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# Documents  | enregistrements trouvés : 200

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## Post-edited  Understanding the growth of Laplace eigenfunctions (part 1 of 2) Canzani, Yaiza (Auteur de la Conférence) | CIRM (Editeur )

In this talk we will discuss a new geodesic beam approach to understanding eigenfunction concentration. We characterize the features that cause an eigenfunction to saturate the standard supremum bounds in terms of the distribution of $L^{2}$ mass along geodesic tubes emanating from a point. We also show that the phenomena behind extreme supremum norm growth is identical to that underlying extreme growth of eigenfunctions when averaged along submanifolds. Using the description of concentration, we obtain quantitative improvements on the known bounds in a wide variety of settings.
In this talk we will discuss a new geodesic beam approach to understanding eigenfunction concentration. We characterize the features that cause an eigenfunction to saturate the standard supremum bounds in terms of the distribution of $L^{2}$ mass along geodesic tubes emanating from a point. We also show that the phenomena behind extreme supremum norm growth is identical to that underlying extreme growth of eigenfunctions when averaged along ...

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## Post-edited  The symplectic type of congruences between elliptic curves Cremona, John (Auteur de la Conférence) | CIRM (Editeur )

In this talk I will describe a systematic investigation into congruences between the mod $p$ torsion modules of elliptic curves defined over $\mathbb{Q}$. For each such curve $E$ and prime $p$ the $p$-torsion $E[p]$ of $E$, is a 2-dimensional vector space over $\mathbb{F}_{p}$ which carries a Galois action of the absolute Galois group $G_{\mathbb{Q}}$. The structure of this $G_{\mathbb{Q}}$-module is very well understood, thanks to the work of J.-P. Serre and others. When we say the two curves $E$ and $E'$ are ”congruent” we mean that $E[p]$ and $E'[p]$ are isomorphic as $G_{\mathbb{Q}}$-modules. While such congruences are known to exist for all primes up to 17, the Frey-Mazur conjecture states that p is bounded: more precisely, that there exists $B$ > 0 such that if $p > B$ and $E[p]$ and $E'[p]$ are isomorphic then $E$ and $E'$ are isogenous. We report on work toward establishing such a bound for the elliptic curves in the LMFDB database. Secondly, we describe methods for determining whether or not a given isomorphism between $E[p]$ and $E'[p]$ is symplectic (preserves the Weil pairing) or antisymplectic, and report on the results of applying these methods to the curves in the database.
This is joint work with Nuno Freitas (Warwick).
In this talk I will describe a systematic investigation into congruences between the mod $p$ torsion modules of elliptic curves defined over $\mathbb{Q}$. For each such curve $E$ and prime $p$ the $p$-torsion $E[p]$ of $E$, is a 2-dimensional vector space over $\mathbb{F}_{p}$ which carries a Galois action of the absolute Galois group $G_{\mathbb{Q}}$. The structure of this $G_{\mathbb{Q}}$-module is very well understood, thanks to the work of ...

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## Post-edited  The conjugacy problem for polynomially growing elements of $Out(F_{n})$ Feighn, Mark (Auteur de la Conférence) | CIRM (Editeur )

(joint work with Michael Handel) $Out(F_{n}) := Aut(F_{n})/Inn(F_{n})$ denotes the outer automorphism group of the rank n free group $F_{n}$. An element $f$ of $Out(F_{n})$ is polynomially growing if the word lengths of conjugacy classes in Fn grow at most polynomially under iteration by $f$. The existence in $Out(F_{n}), n > 2$, of elements with non-linear polynomial growth is a feature of $Out(F_{n})$ not shared by mapping class groups of surfaces.
To avoid some finite order behavior, we restrict attention to the subset $UPG(F_{n})$ of $Out(F_{n})$ consisting of polynomially growing elements whose action on $H_{1}(F_{n}, Z)$ is unipotent. In particular, if $f$ is polynomially growing and acts trivially on $H_{1}(F_{n}, Z_{3})$ then $f$ is in $UPG(F_{n})$ and further every polynomially growing element of $Out(F_{n})$ has a power that is in $UPG(F_{n})$. The goal of the talk is to describe an algorithm to decide given $f,g$ in $UPG(F_{n})$ whether or not there is h in $Out(F_{n})$ such that $hf h^{-1} = g$.
The conjugacy problem for linearly growing elements of $UPG(F_{n})$ was solved by Cohen-Lustig. Krstic-Lustig-Vogtmann solved the case of linearly growing elements of $Out(F_{n})$.
A key technique is our use of train track representatives for elements of $Out(F_{n})$, a method pioneered by Bestvina-Handel in the early 1990s that has since been ubiquitous in the study of $Out(F_{n})$.
(joint work with Michael Handel) $Out(F_{n}) := Aut(F_{n})/Inn(F_{n})$ denotes the outer automorphism group of the rank n free group $F_{n}$. An element $f$ of $Out(F_{n})$ is polynomially growing if the word lengths of conjugacy classes in Fn grow at most polynomially under iteration by $f$. The existence in $Out(F_{n}), n > 2$, of elements with non-linear polynomial growth is a feature of $Out(F_{n})$ not shared by mapping class groups of ...

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## Post-edited  Sampling algorithms and phase transitions Randall, Dana | CIRM (Editeur )

Markov chain Monte Carlo methods have become ubiquitous across science and engineering to model dynamics and explore large combinatorial sets. Over the last 20 years there have been tremendous advances in the design and analysis of efficient sampling algorithms for this purpose. One of the striking discoveries has been the realization that many natural Markov chains undergo phase transitions, whereby they abruptly change from being efficient to inefficient as some parameter of the system is modified. Generating functions can offer an alternative approach to sampling and they play a role in showing when certain Markov chains are efficient or not. We will explore the interplay between Markov chains, generating functions, and phase transitions for a variety of combinatorial problems, including graded posets, Boltzmann sampling, and 3-colorings on $Z^{2}$.
Markov chain Monte Carlo methods have become ubiquitous across science and engineering to model dynamics and explore large combinatorial sets. Over the last 20 years there have been tremendous advances in the design and analysis of efficient sampling algorithms for this purpose. One of the striking discoveries has been the realization that many natural Markov chains undergo phase transitions, whereby they abruptly change from being efficient to ...

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## Post-edited  ​On the Lüroth problem for real varieties Benoist, Olivier (Auteur de la Conférence) | CIRM (Editeur )

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.

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## Post-edited  On the general linear group over arithmetic orders and corresponding cohomology groups Schwermer, Joachim (Auteur de la Conférence) | CIRM (Editeur )

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 ...

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## Post-edited  ​Modelling of errors in photoacoustic tomography Tarvainen, Tanja (Auteur de la Conférence) | CIRM (Editeur )

​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 ...

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## Post-edited  Mathematical topics around fractional quantization - lecture 1 Rougerie, Nicolas (Auteur de la Conférence) | CIRM (Editeur )

The 1983 discovery of the fractional quantum Hall effect marks a milestone in condensed matter physics: systems of “ordinary particles at ordinary energies” displayed highly exotic effects, most notably fractional quantum numbers. It was later recognized that this was due to emergent quasi-particles carrying a fraction of the charge of an electron. It was also conjectured that these quasi-particles had fractional statistics, i.e. a behavior interpolating between that of bosons and fermions, the only two types of fundamental particles.
These lectures will be an introduction to the basic physics of the fractional quantum Hall effect, with an emphasis on the challenges to rigorous many-body quantum mechanics emerging thereof. Some progress has been made on some of these, but lots remains to be done, and open problems will be mentioned.

After the lectures a few references regarding the spectrum of the magnetic Schrödinger operator were suggested to me.
See the bibiography below.

Thanks to Alix Deleporte, Frédéric Faure, Stéphane Nonnenmacher and others for discussions relative to the magnetic Weyl law.
The 1983 discovery of the fractional quantum Hall effect marks a milestone in condensed matter physics: systems of “ordinary particles at ordinary energies” displayed highly exotic effects, most notably fractional quantum numbers. It was later recognized that this was due to emergent quasi-particles carrying a fraction of the charge of an electron. It was also conjectured that these quasi-particles had fractional statistics, i.e. a behavior ...

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## Post-edited  Interactions of analytic number theory and geometry - lecture 1 Schindler, Damaris (Auteur de la Conférence) | CIRM (Editeur )

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.

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## Post-edited  Improved bounds for jet differential operators and the Kobayashi conjecture Demailly, Jean-Pierre (Auteur de la Conférence) | CIRM (Editeur )

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.

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## Post-edited  Graphon mean field games and the GMFG equations Caines, Peter E. (Auteur de la Conférence) | CIRM (Editeur )

Very large networks linking dynamical agents are now ubiquitous and there is significant interest in their analysis, design and control. The emergence of the graphon theory of large networks and their infinite limits has recently enabled the formulation of a theory of the centralized control of dynamical systems distributed on asymptotically infinite networks [Gao and Caines, IEEE CDC 2017, 2018]. Furthermore, the study of the decentralized control of such systems has been initiated in [Caines and Huang, IEEE CDC 2018] where Graphon Mean Field Games (GMFG) and the GMFG equations are formulated for the analysis of non-cooperative dynamical games on unbounded networks. In this talk the GMFG framework will be first be presented followed by the basic existence and uniqueness results for the GMFG equations, together with an epsilon-Nash theorem relating the infinite population equilibria on infinite networks to that of finite population equilibria on finite networks.
Very large networks linking dynamical agents are now ubiquitous and there is significant interest in their analysis, design and control. The emergence of the graphon theory of large networks and their infinite limits has recently enabled the formulation of a theory of the centralized control of dynamical systems distributed on asymptotically infinite networks [Gao and Caines, IEEE CDC 2017, 2018]. Furthermore, the study of the decentralized ...

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## Post-edited  Graph data management - part 1 Martens, Wim (Auteur de la Conférence) | CIRM (Editeur )

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 ...

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## Post-edited  Gradient discretisations : tools and applications Eymard, Robert (Auteur de la Conférence) | CIRM (Editeur )

Some convergence properties for the approximation of second order elliptic problems with a variety of boundary conditions (homogeneous Dirichlet, homogeneous or non-homogeneous Neumann or Fourier boundary conditions), using a given discretisation method, can be obtained when this method is plugged into the Gradient Discretisation Method (GDM) framework.
Instead of defining one GDM framework for each of these boundary conditions, we show that these properties can be stated using the same abstract tools for all the above boundary conditions. Then these tools enable the application of the GDM to a larger class of elliptic problems.
Some convergence properties for the approximation of second order elliptic problems with a variety of boundary conditions (homogeneous Dirichlet, homogeneous or non-homogeneous Neumann or Fourier boundary conditions), using a given discretisation method, can be obtained when this method is plugged into the Gradient Discretisation Method (GDM) framework.
Instead of defining one GDM framework for each of these boundary conditions, we show that ...

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## Post-edited  ​Construction of lattices defining fake projective planes - Lecture 2 Cartwright, Donald I. (Auteur de la Conférence) | CIRM (Editeur )

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## Post-edited  ​Construction of lattices defining fake projective planes - Lecture 1 Cartwright, Donald I. (Auteur de la Conférence) | CIRM (Editeur )

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## Post-edited  Branching random walks and Galton-Watson trees Gantert, Nina (Auteur de la Conférence) | CIRM (Editeur )

We give some results about tree-indexed random walks aka branching random walks. In particular, we investigate the growth of the maximum of such a walk.
Based on joint work with Piotr Dyszewski and Thomas Hofelsauer.

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## Post-edited  Autour de la mesure de Plancherel sur les partitions d'entiers (une introduction aux processus de Schur) - Partie 1 Bouttier, Jérémie (Auteur de la Conférence) | CIRM (Editeur )

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 ...

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## Post-edited  $L^2$ curvature for surfaces in Riemannian manifolds Kuwert, Ernst (Auteur de la Conférence) | CIRM (Editeur )

For surfaces immersed into a compact Riemannian manifold, we consider the curvature functional given by the $L^{2}$ integral of the second fundamental form. We discuss an area bound in terms of the energy, with application to the existence of minimizers. This is joint work with V. Bangert.

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## Multi angle  Zeros, moments and determinants Snaith, Nina (Auteur de la Conférence) | CIRM (Editeur )

For 20 years we have known that average values of characteristic polynomials of random unitary matrices provide a good model for moments of the Riemann zeta function. Now we consider mixed moments of characteristic polynomials and their derivatives, calculations which are motivated by questions on the distribution of zeros of the derivative of the Riemann zeta function.

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## Multi angle  Willmore stability and conformal rigidity of minimal surfaces in $\mathbb{S}^{n}$ Kusner, Rob (Auteur de la Conférence) | CIRM (Editeur )

A minimal surface $M$ in the round sphere $\mathbb{S}^{n}$ is critical for area, as well as for the Willmore bending energy $W=\int\int(1+H^{2})da$. Willmore stability of $M$ is equivalent to a gap between −2 and 0 in its area-Jacobi operator spectrum. We show the $W$-stability of $M$ persists in all higher dimensional spheres if and only if the Laplacian of $M$ has first eigenvalue 2. The square Clifford 2-torus in $\mathbb{S}^{3}$ and the equilateral minimal 2-torus in $\mathbb{S}^{5}$ have this spectral gap, and each is embedded by first eigenfunctions, so both are "persistently” $W$-stable. On the other hand, we discovered the equilateral torus has nontrivial third variation (with vanishing second variation) of $W$, and thus is not a $W$-minimizer (though it is the $W$-minimizer if we fix the conformal type!). This is evidence the Willmore Conjecture holds in every codimension. Another result concerns higher genus minimal surfaces (such as those constructed by Lawson and those by Karcher-Pinkall-Sterling) in $\mathbb{S}^{3}$ which Choe-Soret showed are embedded by first eigenfunctions: we show their first eigenspaces are always 4-dimensional, and that this implies each is (up to Möbius transformations of $\mathbb{S}^{n}$) the unique $W$-minimizer in its conformal class. (Some analogous results hold for free boundary minimal surfaces in the unit ball $\mathbb{B}^{n}$....). This is joint work with Peng Wang.
A minimal surface $M$ in the round sphere $\mathbb{S}^{n}$ is critical for area, as well as for the Willmore bending energy $W=\int\int(1+H^{2})da$. Willmore stability of $M$ is equivalent to a gap between −2 and 0 in its area-Jacobi operator spectrum. We show the $W$-stability of $M$ persists in all higher dimensional spheres if and only if the Laplacian of $M$ has first eigenvalue 2. The square Clifford 2-torus in $\mathbb{S}^{3}$ and the ...

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