Seminars take place in the seminar room, first floor of the building Le Chablais, (see How to come ?).

## Next seminar:

**Friday 11th December 2020 at 14h
**
Mohammad Akil
(Univ Savoie Mont-Blanc),
*Stability results of some coupled wave systems with different kinds of localized damping*

Abstract:(Hide abstracts)

First, we consider a system of two wave equations coupled by velocities in one-dimensional space with one boundary fractional damping and we prove that the energy of our system decays polynomially with different rates. Second, we investigate the stabilization of a locally coupled wave equations with only one internal viscoelastic damping of Kelvin-Voigt type and we prove that the energy of our system decays polynomially with rate 1/t. Finally, we investigate the stabilization of a locally coupled wave equations with local viscoelastic damping of past history type acting only in one equation via non smooth coefficients and we establish the exponential stability of the solution if and only if the two waves have the same speed of propagation. In case of different speed propagation, we prove that the energy of our system decays polynomially with rate 1/t.

The seminar of the team EDPs² is under the responsibility of Jimmy Garnier.

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Other years: 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2021,
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# Year 2020

**Friday 11th December 2020 at 14h
**
Mohammad Akil
(Univ Savoie Mont-Blanc),
*Stability results of some coupled wave systems with different kinds of localized damping*

Abstract:(Hide abstracts)

First, we consider a system of two wave equations coupled by velocities in one-dimensional space with one boundary fractional damping and we prove that the energy of our system decays polynomially with different rates. Second, we investigate the stabilization of a locally coupled wave equations with only one internal viscoelastic damping of Kelvin-Voigt type and we prove that the energy of our system decays polynomially with rate 1/t. Finally, we investigate the stabilization of a locally coupled wave equations with local viscoelastic damping of past history type acting only in one equation via non smooth coefficients and we establish the exponential stability of the solution if and only if the two waves have the same speed of propagation. In case of different speed propagation, we prove that the energy of our system decays polynomially with rate 1/t.

**Friday 27th November 2020 at 14h
**
Hugo Martin
(INSERM, Paris-Sud),
*Periodic asymptotic dynamics of the measure solutions to a growth-fragmentation equation in a critical case*

Abstract:(Hide abstracts)

In the last years, measure solutions to PDE, in particular those modeling populations, have drawn much attention. The talk will be devoted to the presentation of a recent, unusual result in this field, that we obtained with Pierre Gabriel. First, I will expose some wellposedness and asymptotic results for two famous population equations in the L^p and measure frameworks, and explain the critical case that interested us. Then, I will define the notion of solution we used, and if needed, recall some basic definitions about semigroups. Moving to the proof itself, I will present the main steps of the proof of the wellposedness of the problem, that relies on a duality relation used to build a solution expressed as a semigroup acting on an initial measure. Then, I will go a little more into details of the demonstration of the asymptotic behaviour. In particular, I will exhibit how we used Harris' ergodic theorem to obtain a uniform exponential convergence in (weighted) total variation norm toward an oscillating measure.

**Friday 20th November 2020 at 14h
**
Andrea Natale
(Inria (Lille)),
*Lagrangian discretizations of compressible fluids and porous media flow with semi-discrete optimal transport*

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The equations of motion for compressible (barotropic) fluids have the structure of a simple conservative dynamical system when expressed in Lagrangian variables. This can be exposed interpreting the Lagrangian flow as a curve of vector-valued L2 functions, and the internal energy of the fluid as a functional on the same space. Particle methods are a natural discretization strategy in this setting, since in this case the flow is discretized using piecewise constant functions on a given partition of the domain, but they require some form of regularization to define the internal energy of the fluid. In this talk I will describe a particle method in which the internal energy is replaced by its Moreau-Yosida regularization in the L2 space, which can be efficiently computed as a semi-discrete optimal transport problem. I will also show how the convexity of the energy in the Eulerian variables can be exploited in the non-convex Lagrangian setting to prove quantitative convergence estimates towards smooth solution of this problem, and how this result generalizes to dissipative porous media flow.

**Friday 13th November 2020 at 14h
**
Boris Thibert
(Univ Grenoble Alpes, LJK),
*Équations de Monge-Ampère en optique anidolique*

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Le but de l'optique anidolique, aussi appelée optique non imageante, est de construire des composants optiques qui transportent l'énergie lumineuse d'une source de lumière vers une cible de lumière donnée. La modélisation de ce type de problèmes inverses conduit dans certains cas à des équations de type Monge-Ampère. Dans cet exposé, je montrerai comment de telles équations peuvent être résolues numériquement à l'aide d'une discrétisation géométrique particulière appelée semi-discrète. Je présenterai aussi des applications en optique anidolique avec la construction de miroirs et de lentilles.

**Friday 16th October 2020 at 15h15
**
Jimmy Garnier
(Univ Savoie Mont-Blanc),
*Coexistence et invasion dans des communautés mutualistes*

Abstract:(Hide abstracts)

Les champignons endomycorhiziens forment des communautés mutualistes qui aident les plantes à accroître leur système racinaire et donc leur biomasse. Depuis plusieurs décennies, ces champignons sont utilisés comme engrais vert. Cependant quel est l'impact de ces champignons commerciaux sur les communautés sauvages? Afin de comprendre ces interactions j'ai développé en collaboration avec M. Martignoni, R. Tyson et M. Hart (Univ British Columbia) un nouveau modèle mutualiste basé sur des équations aux dérivées partielles. Dans cette exposé, je vous présenterai des critères analytiques d'existence et de stabilité des solutions stationnaires pour lesquels la coexistence apparaît. Ensuite je m'intéresserai à l'invasion spatiale d'une communauté par une autre en montrant l'existence de solutions de type front progressif pour le système et en caractérisant leur vitesse de propagation.

**Friday 26th June 2020 at 14h
**
Kathrin Stollenwerk
(Aachen University),
*à venir*

**Friday 21st February 2020 at 14h
**
Rémi Abgrall
(Univ Zürich, IMCS),
*SUPPRIMER Nouvelle date à venir*

**Friday 31st January 2020 at 14h
**
Marco Picasso
(EPFL, Laussane),
*Eléments finis anisotropes*

**Friday 17th January 2020 at 15h
**
Stefano Spirito
(Gran Sasso Science Institute, Italy),
*Weak solutions of the 2D Euler equations*

Abstract:(Hide abstracts)

In this talk we consider the Cauchy problem for the 2D Euler equations for incompressible inviscid fluids. It is well-known that given a smooth initial datum, the Cauchy problem is well-posed and in particular the energy is conserved and the vorticity is transported by the flow of the velocity. When we consider weak solutions this might not be the case anymore. We will review some recent results obtained in collaboration with Gianluca Crippa and Gennaro Ciampa where we extend those properties to the case of irregular vorticities. In particular, under low integrability assumptions on the vorticity we show that certain approximations important from a physical and a numerical point of view converge to solutions satisfying those properties.

**Friday 17th January 2020 at 14h
**
Paolo Antonelli
(Gran Sasso Science Institute, Italy),
*An intrinsically hydrodynamic approach to one dimensional quantum hydrodynamic systems*

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Quantum hydrodynamic (QHD) systems arise in the effective description of phenomena where quantistic behavior can be seen also at a macroscopic scale. This is the case for instance in Bose-Einstein condensation, superfluidity or in the modeling of semiconductor devices. Standard results for global existence of finite energy weak solutions to the QHD system often exploit the analogy with a nonlinear Schrödinger equation; by using the Madelung transform it is possible to define a solution to the QHD by considering the momenta (mass and current density) associated to a wave function. In particular this argument requires the initial data to be determined by a given wave function. This usual approach hence shows the existence of solutions but can not be used to study their stability properties in a general framework. In this talk I will present some recent developments that overcome those difficulties for the one dimensional QHD system. First of all I will provide an existence result for a large class of initial data, without requiring them to be generated by a wave function. Furthermore, I will prove a stability result for weak solutions. This exploits a novel functional which formally controls the L^2 norm of the chemical potential, weighted with the particle density. This is a joint work with P. Marcati and H. Zheng.

The seminar of the team EDPs² is under the responsibility of Jimmy Garnier.

**Settings:**
See with
increasing date
.
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Other years: 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018, 2019, 2021,
all years together.