications to Counter Mode Encryption

    Résumé : The widely deployed counter mode (CTR) is known for its efficiency and simplicity as it comes with a security proof that guarantees no attack up to the birthday bound and a matching distinguishing attack. However, unlike in CBC mode, a ciphertext collision in CTR mode hardly reveals anything to the attacker. Therefore we define an algorithmic problem, the missing difference problem, and show how its resolution leads to a message recovery attack with complexity close to the birthday bound. As a further result efficiently solving this problem also allows to describe an universal forgery attack against polynomial MACs such as GMAC and Poly1305 in complexity Õ(2^(2n/3)).
    This is a joint work with Gaëtan Leurent.

    Lieu : Bât. Descartes, Salle 301

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