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# Documents  CIRM | enregistrements trouvés : 1 544

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## Zeta functions and monodromy Veys, Wim | CIRM H

Post-edited

Research talks;Algebraic and Complex Geometry;Number Theory

The $p$-adic Igusa zeta function, topological and motivic zeta function are (related) invariants of a polynomial $f$, reflecting the singularities of the hypersurface $f = 0$. The first one has a number theoretical flavor and is related to counting numbers of solutions of $f = 0$ over finite rings; the other two are more geometric in nature. The monodromy conjecture relates in a mysterious way these invariants to another singularity invariant of $f$, its local monodromy. We will discuss in this survey talk rationality issues for these zeta functions and the origins of the conjecture. The $p$-adic Igusa zeta function, topological and motivic zeta function are (related) invariants of a polynomial $f$, reflecting the singularities of the hypersurface $f = 0$. The first one has a number theoretical flavor and is related to counting numbers of solutions of $f = 0$ over finite rings; the other two are more geometric in nature. The monodromy conjecture relates in a mysterious way these invariants to another singularity invariant of ...

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## Wrapping in exact real arithmetic Müller, Norbert | CIRM H

Post-edited

Research talks;Computer Science;Logic and Foundations

A serious problem common to all interval algorithms is that they suffer from wrapping effects, i.e. unnecessary growth of approximations during a computation. This is essentially connected to functional dependencies inside vectors of data computed from the same inputs. Reducing these effects is an important issue in interval arithmetic, where the most successful approach uses Taylor models.
In TTE Taylor models have not been considered explicitly, as they use would not change the induced computability, already established using ordinary interval computations. However for the viewpoint of efficiency, they lead to significant improvements.
In the talk we report on recent improvements on the iRRAM software for exact real arithmetic (ERA) based on Taylor models. The techniques discussed should also easily be applicable to other software for exact real computations as long as they also are based on interval arithmetic.
As instructive examples we consider the one-dimensional logistic map and a few further discrete dynamical systems of higher dimensions
Joint work with Franz Brauße, Trier, and Margarita Korovina, Novosibirsk.
A serious problem common to all interval algorithms is that they suffer from wrapping effects, i.e. unnecessary growth of approximations during a computation. This is essentially connected to functional dependencies inside vectors of data computed from the same inputs. Reducing these effects is an important issue in interval arithmetic, where the most successful approach uses Taylor models.
In TTE Taylor models have not been considered ...

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## Whitney problems and real algebraic geometry Fefferman, Charles | CIRM H

Post-edited

Research talks;Analysis and its Applications;Algebraic and Complex Geometry

This talk sketches connections between Whitney problems and e.g. the problem of deciding whether a given rational function on $\mathbb{R}^n$ belongs to $C^m$.

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## Which geodesic flows are left-handed? Dehornoy, Pierre | CIRM H

Post-edited

Research talks;Dynamical Systems and Ordinary Differential Equations;Topology

Left-handed flows are 3-dimensional flows which have a particular topological property, namely that every pair of periodic orbits is negatively linked. This property (introduced by Ghys in 2007) implies the existence of as many Bikrhoff sections as possible, and therefore allows to reduce the flow to a suspension in many different ways. It then becomes natural to look for examples. A construction of Birkhoff (1917) suggests that geodesic flows are good candidates. In this conference we determine on which hyperbolic orbifolds is the geodesic flow left-handed: the answer is that yes if the surface is a sphere with three cone points, and no otherwise.
dynamical system - geodesic flow - knot - periodic orbit - global section - linking number - fibered knot
Left-handed flows are 3-dimensional flows which have a particular topological property, namely that every pair of periodic orbits is negatively linked. This property (introduced by Ghys in 2007) implies the existence of as many Bikrhoff sections as possible, and therefore allows to reduce the flow to a suspension in many different ways. It then becomes natural to look for examples. A construction of Birkhoff (1917) suggests that geodesic flows ...

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## Were the foundations of measurement without theory laid in the 1920s? Pradier, Pierre-Charles | CIRM H

Post-edited

Research talks;History of Mathematics;Mathematics in Science and Technology;Probability and Statistics

In his 1947 essay, Tjalling Koopmans criticized the development of an empirical science that had no theoretical basis, what he referred to as measurement without theory. The controversy over the status of relations based on mere statistical inference has not ceased since then. Instead of looking for the contemporary consequences, however, I will inquire into its early beginnings. As early as the 1900s, Walras, Pareto and Juglar exchanged views on the status of theory and its relation to economic data. These private exchanges acquired the status of scientific controversy in the aftermath of the First World War, with the dissemination of Pareto’s work. It is precisely this moment that I will try to grasp, when engineers began to read and write pure economic treatises, questioning the relation between theory and empirical problems, the nature of their project and the expectations that the subsequent development of economics has tried to fulfill.

Cournot Centre session devoted to the transformations that took place in mathematical economics during the interwar period.
In his 1947 essay, Tjalling Koopmans criticized the development of an empirical science that had no theoretical basis, what he referred to as measurement without theory. The controversy over the status of relations based on mere statistical inference has not ceased since then. Instead of looking for the contemporary consequences, however, I will inquire into its early beginnings. As early as the 1900s, Walras, Pareto and Juglar exchanged views ...

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## Weak universality of the KPZ equation with arbitrary nonlinearities Hairer, Martin | CIRM H

Post-edited

Research talks;Partial Differential Equations;Probability and Statistics

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## Wall-crossing for Donaldson-Thomas invariants Bridgeland, Tom | CIRM H

Post-edited

Research talks;Algebraic and Complex Geometry;Mathematical Physics

There is a very general story, due to Joyce and Kontsevich-Soibelman, which associates to a CY3 (three-dimensional Calabi-Yau) triangulated category equipped with a stability condition some rational numbers called Donaldson-Thomas (DT) invariants. The point I want to emphasise is that the wall-crossing formula, which describes how these numbers change as the stability condition is varied, takes the form of an iso-Stokes condition for a family of connections on the punctured disc, where the structure group is the infinite-dimensional group of symplectic automorphisms of an algebraic torus. I will not assume any knowledge of stability conditions, DT invariants etc. There is a very general story, due to Joyce and Kontsevich-Soibelman, which associates to a CY3 (three-dimensional Calabi-Yau) triangulated category equipped with a stability condition some rational numbers called Donaldson-Thomas (DT) invariants. The point I want to emphasise is that the wall-crossing formula, which describes how these numbers change as the stability condition is varied, takes the form of an iso-Stokes condition for a family of ...

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## Virtual fundamental cycles and contact homology Pardon, John | CIRM H

Post-edited

Research talks;Geometry;Topology

I will discuss work in progress aimed towards defining contact homology using "virtual" holomorphic curve counting techniques.

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## Victor Havin tribute : 50 years with Hardy spaces. What next... ? Nikolski, Nikolai K. | CIRM H

Post-edited

Research talks;Analysis and its Applications;History of Mathematics

Starting with a personal tribute to Victor Havin (1933-2015), I discuss a dozen achievements of Great Havin’s Analysis Seminar, as well as some challenging still unsolved problems.
The Havin publications list is available in the PDF file at the bottom of the page.

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## Various aspects of the dynamics of the cubic Szegö solutions Grellier, Sandrine | CIRM H

Post-edited

Research talks;Partial Differential Equations;Dynamical Systems and Ordinary Differential Equations

The cubic Szegö equation has been introduced as a toy model for totally non dispersive evolution equations. It turned out that it is a complete integrable Hamiltonian system for which we built a non linear Fourier transform giving an explicit expression of the solutions.
This explicit formula allows to study the dynamics of the solutions. We will explain different aspects of it: almost-periodicity of the solutions in the energy space, uniform analyticity for a large set of initial data, turbulence phenomenon for a dense set of smooth initial data in large Sobolev spaces.
From joint works with Patrick Gérard.
The cubic Szegö equation has been introduced as a toy model for totally non dispersive evolution equations. It turned out that it is a complete integrable Hamiltonian system for which we built a non linear Fourier transform giving an explicit expression of the solutions.
This explicit formula allows to study the dynamics of the solutions. We will explain different aspects of it: almost-periodicity of the solutions in the energy space, uniform ...

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## Variational formulas, Busemann functions, and fluctuation exponents for the corner growth model with exponential weights - Lecture 1 Seppäläinen, Timo | CIRM H

Post-edited

Research School;Mathematical Physics;Probability and Statistics

Variational formulas for limit shapes of directed last-passage percolation models. Connections of minimizing cocycles of the variational formulas to geodesics, Busemann functions, and stationary percolation.

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## Variational and non-Archimedean aspects of the Yau-Tian-Donaldson conjecture Boucksom, Sébastien | CIRM H

Post-edited

Research talks;Algebraic and Complex Geometry

I will discuss some recent developments in the direction of the Yau-Tian-Donaldson conjecture, which relates the existence of constant scalar curvature Kähler metrics to the algebro-geometric notion of $K$-stability. The emphasis will be put on the use of pluripotential theory and the interpretation of $K$-stability in terms of non-Archimedean geometry.

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## Unresolved problems in the theory of integrable systems Zakharov, Vladimir | CIRM H

Post-edited

Research talks;Partial Differential Equations;Mathematical Physics

In spite of enormous success of the theory of integrable systems, at least three important problems are not resolved yet or are resolved only partly. They are the following:
1. The IST in the case of arbitrary bounded initial data.
2. The statistical description of the systems integrable by the IST. Albeit, the development of the theory of integrable turbulence.
3. Integrability of the deep water equations.
These three problems will be discussed in the talk.
In spite of enormous success of the theory of integrable systems, at least three important problems are not resolved yet or are resolved only partly. They are the following:
1. The IST in the case of arbitrary bounded initial data.
2. The statistical description of the systems integrable by the IST. Albeit, the development of the theory of integrable turbulence.
3. Integrability of the deep water equations.
These three problems will be discussed ...

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## Unramified graph covers of finite degree Li, Winnie | CIRM H

Post-edited

Research talks;Combinatorics;Number Theory

Given a finite connected undirected graph $X$, its fundamental group plays the role of the absolute Galois group of $X$. The familiar Galois theory holds in this setting. In this talk we shall discuss graph theoretical counter parts of several important theorems for number fields. Topics include
(a) Determination, up to equivalence, of unramified normal covers of $X$ of given degree,
(c) Chebotarev density theorem.
This is a joint work with Hau-Wen Huang.
Given a finite connected undirected graph $X$, its fundamental group plays the role of the absolute Galois group of $X$. The familiar Galois theory holds in this setting. In this talk we shall discuss graph theoretical counter parts of several important theorems for number fields. Topics include
(a) Determination, up to equivalence, of unramified normal covers of $X$ of given degree,
(c) Chebotarev density ...

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## Unirational varieties - Part 1 Mella, Massimiliano | CIRM H

Post-edited

Research talks;Algebraic and Complex Geometry

The aim of these talks is to give an overview to unirationality problems. I will discuss the behaviour of unirationality in families and its relation with rational connectedness. Then I will concentrate on hypersurfaces and conic bundles. These special classes of varieties are a good place where to test different techniques and try to approach the unirationality problem via rational connectedness.

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## Unique ergodicity for foliations on compact Kähler surfaces Sibony, Nessim | CIRM H

Post-edited

Research talks;Dynamical Systems and Ordinary Differential Equations;Algebraic and Complex Geometry

How to study the dynamics of a holomorphic polynomial vector field in $\mathbb{C}^{2}$? What is the replacement of invariant measure? I will survey some surprising rigidity results concerning the behavior of these dynamical system. It is helpful to consider the extension of this dynamical system to the projective plane.
Consider a foliation in the projective plane admitting a unique invariant algebraic curve. Assume that the foliation is generic in the sense that its singular points are hyperbolic. With T.-C. Dinh, we showed that there is a unique positive $dd^{c}$-closed (1, 1)-current of mass 1 which is directed by the foliation. This is the current of integration on the invariant curve. A unique ergodicity theorem for the distribution of leaves follows: for any leaf $L$, appropriate averages on $L$ converge to the current of integration on the invariant curve (although generically the leaves are dense). The result uses our theory of densities for currents. It extends to Foliations on Kähler surfaces.
I will describe a recent result, with T.-C. Dinh and V.-A. Nguyen, dealing with foliations on compact Kähler surfaces. If the foliation, has only hyperbolic singularities and does not admit a transverse measure, in particular no invariant compact curve, then there exists a unique positive $dd^{c}$-closed (1, 1)-current of mass 1 which is directed by the foliation( it’s like uniqueness of invariant measure for discrete dynamical systems). This improves on previous results, with J.-E. Fornæss, for foliations (without invariant algebraic curves) on the projective plane. The proof uses a theory of densities for positive $dd^{c}$-closed currents (an intersection theory).
How to study the dynamics of a holomorphic polynomial vector field in $\mathbb{C}^{2}$? What is the replacement of invariant measure? I will survey some surprising rigidity results concerning the behavior of these dynamical system. It is helpful to consider the extension of this dynamical system to the projective plane.
Consider a foliation in the projective plane admitting a unique invariant algebraic curve. Assume that the foliation is ...

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## Uniform distribution mod 1, results and open problems Katai, Imre | CIRM H

Post-edited

Research talks;Number Theory

Given a fixed integer $q \geq 2$, an irrational number $\xi$ is said to be a $q$-normal number if any preassigned sequence of $k$ digits occurs in the $q$-ary expansion of $\xi$ with the expected frequency, that is $1/q^k$. In this talk, we expose new methods that allow for the construction of large families of normal numbers. This is joint work with Professor Jean-Marie De Koninck.

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## Une deuxième révolution galiléenne ? Dowek, Gilles | CIRM H

Post-edited

Research talks;Computer Science

L'introduction d'un nouveau concept scientifique permet souvent de donner de nouvelles réponses à des questions anciennes qui n'avaient jusqu'alors reçu que des réponses imparfaites. Cet exposé présente quelques questions qui ont trouvé de nouvelles réponses depuis que nous comprenons mieux la notion d'algorithme : qu'est-ce qu'un aéroport ?, qu'est-ce qu'une cellule, qu'est-ce qu'une loi physique ?, ... La prise de conscience du caractère algorithmique de ces objets scientifiques nous amène à considérer de nouveaux langages pour les décrire. Cette révolution, dans le langage dans lequel la science s'écrit, peut-être comparée à la révolution qui s'est produite, au début du XVIIe siècle, quand le langage mathématique a commencé à être utilisé pour décrire des phénomènes physiques. L'introduction d'un nouveau concept scientifique permet souvent de donner de nouvelles réponses à des questions anciennes qui n'avaient jusqu'alors reçu que des réponses imparfaites. Cet exposé présente quelques questions qui ont trouvé de nouvelles réponses depuis que nous comprenons mieux la notion d'algorithme : qu'est-ce qu'un aéroport ?, qu'est-ce qu'une cellule, qu'est-ce qu'une loi physique ?, ... La prise de conscience du caractère ...

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## Understanding the growth of Laplace eigenfunctions (part 1 of 2) Canzani, Yaiza | CIRM H

Post-edited

Research talks;Partial Differential Equations;Geometry

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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## Two-player perfect-information shift-invariant submixing stochastic games are half-positional Gimbert, Hugo | CIRM H

Post-edited

Research talks;Computer Science

We show that two-player stochastic games with perfect-information and shift-invariant submixing payoff functions are half-positional, i.e. in these games the maximizer has a positional optimal strategy. This extension of our previous result for one-player games relies on an interesting existence result about the existence of epsilon-subgame-perfect strategies.

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