1,409 Works

Reflection of clubs, and forcing principles at $\aleph_2$

Itay Neeman
In 1971 Baumgartner showed it is consistent that any two $\aleph_1$-dense subsets of the real line are order isomorphic. This was important both for the methods of the proof and for consequences of the result. We introduce methods that lead to an analogous result for $\aleph_2$-dense sets. Keywords : forcing - large cardinals - Baumgartner isomorphism - infinitary Ramsey principles - reflection principles

Colouring graphs with no odd holes, and other stories

Paul D. Seymour
The chromatic number $\chi(G)$ of a graph $G$ is always at least the size of its largest clique (denoted by $\omega(G)$), and there are graphs $G$ with $\omega(G)=2$ and $\chi(G)$ arbitrarily large. On the other hand, the perfect graph theorem asserts that if neither $G$ nor its complement has an odd hole, then $\chi(G)=\omega(G)$ . (A "hole" is an induced cycle of length at least four, and "odd holes" are holes of odd length.) What...

Tilings and non-intersecting paths beyond integrable cases

Vadim Gorin
The talk is about a class of systems of 2d statistical mechanics, such as random tilings, noncolliding walks, log-gases and random matrix-type distributions. Specific members in this class are integrable, which means that available exact formulas allow delicate asymptotic analysis leading to the Gaussian Free Field, sine-process, Tracy-Widom distributions. Extending the results beyond the integrable cases is challenging. I will speak about a recent progress in this direction: about universal local limit theorems for a...

Overdamping in gyroscopic systems composed of high-loss and lossless components

Alexander Figotin
Using a Lagrangian framework, we study overdamping phenomena in gyroscopic systems composed of two components, one of which is highly lossy and the other is lossless. The losses are accounted for by a Rayleigh dissipative function. We prove that selective overdamping is a generic phenomenon in Lagrangian systems with gyroscopic forces and give an analysis of the overdamping phenomena in such systems. Central to the analysis is the introduction of the notion of a dual...

Finite-energy solutions for compressible Euler and Navier-Stokes with nonlocal forces

Ewelina Zatorska
In this talk I will present a couple of results for the existence of solutions to the one-dimensional Euler, Navier-Stokes and multi-dimensional Navier-Stokes systems. The purpose of the talk is to focus on the role of the pressure in the compressible fluid equations, and to understand whether or not it can be replaced by the nonlocal attraction-repulsion terms arising in the models of collective behaviour.

Da Prato and Grisvard meet global Lagrangian coordinates

Patrick Tolksdorf
A common way to prove global well-posedness of free boundary problems for incompressible viscous fluids is to transform the equations governing the fluid motion to a fixed domain with respect to the time variable. An elegant and physically reasonable way to do this is to introduce Lagrangian coordinates. These coordinates are given by the transformation rule $x(t)=\xi +\int_{0}^{t}u(\tau ,\xi ) d\tau $ where $u(\tau ,\xi )$ is the velocity vector of the fluid particle at...

Relative entropy for the Euler-Korteweg system with non-monotone pressure

Jan Giesselmann
In this joint work with Athanasios Tzavaras (KAUST) and Corrado Lattanzio (L’Aquila) we develop a relative entropy framework for Hamiltonian flows that in particular covers the Euler-Korteweg system, a well-known diffuse interface model for compressible multiphase flows. We put a particular emphasis on extending the relative entropy framework to the case of non-monotone pressure laws which make the energy functional non-convex.The relative entropy computation directly implies weak (entropic)-strong uniqueness, but we will also outline how...

Existence and stability of partially congested fronts

Charlotte Perrin
In this talk, I will present a recent study on traveling waves solutions to a 1D biphasic Navier-Stokes system coupling compressible and incompressible phases. With this original fluid equations, we intend to model congestion (or saturation) phenomena in heterogeneous flows (mixtures, collective motion, etc.). I will first exhibit explicit partially congested propagation fronts and show that these solutions can be approached by profiles which are solutions to a singular compressible Navier-Stokes system. The last part...

Multiple traveling waves of the Euler-Korteweg system

Corentin Audiard
The Euler-Korteweg system corresponds to compressible, inviscid fluids with capillary forces. It can be used to model diffuse interfaces. Mathematically it reads as the Euler equations with a third order dispersive perturbation corresponding to the capillary tensor. In dimension one there exists traveling waves with equal or different limit at infinity, respectively solitons and kinks. Their stability is ruled by a simple criterion a la Grillakis-Shatah-Strauss. This talk is devoted to the construction of multiple...

Chain conditions, unbounded colorings and the $C$-sequence spectrum

Assaf Rinot
The productivity of the $\kappa $-chain condition, where $\kappa $ is a regular, uncountable cardinal, has been the focus of a great deal of set-theoretic research. In the 1970’s, consistent examples of $kappa-cc$ posets whose squares are not $\kappa-cc$ were constructed by Laver, Galvin, Roitman and Fleissner. Later, ZFC examples were constructed by Todorcevic, Shelah, and others. The most difficult case, that in which $\kappa = \aleph{_2}$, was resolved by Shelah in 1997. In the...

Towards the right generalization of descriptive set theory to uncountable cardinals

Luca Motto Ros
Generalized descriptive set theory has mostly been developed for uncountable cardinals satisfying the condition $\kappa ^{< \kappa }=\kappa$ (thus in particular for $\kappa$ regular). More recently the case of uncountable cardinals of countable cofinality has attracted some attention, partially because of its connections with very large cardinal axioms like I0. In this talk I will survey these recent developments and propose a unified approach which potentially could encompass all possible scenarios (including singular cardinals of...

Some results on set mappings

Péter Komjáth
I give a survey of some recent results on set mappings

Lower bounds for the perfect set property at weakly compact cardinals

Sandra Müller
By the Cantor-Bendixson theorem, subtrees of the binary tree on $\omega$ satisfy a dichotomy - either the tree has countably many branches or there is a perfect subtree (and in particular, the tree has continuum manybranches, regardless of the size of the continuum). We generalize this to arbitrary regular cardinals $\kappa$ and ask whether every $\kappa$-tree with more than $\kappa$ branches has a perfect subset. From large cardinals, this statement isconsistent at a weakly compact...

Borel sets of Rado graphs are Ramsey

Natasha Dobrinen
The Galvin-Prikry theorem states that Borel partitions of the Baire space are Ramsey. Thus, given any Borel subset $\chi$ of the Baire space and an infinite set $N$, there is an infinite subset $M$ of $N$ such that $\left [M \right ]^{\omega }$ is either contained in $\chi$ or disjoint from $\chi$ . In their 2005 paper, Kechris, Pestov and Todorcevic point out the dearth of similar results for homogeneous relational structures. We have attained...

The evolution of cooperation in an iterated survival game

John Wakeley
A new type of a simple iterated game with natural biological motivation is introduced. Two individuals are chosen at random from a population. They must survive a certain number of steps. They start together, but if one of them dies the other one tries to survive on its own. The only payoff is to survive the game. We only allow two strategies: cooperators help the other individual, while defectors do not. There is no strategic...

Reconstruction by optimal transport: applications in cosmology and finance

Grégoire Loeper
Following the seminal work by Benamou and Brenier on the time continuous formulation of the optimal transport problem, we show how optimal transport techniques can be used in various areas, ranging from "the reconstruction problem" cosmology to a problem of volatility calibration in finance.

Interview au CIRM : Nicola Kistler

Nicola Kistler
Swiss-born mathematician Nicola Kistler was the first holder of the Jean-Morlet Chair for mathematical sciences at CIRM and, in that capacity, became the first visiting researcher in residence for six months at the Centre. His stay at CIRM lasted from early February till July 2013. He set up a program of mathematical events focusing on 'Probability', with the collaboration of Véronique Gayrard, local project leader working at Marseille's Laboratoire d'Analyse, Topologie, Probabilités (ex LATP -...

Structure of hyperbolic manifolds - Lecture 1

Jessica Purcell
In these lectures, we will review what it means for a 3-manifold to have a hyperbolic structure, and give tools to show that a manifold is hyperbolic. We will also discuss how to decompose examples of 3-manifolds, such as knot complements, into simpler pieces. We give conditions that allow us to use these simpler pieces to determine information about the hyperbolic geometry of the original manifold. Most of the tools we present were developed in...

Mean field type control with congestion

Mathieu Laurière
The theory of mean field type control (or control of MacKean-Vlasov) aims at describing the behaviour of a large number of agents using a common feedback control and interacting through some mean field term. The solution to this type of control problem can be seen as a collaborative optimum. We will present the system of partial differential equations (PDE) arising in this setting: a forward Fokker-Planck equation and a backward Hamilton-Jacobi-Bellman equation. They describe respectively...

Set-valued risk measures of non-convex portfolios

Ilya Molchanov
Non-convex random sets of admissible positions naturally arise in the setting of fixed transaction costs or when only a finite range of possible transactions is considered. The talk defines set-valued risk measures in such cases and explores the situations when they return convex result, namely, when Lyapunov's theorem applies. The case of fixed transaction costs is analysed in greater details. Joint work with Andreas Haier (FINMA, Switzerland).

Poisson-Lie duality and Langlands duality via Bohr-Sommerfeld

Anton Alekseev
Let $G$ be a connected semisimple Lie group with Lie algebra $\mathfrak{g}$. There are two natural duality constructions that assign to it the Langlands dual group $G^\lor$ (associated to the dual root system) and the Poisson-Lie dual group $G^∗$. Cartan subalgebras of $\mathfrak{g}^\lor$ and $\mathfrak{g}^∗$ are isomorphic to each other, but $G^\lor$ is semisimple while $G^∗$ is solvable. In this talk, we explain the following non-trivial relation between these two dualities: the integral cone defined...

Quasi-Cocycles Detect Hyperbolically Embedded Subgroups

Alessandro Sisto
Hyperbolically embedded subgroups have been defined by Dahmani-Guirardel-Osin and they provide a common perspective on (relatively) hyperbolic groups, mapping class groups, Out(F_n), CAT(0) groups and many others. I will sketch how to extend a quasi-cocycle on a hyperbolically embedded subgroup H to a quasi-cocycle on the ambient group G. Also, I will discuss how some of those extended quasi-cocycles (of dimension 2 and higher) "contain" the information that H is hyperbolically embedded in G. This...

Stochastic solutions of 2D fluids​

Franco Flandoli
We revise recent contributions to 2D Euler and Navier-Stokes equations with and without noise, but always in the case of stochastic solutions. The role of white noise initial conditions will be stressed and related to some questions about turbulence.

Grothendieck residue in the Jacobian algebra and cup product in vanishing cohomology

Xavier Gómez-Mont
The Jacobian algebra, obtained from the ring of germs of functions modulo the partial derivatives of a function $f$ with an isolated singularity, has a non-degenerate bilinear form, Grothendieck Residue, for which multiplication by $f$ is a symmetric nilpotent operator. The vanishing cohomology of the Milnor Fibre has a bilinear form induced by cup product for which the nilpotent operator $N$, the logarithm of the unipotent part of the monodromy, is antisymmetric. Using the nilpotent...

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