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# Videothèque  | enregistrements trouvés : 7

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## Multi angle  Time changes of stochastic processes: convergence and heat kernel estimates Kumagai, Takashi (Auteur de la Conférence) | CIRM (Editeur )

In recent years, interest in time changes of stochastic processes according to irregular measures has arisen from various sources. Fundamental examples of such time-changed processes include the so-called Fontes-Isopi-Newman (FIN) diffusion and fractional kinetics (FK) processes, the introduction of which were partly motivated by the study of the localization and aging properties of physical spin systems, and the two- dimensional Liouville Brownian motion, which is the diffusion naturally associated with planar Liouville quantum gravity.
This FIN diffusions and FK processes are known to be the scaling limits of the Bouchaud trap models, and the two-dimensional Liouville Brownian motion is conjectured to be the scaling limit of simple random walks on random planar maps.
In the first part of my talk, I will provide a general framework for studying such time changed processes and their discrete approximations in the case when the underlying stochastic process is strongly recurrent, in the sense that it can be described by a resistance form, as introduced by J. Kigami. In particular, this includes the case of Brownian motion on tree-like spaces and low-dimensional self-similar fractals.
In the second part of my talk, I will discuss heat kernel estimates for (generalized) FIN diffusions and FK processes on metric measure spaces.
This talk is based on joint works with D. Croydon (Warwick) and B.M. Hambly (Oxford) and with Z.-Q. Chen (Seattle), P. Kim (Seoul) and J. Wang (Fuzhou).
In recent years, interest in time changes of stochastic processes according to irregular measures has arisen from various sources. Fundamental examples of such time-changed processes include the so-called Fontes-Isopi-Newman (FIN) diffusion and fractional kinetics (FK) processes, the introduction of which were partly motivated by the study of the localization and aging properties of physical spin systems, and the two- dimensional Liouville ...

## Multi angle  Introduction to hyperbolic sigma models and Edge Reinforced Random Walk Spencer, Tom (Auteur de la Conférence) | CIRM (Editeur )

This talk will introduce two statistical mechanics models on the lattice. The spins in these models have a hyperbolic symmetry. Correlations for these models can be expressed in terms of a random walk in a highly correlated random environment. In the SUSY hyperbolic case these walks are closely related to the vertex reinforced jump process and to the edge reinforced random walk. (Joint work with M. Disertori and M. Zirnbauer.)

## Multi angle  Internal diffusion-limited aggregation with random starting points Kozma, Gady (Auteur de la Conférence) | CIRM (Editeur )

We consider a model for a growing subset of a euclidean lattice (an "aggregate") where at each step one choose a random point from the existing aggregate, starts a random walk from that point, and adds the point of exit to the aggregate. We show that the limiting shape is a ball. Joint work with Itai Benjamini, Hugo Duminil-Copin and Cyril Lucas.

## Multi angle  The invariant subspace problem: a concrete operator theory approach Gallardo-Gutiérrez, Eva (Auteur de la Conférence) | CIRM (Editeur )

The Invariant Subspace Problem for (separable) Hilbert spaces is a long-standing open question that traces back to Jonhn Von Neumann's works in the fifties asking, in particular, if every bounded linear operator acting on an infinite dimensional separable Hilbert space has a non-trivial closed invariant subspace. Whereas there are well-known classes of bounded linear operators on Hilbert spaces that are known to have non-trivial, closed invariant subspaces (normal operators, compact operators, polynomially compact operators,...), the question of characterizing the lattice of the invariant subspaces of just a particular bounded linear operator is known to be extremely difficult and indeed, it may solve the Invariant Subspace Problem.

In this talk, we will focus on those concrete operators that may solve the Invariant Subspace Problem, presenting some of their main properties, exhibiting old and new examples and recent results about them obtained in collaboration with Prof. Carl Cowen (Indiana University-Purdue University).
The Invariant Subspace Problem for (separable) Hilbert spaces is a long-standing open question that traces back to Jonhn Von Neumann's works in the fifties asking, in particular, if every bounded linear operator acting on an infinite dimensional separable Hilbert space has a non-trivial closed invariant subspace. Whereas there are well-known classes of bounded linear operators on Hilbert spaces that are known to have non-trivial, closed ...

## Multi angle  New spectral bounds for damped systems Tretter, Christiane (Auteur de la Conférence) | CIRM (Editeur )

In this talk new enclosures for the spectra of operators associated with second order Cauchy problems are presented for non-selfadjoint damping. Our new results yield much better bounds than the numerical range of these non-selfadjoint operators for both uniformly accretive and sectorial damping.
(joint work with B. Jacob, Carsten Trunk and H. Vogt)

## Multi angle  A new complex spectrum associated to invisibility in waveguides Bonnet-Ben Dhia, Anne-Sophie (Auteur de la Conférence) | CIRM (Editeur )

We consider an acoustic waveguide modeled as follows:

$\left \{\begin {matrix} \Delta u+k^2(1+V)u=0& in & \Omega= \mathbb{R} \times]0,1[\\ \frac{\partial u}{\partial y}=0& on & \partial \Omega \end{matrix}\right.$

where $u$ denotes the complex valued pressure, k is the frequency and $V \in L^\infty(\Omega)$ is a compactly supported potential.
It is well-known that they may exist non trivial solutions $u$ in $L^2(\Omega)$, called trapped modes. Associated eigenvalues $\lambda = k^2$ are embedded in the essential spectrum $\mathbb{R}^+$. They can be computed as the real part of the complex spectrum of a non-self-adjoint eigenvalue problem, defined by using the so-called Perfectly Matched Layers (which consist in a complex dilation in the infinite direction) [1].
We show here that it is possible, by modifying in particular the parameters of the Perfectly Matched Layers, to define new complex spectra which include, in addition to trapped modes, frequencies where the potential $V$ is, in some sense, invisible to one incident wave.
Our approach allows to extend to higher dimension the results obtained in [2] on a 1D model problem.
We consider an acoustic waveguide modeled as follows:

$\left \{\begin {matrix} \Delta u+k^2(1+V)u=0& in & \Omega= \mathbb{R} \times]0,1[\\ \frac{\partial u}{\partial y}=0& on & \partial \Omega \end{matrix}\right.$

where $u$ denotes the complex valued pressure, k is the frequency and $V \in L^\infty(\Omega)$ is a compactly supported potential.
It is well-known that they may exist non trivial solutions $u$ in $L^2(\Omega)$, called trapped ...

## Multi angle  Truncated Toeplitz operators Câmara, Cristina (Auteur de la Conférence) | CIRM (Editeur )

Toeplitz matrices and operators constitute one of the most important and widely studied classes of non-self-adjoint operators. In this talk we consider truncated Toeplitz operators, a natural generalisation of finite Toeplitz matrices. They appear in various contexts, such as the study of finite interval convolution equations, signal processing, control theory, diffraction problems, hydrodynamics, elasticity, and they play a fundamental role in the study of complex symmetric operators. We will focus mainly on their invertibility and Fredholmness properties, showing in particular that they are equivalent after extension to block Toeplitz operators, and how this can be used to study the spectra of several classes of truncated Toeplitz operators. Toeplitz matrices and operators constitute one of the most important and widely studied classes of non-self-adjoint operators. In this talk we consider truncated Toeplitz operators, a natural generalisation of finite Toeplitz matrices. They appear in various contexts, such as the study of finite interval convolution equations, signal processing, control theory, diffraction problems, hydrodynamics, elasticity, and they play a fundamental role in ...

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