### WADE: from Lisbon to the world

The * Lisbon Webinar in Analysis in Differential Equations *is a joint iniciative of CAMGSD, CMAFcIO and GFM, three research centers of the University of Lisbon. It is aimed at filling the absence of face-to-face seminars and wishes to be a meeting point of mathematicians working in the field.

All seminars will take place in the Zoom platform. The links for the planned seminars can be found here. In order to get the password to access the seminars, please subscribe the announcements in the registration menu, or contact one of the organizers.

**Next webinar** **| https://wade.ulisboa.pt/**

To be held online at: https://videoconf-colibri.zoom.us/j/81421184299

Online access password: lisbonwade

**25.02.2021 | 14:00 — 15:00 Europe/Lisbon**

**Boyan Sirakov **(PUC - Rio)

** **

**The Vázquez maximum principle and the Landis conjecture for elliptic PDE with unbounded coefficients**

In this joint work with P. Souplet we develop a new, unified approach to the following two classical questions on elliptic PDE:

(i) the strong maximum principle for equations with non-Lipschitz nonlinearities; and

(ii) the at most exponential decay of solutions in the whole space or exterior domains.

Our results apply to divergence and nondivergence operators with locally unbounded lower-order coefficients, in a number of situations where all previous results required bounded ingredients. Our approach, which allows for relatively simple and short proofs, is based on a (weak) Harnack inequality with optimal dependence of the constants in the lower-order terms of the equation and the size of the domain, which we establish.

**18.02.2021 | Thursday | 14:00 — 15:00 Europe/Lisbon**

**Paolo Gidoni **(Czech Academy of Sciences)

** **

**A vanishing inertia analysis for finite dimensional rate-independent systems and an application to soft crawlers**

The quasistatic limit is a convenient approximation in the modelling of several (suitable) mechanical systems, when the evolution occurs at a sufficiently slow time-scale. In this talk we discuss the validity of the quasistatic approximation in finite-dimensional rate-independent systems via a vanishing-inertia asymptotic analysis of dynamic evolutions. More precisely, we show the uniform convergence of dynamic solutions to the quasistatic one, employing the concept of energetic solution. Our work is motivated by the application to a family of models for biological and bio-inspired crawling locomotion. Hence a part of the seminar will focus on modelling: we will discuss how soft crawlers can be effectively described in our theoretical framework and briefly survey the relevance, or lack thereof, of inertia in some locomotion strategies. By a technical point of view, our application requires time-dependence of the dissipation potential and translation invariance of the potential energy.

**11.02.2021 | Thursday | 14:00 — 15:00 Europe/Lisbon**

**Harbir Antil **(George Mason University)

** **

**Fractional PDEs: Control, Numerics, and Applications**

Fractional calculus and its application to anomalous diffusion has recently received a tremendous amount of attention. In complex/heterogeneous material mediums, the long-range correlations or hereditary material properties are presumed to be the cause of such anomalous behavior. Owing to the revival of fractional calculus, these effects are now conveniently modeled by fractional-order differential operators and the governing equations are reformulated accordingly. Similarly, the potential of fractional operators has been harnessed in various scientific domains like geophysical electromagnetics, imaging science, deep learning, etc.

In this talk, fractional operators will be introduced and both linear and nonlinear, fractional-order differential equations will be discussed. New notions of optimal control and optimization under uncertainty will be presented. Several applications from geophysics, imaging science, and deep learning will be presented.

**04.02.2021 | Thursday | 14:00 — 15:00 Europe/Lisbon**

**Yvan Martel **(École Polytechnique)

** **

**On the asymptotic stability of kinks for (1+1)-scalar field models**

The talk concerns stability properties of kinks for (1+1)-dimensional nonlinear scalar field models of the form

We establish a simple and explicit sufficient condition on the potential for the asymptotic stability of a given moving or standing kink.

We present applications of the criterion to some models of the Physics literature.

Work in collaboration with Michał Kowalczyk, Claudio Muñoz and Hanne Van Den Bosch. See also the related work with Michał Kowalczyk and Claudio Muñoz.

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**28.01.2021 | ****Thursday**** | 14:00 — 15:00 Europe/Lisbon**

**Enrico Serra **(Politecnico di Torino)

** **

**NLS ground states on metric trees: existence results and open questions**

We consider the minimization of the NLS energy on a metric tree, either rooted or unrooted, subject to a mass constraint. With respect to the same problem on other types of metric graphs, several new features appear, such as the existence of minimizers with positive energy, and the emergence of unexpected threshold phenomena. We also study the problem with a radial symmetry constraint that is in principle different from the free problem due to the failure of the Polya-Szego inequality for radial rearrangements. A key role is played by a new Poincaré inequality with remainder.

**21.01.2021 | 14:00 — 15:00 Europe/Lisbon**

**Charles Collot, **(Cergy Paris Université)

** **

**On the stability of equilibria for infinitely many particles**

We study the evolution of a system of particles. Instead of the usual Hartree equation for density matrices, we consider the following equivalent model, proposed by de Suzzoni, of a Hartree type equation but for a random field:

Above, is a time-dependent random field, a pair interaction potential, *the convolution product and the expectation. This equation admits equilibria which are random Gaussian fields whose laws are invariant by time and space translations. They are hence not localised and represent an infinite number of particles. We give a stability result under certain hypotheses, by showing that small perturbations scatter as to linear waves. This is joint work with de Suzzoni.

**14.01.2021 | 10:00 — 12:00 Europe/Lisbon**

**Serena Dipierro & Enrico Valdinoci **(University of Western Australia)

** **

**Nonlocal Minimal Surfaces: interior regularity, boundary behavior and stickiness phenomena**

Surfaces which minimize a nonlocal perimeter functional exhibit quite different behaviors than the ones minimizing the classical perimeter. We will investigate some structural properties of nonlocal minimal surfaces both in the interior of a given domain and in the vicinity of its boundary. Among these peculiar features, an interesting property, which is also in contrast with the pattern produced by the solutions of linear equations, is given by the capacity, and the strong tendency, of adhering at the boundary. We will also discuss this phenomenon and present some recent results.

(These are two consecutive talks: Part I is given by Serena Dipierro, Part II by Enrico Valdinoci)

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**07.01.2021 | 16:30 — 17:30 Europe/Lisbon**

**Nilima Nigam **(Simon Fraser University)

** **

**Boundary integral strategies for the Steklov eigenproblem**

In Steklov eigenproblems for elliptic operators, the spectral parameter links boundary traces of eigenfunctions to traces of the Neumann data. It is natural, therefore, to reformulate such eigenproblems in terms of boundary integral operators, which allow for nonsmooth boundaries. In this talk we describe such strategies in the context of Steklov problems for the Laplacian as well as the Helmholtz operator, and their use in studying questions arising in spectral geometry.

**17.12.2020 | 14:30 — 15:30 Europe/Lisbon**

**Svetlana Roudenko **(Florida International University)

** **

**Zakharov-Kuznetsov equation: toward soliton resolution**

We consider Zakharov-Kuznetsov (ZK) equation, which is a higher-dimensional version of the Korteweg-de Vries (KdV) equation, and investigate the dynamics of solutions, especially questions about the soliton stability. We first discuss the situation in two dimensions, in particular, the instability of solitons in the 2d cubic (critical) ZK equation, which leads to blow-up. Then we consider the 3d quadratic ZK equation, originally introduced by Zakharov and Kuznetsov in early 1970's, and discuss the asymptotic stability of solitons. We will also show numerical findings on the formation of solitons and radiation in this equation. This talk will be based on joint works with L.G. Farah, J. Holmer, C. Klein, N. Stoilov, K. Yang.

**10.12.2020 | 14:30 — 15:30 Europe/Lisbon**

**Qiao Huang **(GFM, Universidade de Lisboa)

** **

**Stochastic Cucker-Smale model: collision-avoidance and flocking**

In this talk, we consider the Cucker-Smale flocking model involving both singularity and noise. We first show the local strong well-posedness for the system, in which the communication weight is locally Lipschitz beyond the origin. Then, for the special case that the communication weight has a strong singularity at the origin, we establish the global well-posedness by showing the finite time collision-avoidance. Finally, we study the large time behavior of the system when the communication weight is of zero lower bound. The conditional flocking emerges for the case of constant noise intensity, while the unconditional flocking emerges for various time-varying intensities and long-range communications.

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**03.12.2020 | 14:30 — 15:30 Europe/Lisbon**

**Youcef Mammeri **(Université de Picardie Jules Verne)

** **

**A SIR-type model with diffusion to describe the spatial spread of Covid-19**

We all have to deal with the coronavirus epidemic. Many strategies have been put in place to try to contain the disease, with varying success.

I will present an SIR-type mathematical model to predict the state of the epidemic. The effect of distancing, isolation of exposed individuals and treatment of symptoms will be compared.

I will begin with a simple explanation of SIR models, then discuss a PDE model and its resolution.

**26.11.2020 | 13:30 — 14:30 Europe/Lisbon**

**Jean-Francois Babadjian **(Université Paris Saclay)

** **

**Concentration versus oscillation effects in brittle damage**

This talk is concerned with the asymptotic analysis of a variational model of brittle damage, when the damaged zone concentrates into a set of zero Lebesgue measure, and, at the same time, the stiffness of the damaged material becomes arbitrarily small. In a particular non-trivial regime, concentration leads to a limit energy with linear growth as typically encountered in perfect plasticity. While the singular part of the limit energy can be easily described, the identification of the bulk part of the limit energy requires a subtler analysis of the interplay between concentration and oscillation properties of the displacements.

This is a joint work with F. Iurlano and F. Rindler.

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**26.11.2020 | 14:30 — 15:30 Europe/Lisbon**

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**Marco Caroccia **(Università degli studi di Roma Tor Vergata)

** **

**Contact surface of Cheeger sets**

Geometrical properties of Cheeger sets have been deeply studied by many authors since their introduction, as a way of bounding from below the first Dirichlet (p)-Laplacian eigenvualue. They represents the first eigenfunction of the Dirichlet (1)-Laplacian of a domain. In this talk we will introduce a recent property, studied in collaboration with Simone Ciani (Università degli studi di Firenze), concerning their contact surface with the ambient space. In particular we will show that the contact surface cannot be too small, with a lower bound on the dimension strictly related to the regularity of the ambient space. The talk will focus on the introduction of the problem. It will start with a brief explanation of its connection with the Dirichlet (p)-Laplacian eigenvalue problem. Then a brief sketch of the proof is given. Functional to the whole argument is the notion of removable singularity, as a tool for extending solutions of pdes under some regularity constraint.

**26.11.2020 | 13:30 — 14:30 Europe/Lisbon**

**Jean-Francois Babadjian **(Université Paris Saclay)

** **

**Concentration versus oscillation effects in brittle damage**

This talk is concerned with the asymptotic analysis of a variational model of brittle damage, when the damaged zone concentrates into a set of zero Lebesgue measure, and, at the same time, the stiffness of the damaged material becomes arbitrarily small. In a particular non-trivial regime, concentration leads to a limit energy with linear growth as typically encountered in perfect plasticity. While the singular part of the limit energy can be easily described, the identification of the bulk part of the limit energy requires a subtler analysis of the interplay between concentration and oscillation properties of the displacements.

This is a joint work with F. Iurlano and F. Rindler.

* *

* *

**26.11.2020 | 14:30 — 15:30 Europe/Lisbon**

* *

**Marco Caroccia **(Università degli studi di Roma Tor Vergata)

** **

**Contact surface of Cheeger sets**

Geometrical properties of Cheeger sets have been deeply studied by many authors since their introduction, as a way of bounding from below the first Dirichlet (p)-Laplacian eigenvualue. They represents the first eigenfunction of the Dirichlet (1)-Laplacian of a domain. In this talk we will introduce a recent property, studied in collaboration with Simone Ciani (Università degli studi di Firenze), concerning their contact surface with the ambient space. In particular we will show that the contact surface cannot be too small, with a lower bound on the dimension strictly related to the regularity of the ambient space. The talk will focus on the introduction of the problem. It will start with a brief explanation of its connection with the Dirichlet (p)-Laplacian eigenvalue problem. Then a brief sketch of the proof is given. Functional to the whole argument is the notion of removable singularity, as a tool for extending solutions of pdes under some regularity constraint.

* *

**16.11.2020**** | 14:30 — 15:30 Europe/Lisbon**

**Hynek Kovařík **(Università degli studi di Brescia)

** **

**Absence of positive eigenvalues of magnetic Schroedinger operators**

We study sufficient conditions for the absence of positive eigenvalues of magnetic Schroedinger operators in . In our main result we prove the absence of eigenvalues above certain threshold energy which depends explicitly on the magnetic and electric field. A comparison with the examples of Miller-Simon shows that our result is sharp as far as the decay of the magnetic field is concerned.

The talk is based on a joint work with Silvana Avramska-Lukarska and Dirk Hundertmark.

**Please note that this seminar will take place Monday, not the usual day of the Lisbon WADE.**

**12.11.2020 | 14:30 — 15:30 Europe/Lisbon**

**Rainer Mandel **(Karlsruhe Institute of Technology)

**New dual variational methods for Nonlinear Helmholtz Equations and polychromatic solutions of Nonlinear wave equations**

In the first part of my talk, I present the classical dual variational method in the context of Nonlinear Helmholtz equations that describe monochromatic waves in nonlinear materials. Afterwards, I discuss two recent generalizations of the method. The first deals with an extension to Nonlinear Helmholtz equations with sign-changing nonlinearities. For these problems we construct solutions that have infinite Morse-Index in the dual variational formulation. The second generalization concerns dual variational methods for the construction of breathers, i.e., polychromatic, spatially localized and time-periodic solutions of nonlinear wave equations.

The results were obtained in collaboration with D. Scheider and T. Yesil.

**5/11/2020, 14:30 — 15:30 Europe/Lisbon**

**Wladimir Neves **(Universidade Federal do Rio de Janeiro)** **

**Homogenization of Schrödinger equations. Extended Effective Mass Theorems for non-crystalline matter.**

In this talk, we study the homogenization of the Schrödinger equation beyond the periodic setting. More precisely, rigorous derivation of the effective mass theorems in solid state physics for non-crystalline materials are obtained.

This is a joint work with Vernny Ccajma and Jean Silva.

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**29/10/2020, 13:30 — 14:30 Europe/Lisbon**

**Lorenzo Mazzieri and Piotr Chruściel **(Università di Trento / University of Vienna)

**Static vacuum black holes and Lambda**

We will review the status of the uniqueness theory of static vacuum black holes, with or without a cosmological constant and we will outline the proof of a uniqueness theorem with , proved jointly in collaboration with Stefano Borghini.

**22/10/2020, 14:30 — 15:30 Europe/Lisbon**

**Jean-Baptiste Casteras **(CMAFcIO, Universidade de Lisboa)

**Standing wave and travelling wave solutions for a fourth order Schrödinger equation**

In this talk, we will be interested in standing wave solutions to a fourth order nonlinear Schrödinger equation having second and fourth order dispersion terms. This kind of equation naturally appears in nonlinear optics. In a first time, we will establish the existence of ground-state and renormalized solutions. We will then be interested in their qualitative properties, in particular their stability.

Joint works with Denis Bonheure, Ederson Moreira Dos Santos, Tianxiang Gou, Louis Jeanjean and Robson Nascimento.

**04/09/2020, ****11:00–12:00**** Europe/Lisbon — Online ***

**Marco Morandotti **(Politecnico di Torino)

**Spatially inhomogeneous evolutionary games**

We study an interaction model of a large population of players based on an evolutionary game, which describes the dynamical process of how the distribution of strategies changes in time according to their individual success.

We assume that the population of players is distributed over a state space and that they are each endowed with probability distributions of pure strategies, which they draw at random to evolve their states. Simultaneously, the mixed strategies evolve according to a replicator dynamics, modeling the success of pure strategies according to a payoff functional.

We establish existence, uniqueness, and stability of Lagrangian and Eulerian solutions of this dynamical game by using methods of ODE and optimal transport on Banach spaces.

We apply the general theoretical framework to perform the mean-field analysis of a multi-population agent-based model, where a particle dynamics derived by a nonlocal velocity and a Markov-type jump process for the label s are coupled.

An alternate Lagrangian approximation scheme at discrete times is also proposed.

This research is from joint works with S. Almi, L. Ambrosio, M. Fornasier, G. Savaré, and F. Solombrino.

**14/07/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Pavel Exner **(Doppler Institute for Mathematical Physics and Applied Mathematics, Prague)

**Vertex coupling and spectra of periodic quantum graphs.**

The talk focuses on the influence of the vertex coupling on spectral properties of periodic quantum graphs. Specifically, two questions will be addressed. The first concerns the number of open spectral gaps; it is shown that graphs with a nontrivial coupling can have finite but nonzero number of them. Secondly, motivated by recent attempts to model the anomalous Hall effect, we investigate a class of vertex couplings that violate the time reversal invariance. For the simplest coupling of this type we show that its high-energy properties depend on the parity of the lattice vertices, and discuss various consequences of this property.

*****to get access to the password, please register on the website or contact one of the organizers.

**09/07/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Elvira Zappale **(Università Degli Studi di Salerno)

**Optimal design problems**I will present several integral representation results for certain functionals arising in the context of optimal design and damage models, in presence of a perimeter penalization term.

I will consider several frameworks, and I will also discuss the case with non-standard growth conditions.

*****to get access to the password, please register on the website or contact one of the organizers.

**30/06/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Barbara Brandolini **(Università Degli Studi di Napoli Federico II)

**Sharp lower bounds for Neumann eigenvalues**

*****to get access to the password, please register on the website or contact one of the organizers.

**23/06/2020, Tuesday, 11:00–12:00 Europe/Lisbon — Online ***

**Tatsuya Miura **(Tokyo Institute of Technology)

**On the isoperimetric inequality and surface diffusion flow for multiply winding curves.**

In this talk we discuss dynamical stability of multiply covered circles under the surface diffusion flow. To this end we first establish a general form of the isoperimetric inequality for immersed closed curves under rotational symmetry, which would be of independent interest. We then apply it to obtaining a certain class of rotationally symmetric initial curves from which solutions to the surface diffusion flow exist globally-in-time and converge to multiply covered circles. This talk is based on a joint work with Shinya Okabe at Tohoku University.

*****to get access to the password, please register on the website or contact one of the organizers.

**16/06/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Riccardo Adami **(Politecnico di Torino)

**Ground states of the Nonlinear Schroedinger Equation on Graphs: an overview.**

Driven by physical and technological applications, during the last five years the study of nonlinear evolution on branched structures (graphs, networks) has undergone a fast development. We review on the main achievements and on the open problems. This is a joint project with several people, among which Simone Dovetta, Enrico Serra, Lorenzo Tentarelli, and Paolo Tilli.

*****to get access to the password, please register on the website or contact one of the organizers.

**09/06/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Lucio Boccardo **(Università di Roma La Sapienza)

**Recent developments on Dirichlet problems with singular convection/drift terms.**

*****to get access to the password, please register on the website or contact one of the organizers.

* *

**02/06/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Maria Colombo **(École Polytechnique Fédérale de Lausanne)

**Nonunique characteristic curves of Sobolev vector fields**

Given a vector field in , the classical Cauchy-Lipschitz theorem shows existence and uniqueness of its flow provided the vector field is sufficiently smooth; this, in turn, translates in existence and uniqueness results for the transport equation. In 1989, Di Perna and Lions proved that Sobolev regularity for vector fields, with bounded divergence and a growth assumption, is sufficient to establish existence, uniqueness and stability of a generalized notion of flow, consisting of a suitable selection among the trajectories of the associated ODE. A long-standing open question is whether the uniqueness of the regular Lagrangian flow is a corollaryof the uniqueness of the trajectory of the ODE for a.e. initial datum. In this talk we give an overview of the topic and we provide a negative answer to this question. To show this result we exploit the connection with the transport equation, based on Ambrosio’s superposition principle, and a new ill-posedness result for positive solutions of the continuity equation.

*****to get access to the password, please register on the website or contact one of the organizers.

**26/05/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Luis Vega** (Basque Center for Applied Mathematics)

**The Vortex Filament Equation, the Talbot effect, and non-circular jets**

We will propose the vortex filament equation as a possible toy model for turbulence, in particular because of its striking similarity to the dynamics of non-circular jets. This similarity implies the existence of some type of Talbot effect due to the interaction of non-linear waves that propagate along the filament. Another consequence of this interaction is the existence of a new class of multi-fractal sets that can be seen as a generalization of the graph of Riemann’s non-differentiable function. Theoretical and numerical arguments about the transfer of energy will be also given. This a joint work with V. Banica and F. de la Hoz.

*****to get access to the password, please register on the website or contact one of the organizers.

**19/05/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Bernold Fiedler** (Institute of Mathematics, Freie Universität Berlin)

**Sturm meanders: global attractors, Temperley-Lieb algebras, and black holes**

Fusco and Rocha studied Neumann boundary value problems for scalar ODEs of second order via a shooting approach. They introduced the notion of what we now call Sturm permutations. These permutations relate, on the one hand, to a special class of meandering curves as introduced by Arnol’d in a singularity theory context. On the other hand, they became central in the study of global attractors of nonlinear parabolic partial differential equations of Sturm type.

We discuss relations of Sturm meanders with further areas: the multiplicative and trace structure in Temperley-Lieb algebras, discrete versions of Cartesian billiards, and the problem of constructing initial conditions for black hole dynamics which satisfy the Einstein constraints. We also risk a brief glimpse at the long and meandric history of meander patterns themselves.

We report on joint work with Pablo Castañeda, Juliette Hell, Carlos Rocha, and Brian Smith. See also http://dynamics.mi.fu-berlin.de/

For further material we recommend the beautifully illustrated book “Meanders” by Anna Karnauhova, de Gruyter 2017.

*****to get access to the password, please register on the website or contact one of the organizers.

**12/05/2020, Tuesday, 16:30–17:30 Europe/Lisbon — Online ***

**Diogo Oliveira e Silva **(University of Birmingham)

**Global maximizers for spherical restriction**

*to get access to the password, please register on the website or contact one of the organizers.

**05/05/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online ***

**Filippo Santambrogio** (Université Claude Bernard - Lyon 1)

**Optimal transport methods for the regularity of 2D functions of least gradient**

The least gradient problem (minimizing the BV norm with given boundary data), motivated by both image processing applications and connections with minimal surfaces, is known to be equivalent, in the plane, to the Beckmann minimal-flow problem (an alternative formulation of the *L*^{1} Monge-Kantorovich optimal transport problem) with source and target measures located on the boundary of the domain. Hence, Sobolev regularity of functions of least gradient is equivalent in this setting to *L ^{p} *bounds on the solution of the Beckmann problem (i.e. on what is called the transport density) and can be attacked with techniques which are now standard in optimal transport. From the transport point of view, the novelty of the estimates that I will present, coming from a joint paper with S. Dweik, lies in the fact they are obtained for transport between measures which are concentrated on the boundary. From the BV point of view, a new result is the

*W*

^{1,p}^{ }regularity of the least gradient function whenever the boundary datum is

*W*

^{1,p}^{ }as a 1

*D*function: moreover, the optimal transport framework is strong enough to deal with arbitrary strictly convex norms instead of the Euclidean one with almost no effort.

*to get access to the password, please register on the website or contact one of the organizers.

* *

*The Lisbon Webinar in Analysis in Differential Equations is a joint iniciative of CAMGSD, CMAFcIO and GFM, three research centers of the University of Lisbon. It is aimed at filling the absence of face-to-face seminars and wishes to be a meeting point of mathematicians working in the field.*

**28/04/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online**

**Michael Goldman** (Laboratoire Jacques-Louis Lions and Université Paris 7)

**On an old conjecture of Almgren**

In this talk I will give an overview on the few results available on the conjecture of Almgren regarding the convexity of drops subject to the action of an external potential. In particular I will present recent progress in this direction obtained with G. De Philippis on their connectedness. Together with an older result of McCann, this answers positively the conjecture in dimension two. The proof is inspired by the two-point function technique introduced by B. Andrews and is reminiscent of the doubling of variables trick in the context of viscosity solutions.

**21/04/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online**

**Matthias Röger**, Technische Universität Dortmund

**An obstacle type problem as a limit of a model for cell polarization**

We consider the polarization of a cell in response to an outer signal. The mathematical model consists of a diffusion equation in the inner volume coupled to a reaction diffusion system on the cell membrane. In a certain asymptotic limit we rigorously prove the convergence towards a generalized obstacle problem. In term of this limit system we derive conditions for the onset of polarization. The results will be mainly presented for the stationary case, we will briefly discuss some extensions to the time-dependent case.

(This is joint work with Barbara Niethammer, Juan Velazquez, and Anna Logioti)

**14/04/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online**

**Dorin Bucur,** Université de Savoie.

**Boundary behaviour of Robin problems and isoperimetric spectral inequalities.**

Consider the Poisson equation with Robin boundary conditions in a (nonsmooth) domain with a bounded, nonnegative right hand side. Given a point on the boundary, the question is whether the solution has a strictly positive lower limit at this point. If the domain is smooth the answer is positive as a consequence of the Hopf maximum principle. If the domain is not smooth, the answer may be positive or negative, depending on the geometry of the domain around the point. This question was raised in a probabilistic context by Bass, Burdzy and Chen in 2008, when they obtained results for Lipschitz sets and cuspidal domains.

Our motivation is related to the fact that positive answers to the question above, together with a control of the infimum of the boundary values, lead to sharp quantitative forms of isoperimetric inequalities of spectral type for the Robin Laplacian.

In this talk, I will make the point on recent advances on isoperimetric inequalities involving the Robin Laplacian and I will show how the boundary behaviour pops up in the quantitative form of the inequalities. If times remains, I will present a variational approach to deal with the boundary behaviour for general elliptic operators and various geometric contexts.

The results were obtained together with A. Giacomini and M. Nahon.

**07/04/2020, Tuesday, 16:00–17:00 Europe/Lisbon — Online**

Susanna Terracini, Università di Torino.

Pattern Formation Through Spatial Segregation.

*Video available here.*

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