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Research talks

The Lüroth problem asks whether every unirational variety is rational. Over the complex numbers, it has a positive answer for curves and surfaces, but fails in higher dimensions. In this talk, I will consider the Lüroth problem for real algebraic varieties that are geometrically rational, and explain a counterexample not accounted for by the topology of the real locus or by unramified cohomology. This is joint work with Olivier Wittenberg.

14M20 ; 14E08

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Research talks

Orders in finite-dimensional algebras over number fi give rise to interesting locally symmetric spaces and algebraic varieties. Hilbert modular varieties or arithmetically defined hyperbolic 3-manifolds, compact ones as well as noncompact ones, are familiar examples. In this talk we discuss various cases related to the general linear group $GL(2)$ over orders in division algebras defined over some number field. Geometry, arithmetic, and the theory of automorphic forms are interwoven in a most fruitful way in this work. Finally we indicate a construction of non-vanishing square-integrable cohomology classes for such arithmetically defined groups. Orders in finite-dimensional algebras over number fi give rise to interesting locally symmetric spaces and algebraic varieties. Hilbert modular varieties or arithmetically defined hyperbolic 3-manifolds, compact ones as well as noncompact ones, are familiar examples. In this talk we discuss various cases related to the general linear group $GL(2)$ over orders in division algebras defined over some number field. Geometry, arithmetic, and the ...

11F75 ; 11F55

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Research talks

Let $X$ be a compact Kähler manifold. The so-called Kodaira problem asks whether $X$ has arbitrarily small deformations to some projective varieties. While Kodaira proved that such deformations always exist for surfaces. Starting from dimension 4, there are examples constructed by Voisin which answer the Kodaira problem in the negative. In this talk, we will focus on threefolds, as well as compact Kähler manifolds of algebraic dimension $a(X) = dim(X) -1$. We will explain our positive solution to the Kodaira problem for these manifolds. Let $X$ be a compact Kähler manifold. The so-called Kodaira problem asks whether $X$ has arbitrarily small deformations to some projective varieties. While Kodaira proved that such deformations always exist for surfaces. Starting from dimension 4, there are examples constructed by Voisin which answer the Kodaira problem in the negative. In this talk, we will focus on threefolds, as well as compact Kähler manifolds of algebraic dimension $a(X) = ...

32J17 ; 32J27 ; 32J25 ; 32G05 ; 14D06 ; 14E30

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Research talks

​We consider photoacoustic tomography in the presence of approximation and modelling errors. The inverse problem, i.e. estimation of the initial pressure from photoacoustic time-series measured on the boundary of the target, is approached in the framework of Bayesian inverse problems. The posterior distribution is examined in situations in which the forward model contains errors or uncertainties for example due to numerical approximations or uncertainties in the acoustic parameters. Modelling of these errors and its impact on the posterior distribution are investigated.
This is joint work with Teemu Sahlstrm, Jenni Tick and Aki Pulkkinen. ​We consider photoacoustic tomography in the presence of approximation and modelling errors. The inverse problem, i.e. estimation of the initial pressure from photoacoustic time-series measured on the boundary of the target, is approached in the framework of Bayesian inverse problems. The posterior distribution is examined in situations in which the forward model contains errors or uncertainties for example due to numerical approximations or ...

35R30 ; 35Q60 ; 65R32 ; 65C20 ; 92C55

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Research talks

​I will discuss recent developments concerning the non-uniqueness of distributional solutions to the Navier-Stokes equation.

35Q30 ; 76D05 ; 35Q35

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Research schools

A general introduction to the state of the art in counting of rational and integral points on varieties, using various analytic methods with the Brauer-Manin obstruction.

14G05 ; 14F22

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Research schools

68Q25 ; 68Q42 ; 90C39 ; 92D20 ; 92E10

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Research talks

We will discuss several new ideas that can show the existence of jet differential operators on arbitrary projective varieties, and also on general hypersurfaces of $\mathbb{P}^n$ of sufficiently high degree. These results can be applied to improve degree bounds in several hyperbolicity problems and especially in the proof of the Kobayashi conjecture.

32Q45 ; 32L10 ; 53C55 ; 14J40

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Ecoles de recherche

After giving a motivation of graph databases and an overview of the main data models, we will dive into foundational aspects of graph database query languages, with a strong focus on regular path queries (RPQs) and conjunctive regular path queries (CRPQs). We will consider the different semantics that graph database systems use for such queries (every path, simple path, trail), and we will look into the computational complexities of query evaluation and query containment.
After having gone through these foundations, we plan to do some excursions into connections between tree-structured and graph-structured data, adding data value comparisons, and aspects of real-life queries. After giving a motivation of graph databases and an overview of the main data models, we will dive into foundational aspects of graph database query languages, with a strong focus on regular path queries (RPQs) and conjunctive regular path queries (CRPQs). We will consider the different semantics that graph database systems use for such queries (every path, simple path, trail), and we will look into the computational complexities of query ...

68P15 ; 68Q19

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Research talks

14J29 ; 11R29 ; 22E40

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Research talks

14J29 ; 11R29 ; 22E40

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Research schools

This course presents an overview of modern Bayesian strategies for solving imaging inverse problems. We will start by introducing the Bayesian statistical decision theory framework underpinning Bayesian analysis, and then explore efficient numerical methods for performing Bayesian computation in large-scale settings. We will pay special attention to high-dimensional imaging models that are log-concave w.r.t. the unknown image, related to so-called “convex imaging problems”. This will provide an opportunity to establish connections with the convex optimisation and machine learning approaches to imaging, and to discuss some of their relative strengths and drawbacks. Examples of topics covered in the course include: efficient stochastic simulation and optimisation numerical methods that tightly combine proximal convex optimisation with Markov chain Monte Carlo techniques; strategies for estimating unknown model parameters and performing model selection, methods for calculating Bayesian confidence intervals for images and performing uncertainty quantification analyses; and new theory regarding the role of convexity in maximum-a-posteriori and minimum-mean-square-error estimation. The theory, methods, and algorithms are illustrated with a range of mathematical imaging experiments. This course presents an overview of modern Bayesian strategies for solving imaging inverse problems. We will start by introducing the Bayesian statistical decision theory framework underpinning Bayesian analysis, and then explore efficient numerical methods for performing Bayesian computation in large-scale settings. We will pay special attention to high-dimensional imaging models that are log-concave w.r.t. the unknown image, related to ...

49N45 ; 65C40 ; 65C60 ; 65J22 ; 68U10 ; 62C10 ; 62F15 ; 94A08

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Research schools

Le but de ce cours sera de présenter quelques techniques liées aux processus de Schur, dans le cadre le plus simple de la mesure de Plancherel sur les partitions d'entiers.
La mesure de Plancherel est une mesure sur l'ensemble des partitions d'un entier n, où une partition donnée apparaît avec une probabilité proportionnelle au carré de son nombre de tableaux de Young standard. Cette mesure apparaît très naturellement en lien avec le fameux problème de Ulam-Hammersley, qui consiste à étudier la longueur d'une plus longue sous-suite croissante d'une permutation uniforme de {1,...,n}. Il est en fait fructueux de travailler avec une version "poissonisée" du problème, où la taille n est tirée selon une loi de Poisson, dont on fera tendre le paramètre vers l'infini afin d'étudier les asymptotiques.
Dans la première séance, nous verrons que la mesure de Plancherel poissonisée est en fait un processus déterminantal, dont le noyau de corrélation fait intervenir les fonctions de Bessel. Nous utiliserons pour cela le formalisme de l'espace de Fock fermionique. (Toutes les notions nécessaires seront introduites au fur et à mesure, de la manière la plus élémentaire possible.)
Dans la seconde séance, nous étudierons les différentes asymptotiques du noyau de corrélation, par une application élégante de la méthode du col due à Okounkov et Reshetikhin. Nous verrons en particulier apparaître un phénomène de forme-limite, le noyau sinus discret dans le cas des limites "bulk" et le noyau d'Airy dans la limite "edge". In fine, nous aboutirons à une preuve du théorème de Baik-Deift-Johansson (1998) énonçant que les fluctuations de la longueur d'une plus longue sous-suite croissante d'une permutation uniforme ont asymptotiquement la même distribution que la plus grande valeur propre d'une matrice hermitienne aléatoire. Le but de ce cours sera de présenter quelques techniques liées aux processus de Schur, dans le cadre le plus simple de la mesure de Plancherel sur les partitions d'entiers.
La mesure de Plancherel est une mesure sur l'ensemble des partitions d'un entier n, où une partition donnée apparaît avec une probabilité proportionnelle au carré de son nombre de tableaux de Young standard. Cette mesure apparaît très naturellement en lien avec le fameux ...

05A17 ; 05E10 ; 60C05 ; 60G55

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Research talks

We proved that any K-semistable log Fano cone admits a special degeneration to a uniquely determined K-polystable log Fano cone. This confirms a conjecture of Donaldson-Sun stating that the metric tangent cone of any close point appearing on a Gromov-Hausdorff limit of Kähler-Einstein Fano manifolds depends only on the algebraic structure of the singularity. This is a joint work with Chi Li and Chenyang Xu.

14J45 ; 32Q15 ; 32Q20 ; 53C55

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- xvii; 261 p.
ISBN 978-2-85629-895-4

Séminaires et congrès , 0032

Localisation : Collection 1er étage

théorie spectrale # opérateur non borné # opérateur discret # extension auto-adjointe # théorie des perturbations # opérateur de Schrödinger # graphe combinatoire # graphe quantique # variété CR # graphe aléatoire

35Pxx ; 39Axx ; 58JXX ; 05Cxx ; 47Axx ; 81Q35 ; 32V30 ; 82B44

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- 258 p.
ISBN 978-83-86806-38-6

Banach center publications , 0116

Localisation : Périodique 1er étage

géométrie algébrique # géométrie différentielle # variété hyper-kählérienne # surface K3 # variété de Calabi-Yau # courbe # hyperplan # conjecture de Nagata # corps de Newton-Okounkov

14-06 ; 53-06 ; 14J28 ; 14E15 ; 14J32 ; 14C20 ; 53C26 ; 32L05 ; 14J99 ; 32Q15 ; 13F30

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- vi; 139 p.
ISBN 978-2-271-12611-5

Localisation : Colloque 1er étage (MARS);Ouvrage RdC (INFO)

informatique mathématique # transduction # mot sturmien # maillage 3D # poset, polynôme, polytope (popopo) # algorithmique distribuée # système d'agents mobiles

68Q45 ; 68R15 ; 68P20 ; 94A08 ; 52A25 ; 06A07

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- 258 p.
ISBN 978-83-86806-43-0

Banach center publications , 0117

Localisation : Périodique 1er étage

grassmannienne lagrangienne # système différentiel extérieur # géométrie de la boule de Lie # dualité projective

58A15 ; 35A30 ; 53C30 ; 53D10 ; 14N05 ; 14M17 ; 53A40 ; 53A35

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- ix; 231 p.

Localisation : The University of Wisconsin Mathematical Research Center series

théorie des codes # histoire de la théorie des codes # topologie # code convolutionnel # code avec retour d'information # code algébrique

00B25 ; 94-06

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