News & Events 2024
Activities of the RTG

Weekly Seminar
TBA
John Simon (Austrian Academy of Sciences, Linz (Austria))
Thu, 12 Dec 2024 • 10:45-11:45h • Pontdriesch 14, room 008 SeMath (host: Niklas Kolbe)

Abstract

TBA

Weekly Seminar
A general framework of implicit high-order schemes for hyperbolic systems
Giuseppe Visconti (Sapienza University of Rome (Italy))
Thu, 05 Dec 2024 • 10:15 coffee/tea in 001 // 10:45-11:45 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Michael Herty)

Abstract

This talk addresses the challenges of designing high-order implicit schemes for systems of hyperbolic conservation laws, particularly in the context of stiff problems where wave speeds span several orders of magnitude.

In such cases, explicit schemes necessitates small time-steps due to the stringent CFL stability criterion. Implicit time integration schemes leverage superior stability properties enabling the selection of time-steps based solely on accuracy requirements, thereby bypassing the need for minute time-steps. This feature is particularly beneficial when tracking phenomena of interest which evolve on slower time scales than the fastest waves.

However, implicit methods are computationally expensive, as they require solving a system of equations, typically nonlinear, at each time-step. Achieving high-order accuracy adds another layer of complexity due to the need for nonlinear space limiters that prevent spurious oscillations, which further increase the nonlinearity of the problem.

The novel approach proposed here uses a first-order implicit scheme to pre-compute the nonlinearities introduced by the space-limiting procedure. This allows the high-order implicit scheme to remain nonlinear only due to the flux function, thereby reducing computational complexity. The method is investigated using DIRK (Diagonally Implicit Runge-Kutta) schemes for time integration, along with CWENO (Central Weighted Essentially Non-Oscillatory) or Discontinuous Galerkin reconstructions for space approximation. We show that this approach enables accurate resolution of slow waves, even with large time-steps, offering a significant advantage in stiff regimes. Joint work with: Maya Briani, Gabriella Puppo, Matteo Semplice

Weekly Seminar
An introduction to the turnpike phenomenon with examples and links to recent results
Martin Gugat (Friedrich-Alexander-Universität Erlangen)
Thu, 28 Nov 2024 • 10:45-11:45h • Pontdriesch 14, room 008 SeMath (host: Michael Herty)

Abstract

The turnpike phenomenon concerns structural properties of the solutions of optimal control problems where the state is governed by an evolution equation over a certain time horizon and the objective functional is of integral type, for example the sum of a tracking term and a control cost.

In this situation for large time horizons the optimal controls and optimal states often come close to the solution of the corresponding static problem, where the feasible states are the static states corresponding to the evolution equation.

In particular, the influence of the given initial state becomes smaller as time proceeds.

Turnpike results provide valuable insights about the types of optimal controls that we can expect in a wide range of applications.

Joint Seminar CRC/EDDy
Quantitative uniqueness to parabolic operators with applications to nodal sets
Vedansh Arya (University of Jyväskylä (Finland))
Thu, 17 Oct 2024 • 10:45-11:45 • Pontdriesch 14, room 008 SeMath (host: Wenhui Shi)

Abstract

In this talk, we will discuss sharp estimate of the order of vanishing of solutions to parabolic equations with variable coefficients. For real-analytic leading coefficients, we will present localised estimate of the nodal set, at a given time-level, that generalises the celebrated one of Donnelly and Fefferman. We will also discuss Landis type results for global solutions. This is based on joint work with Agnid Banerjee and Nicola Garofalo.

Weekly Seminar
Monotonicity of the jump set for denoising model
Riccardo Cristoferi (Radboud University (The Netherlands))
Thu, 10 Oct 2024 • 10:15 coffee/tea in 001 // 10:45-11:45 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Likhit Ganedi)

Abstract

We consider the minimizer of the ROF functional for denoising, which is the sum of the total variation and the square of the $L^2$ distance from a fixed function $f$. The former term is weighted by a positive parameter alpha. We focus on the one dimensional scalar case. Under mild regularity assumptions on the function $f$, we prove that the jump set of the minimizer is monotone with respect to the parameter alpha. This proves a case of a long-standing conjecture in the field.

This is a joint work in collaboration with Rita Ferreira (KAUST), Irene Fonseca (CMU), and José Iglesias (U. Twente).

Weekly Seminar
Local well-posedness of the Landau-Lifshitz equation with helicity
Ikkei Shimizu (Osaka University (Japan))
Thu, 22 Aug 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath (host: Christof Melcher)

Abstract

In this talk, we consider the dynamical Landau-Lifshitz equation (LL) without Gilbert damping, which is a PDE arising in micromagnetism. From PDE point of view, LL can be considered as geometric version of the Schrödinger equation, and has been studied in the community of nonlinear dispersive PDEs, referred to as Schrödinger maps. Starting with a short review on known results, I will introduce my result on the local well-posedness of initial value problem for LL with helicity term (Dzyaloshinskii-Moriya interaction). The difficulty is that the helicity term has quadratic derivative nonlinearity. To handle this, we exploit the skew-adjoint structure of helicity term to cancel bad part of the nonlinearity in the energy method.

Weekly Seminar
Uncertainty quantification in traffic flow models
Elisa Iacomini (University of Ferrara (Italy))
Thu, 08 Aug 2024 • 10:00 coffee/tea in 001 // 10:30-11:30 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Sophie Hörnschemeyer)

Abstract

Vehicular traffic models as complex dynamical systems have been widely studied, yet challenges in obtaining reliable forecasts persist, due to the inherent uncertainty in real-world traffic caused by noises in the measurements, fluctuating demand, unforeseen incidents, and varied driver behaviors. This uncertainty significantly impacts the accuracy and reliability of traffic flow models, making it essential to integrate uncertainty into these models for more realistic solutions.

In this talk, we will focus on investigating the propagation of uncertainties in traffic flow models. Two main approaches to quantify uncertainty will be discussed: non-intrusive methods, such as Monte Carlo techniques, which solve the model for a fixed number of samples using deterministic algorithms, and intrusive methods, like the stochastic Galerkin method, which modify the governing equations to incorporate probabilistic elements. Both methodologies will be presented, highlighting their advantages and limitations, to provide a comprehensive analysis of how they contribute to more accurate and reliable traffic flow predictions.

Weekly Seminar
Dispersive behaviour of hyperbolic waves on periodic media
Giovanni Russo (University of Catania (Italy))
Thu, 18 Jul 2024 • 10:00 coffee/tea in 001 // 10:30-11:30 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Sebastian Noelle)

Short Course
Case studies on dynamics in infinite dimensions (3/3)
Bob Pego (Carnegie Mellon University, Pittsburgh (U.S.A.))
Tue, 16 Jul 2024 • 10:30-12:00h • Templergraben 55, seminar room 114 (host: Likhit Ganedi)

Abstract

The overall aim of the course is to review a number of recent results illustrating the great variety of dynamic behavior possible in infinite-dimensional problems, going into some detail on several of the key aspects.

Convergence and nonconvergence of gradient flows: Gradient-flow structure is widely investigated and is nowadays of great importance in optimization and machine learning. Despite having finite dissipation, gradient flows can fail to converge in large time. We’ll discuss criteria that can ensure convergence, including Lojasiewicz gradient estimates, surprising examples showing how convergence can fail in infinite dimensions, and some open problems. Models discussed will come mainly from population biology and phase transition dynamics.

Optimal breaking flows and Minkowski’s problem for polytopes: Least-action principles for dynamics have seen much activity in the last few decades related to geodesic flows of diffeomorphisms and Wasserstein distance. Motivated by results relating relaxation of action minimization for free-boundary incompressible flows to Wasserstein distance, we study optimizing incompressible flows. Such flows are locally rigid and characterized by a countable Alexandrov theorem related to Minkowski’s study of convex bodies and Gauss curvature. Examples and conjectures will be discussed.

Long waves and solitons in lattices with forces of infinite range: Solitary waves have been known to exist in particle lattices with nearest-neighbor forces since the 1994 work of Friesecke and Wattis.
Small-amplitude, near-sonic waves are well described by the KdV equation in lattices with finite-range forces. For infinite-range, power-law forces that decay slower than the fourth power of distance, nonlocal effects appear in the continuum limit. We describe this through formal asymptotics, describe remarkable formulas for solitons in the inverse-cube case (Calogero-Moser), and discuss a new existence theorem for near-sonic solitary waves in this regime.

Weekly Seminar
Long-time dynamics and photon loss in the Kompaneets model of Compton scattering
Bob Pego (Carnegie Mellon University, Pittsburgh (U.S.A.))
Thu, 11 Jul 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath (host: Likhit Ganedi)

Abstract

The Kompaneets equation models the energy spectrum of photons interacting with a gas of electrons by Compton scattering. It is fundamental in modern cosmology for explaining the Sunyaev-Zeldovich effect which involves deformation of the cosmic microwave background. We establish L1 convergence to Bose-Einstein equilibria in large time and prove several results on the existence and behavior of a `condensate' of photons at the zero-energy boundary.

The Kompaneets equation is a scalar conservation law with a degenerate parabolic nature that permits a loss of photons in finite time due to shock formation at the zero-energy boundary. Solutions satisfy entropy decay, an Oleinik one-sided slope bound, and L1 contraction and comparison principles.

This is joint work with Gautam Iyer, Hailiang Liu, Josh Ballew, and Dave Levermore.

Short Course
Case studies on dynamics in infinite dimensions (2/3)
Bob Pego (Carnegie Mellon University, Pittsburgh (U.S.A.))
Wed, 10 Jul 2024 • 10:30-12:00h • Pontdriesch 14, room 008 SeMath (host: Likhit Ganedi)

Abstract

The overall aim of the course is to review a number of recent results illustrating the great variety of dynamic behavior possible in infinite-dimensional problems, going into some detail on several of the key aspects.

Convergence and nonconvergence of gradient flows: Gradient-flow structure is widely investigated and is nowadays of great importance in optimization and machine learning. Despite having finite dissipation, gradient flows can fail to converge in large time. We’ll discuss criteria that can ensure convergence, including Lojasiewicz gradient estimates, surprising examples showing how convergence can fail in infinite dimensions, and some open problems. Models discussed will come mainly from population biology and phase transition dynamics.

Optimal breaking flows and Minkowski’s problem for polytopes: Least-action principles for dynamics have seen much activity in the last few decades related to geodesic flows of diffeomorphisms and Wasserstein distance. Motivated by results relating relaxation of action minimization for free-boundary incompressible flows to Wasserstein distance, we study optimizing incompressible flows. Such flows are locally rigid and characterized by a countable Alexandrov theorem related to Minkowski’s study of convex bodies and Gauss curvature. Examples and conjectures will be discussed.

Long waves and solitons in lattices with forces of infinite range: Solitary waves have been known to exist in particle lattices with nearest-neighbor forces since the 1994 work of Friesecke and Wattis.
Small-amplitude, near-sonic waves are well described by the KdV equation in lattices with finite-range forces. For infinite-range, power-law forces that decay slower than the fourth power of distance, nonlocal effects appear in the continuum limit. We describe this through formal asymptotics, describe remarkable formulas for solitons in the inverse-cube case (Calogero-Moser), and discuss a new existence theorem for near-sonic solitary waves in this regime.

Short Course
Case studies on dynamics in infinite dimensions (1/3)
Bob Pego (Carnegie Mellon University, Pittsburgh (U.S.A.))
Tue, 09 Jul 2024 • 10:30-12:00h • Templergraben 55, seminar room 114 (host: Likhit Ganedi)

Abstract

The overall aim of the course is to review a number of recent results illustrating the great variety of dynamic behavior possible in infinite-dimensional problems, going into some detail on several of the key aspects.

Convergence and nonconvergence of gradient flows: Gradient-flow structure is widely investigated and is nowadays of great importance in optimization and machine learning. Despite having finite dissipation, gradient flows can fail to converge in large time. We’ll discuss criteria that can ensure convergence, including Lojasiewicz gradient estimates, surprising examples showing how convergence can fail in infinite dimensions, and some open problems. Models discussed will come mainly from population biology and phase transition dynamics.

Optimal breaking flows and Minkowski’s problem for polytopes: Least-action principles for dynamics have seen much activity in the last few decades related to geodesic flows of diffeomorphisms and Wasserstein distance. Motivated by results relating relaxation of action minimization for free-boundary incompressible flows to Wasserstein distance, we study optimizing incompressible flows. Such flows are locally rigid and characterized by a countable Alexandrov theorem related to Minkowski’s study of convex bodies and Gauss curvature. Examples and conjectures will be discussed.

Long waves and solitons in lattices with forces of infinite range: Solitary waves have been known to exist in particle lattices with nearest-neighbor forces since the 1994 work of Friesecke and Wattis.
Small-amplitude, near-sonic waves are well described by the KdV equation in lattices with finite-range forces. For infinite-range, power-law forces that decay slower than the fourth power of distance, nonlocal effects appear in the continuum limit. We describe this through formal asymptotics, describe remarkable formulas for solitons in the inverse-cube case (Calogero-Moser), and discuss a new existence theorem for near-sonic solitary waves in this regime.

Joint Seminar CRC/EDDy
Optimal Korn-Maxwell-Sobolev inequalities
Peter Lewintan (Karlsruhe Institute of Technology)
Thu, 04 Jul 2024 • 10:00 coffee/tea in 001 // 10:30-11:30 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Lambert Theisen)

Abstract

We present a complete picture of coercive Korn-type inequalities for generalised incompatible fields, and optimally extend and unify several previously known inequalities that are crucial to the existence theory for a variety of models in continuum mechanics.

CANCELLED: Joint Seminar CRC/EDDy (special time/location)
Structure-preserving artificial neural networks for entropy-based moment closures
Martin Frank (Karlsruhe Institute of Technology)
Thu, 27 Jun 2024 • 15:00-16:00h • CANCELLED

Abstract

We present how artificial neural networks can be used to substitute an otherwise costly moment closure. Specifically, we study entropy closures which have many desirable properties (among them guaranteed hyperbolicity, positivity and entropy decay). We will need to construct very specific artificial neural networks that are guaranteed to preserve these advantages. Furthermore, we address the question of data sampling. Several numerical results are shown.

Weekly Seminar
Geometric regularization in three-dimensional inverse obstacle scattering
Jannik Rönsch (Georg-August-Universität Göttingen)
Thu, 27 Jun 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath (host: Axel Wings)

Abstract

We study the classical inverse problem to determine the shape of a three-dimensional scattering obstacle from measurements of scattered waves or their far-field patterns. Previous research on this subject has mostly assumed the object to be star-shaped and imposed a Sobolev penalty on the radial function or has defined the penalty term in some other ad-hoc manner which is not invariant under coordinate transformations.

For the case of curves in $\mathbb{R}^2$, Julian Eckardt suggests in his PhD thesis to use the bending energy as regularisation functional and proposes Tikhonov regularization and regularized Newton methods on a shape manifold. The case of surfaces in $\mathbb{R}^3$ is considerably more demanding. First, a suitable space (manifold) of shapes is not obvious. The second problem is to find a

stabilizing functional for generalised Tikhonov regularisation which on the

one hand should be bending-sensitive and on the other hand prevent the surface from self-intersections during the reconstruction.

The tangent-point energy is a parametrization-invariant and repulsive surface energy that is constructed as the double integral over a power of the tangent point radius with respect to two points on the surface, i.e. the smallest radius of a sphere being tangent to the first point and intersecting the other. The finiteness of this energy also provides $C^{1,\alpha}$ Hölder regularity of the surfaces. Using this energy as the stabilising functional, we choose general surfaces of Sobolev-Slobodeckij reguality, which are naturally connected to this energy.

The proposed approach works for surfaces of arbitrary (known) topology.

In numerical examples we demonstrate that the flexibility of our approach in handling the reconstruction of rather general shapes.

Authors: Jannik Rönsch, Henrik Schumacher, Max Wardetzky, and Thorsten Hohage

Joint Seminar CRC/EDDy
Multiscale-Spectral Generalized Finite Elements: Efficient Localized Model Order Reduction
Robert Scheichl (University of Heidelberg)
Fri, 21 Jun 2024 • 14:30-15:30h • Pontdriesch 14, room 008 SeMath (host: Markus Bachmayr)

Abstract

In this talk, I will present an efficient generalized finite element method with optimal (multiscale spectral) local approximation spaces (MS-GFEM) for PDEs with heterogeneous coefficients. In practice, the local approximation spaces are constructed from local eigenproblems solved on some sufficiently fine finite element mesh with mesh size h. In this work, we will provide rigorous error estimates for the fully discrete method. The error bound of the discrete MS-GFEM approximation is proved to converge as h tends to 0 towards that of the continuous MS-GFEM approximation, which was shown to decay nearly exponentially in previous works. Moreover, even for fixed mesh size h, a nearly exponential rate of convergence of the local approximation errors with respect to the dimension of the local spaces is established. The method can also be used as an effective preconditioner in an iterative solver with a ’tuneable’ convergence rate. On the practical side, an efficient and accurate method for solving the discrete eigenvalue problems is proposed by incorporating the discrete-harmonic constraint directly into the eigenproblem via a Lagrange multiplier approach. Numerical experiments that confirm the theoretical results and demonstrate the effectiveness of the method are presented for a second-order elliptic problem, for a large-scale problem of linear elasticity in composite aerospace applications and for a high-frequency heterogeneous Helmholtz problem. Even in this last example, a quasi-optimal and nearly exponential (wavenumber-explicit) global convergence of the method can be theoretically proved, provided the size of the subdomains is O(1/k) (where k is the wavenumber).

Joint Seminar CRC/EDDy
Optimal control for a class of hypocoercive Fokker-Planck equations
Tobias Breiten (TU Berlin)
Fri, 14 Jun 2024 • 14:30-15:30h • Schinkelstr. 1, Templergraben 51, bldg 1080, room R5 (host: Mathias Oster)

Abstract

The transient and long time behavior of Langevin equations has received increased attention over the last years. Difficulties arise from a lack of coercivity, usually termed hypocoercivity, of the underlying kinetic Fokker-Planck operator which is a consequence of the partially deterministic nature of a second order stochastic differential equation. Similar challenges arise within the study of the continuous version of (stochastic) Cucker-Smale type flocking models where the resulting hypocoercive PDE additionally becomes nonlinear. Introducing controls on the finite-dimensional level naturally lead to abstract infinite-dimensional bilinear control problems with an unbounded but admissible control operator. By means of an artificial diffusion approach, solutions to a class of hypoercives PDEs as well as to associated optimal control problems are analyzed under smallness assumptions on the initial data.

Short Course
Topological Obstructions for Sobolev Spaces (3/3)
Armin Schikorra (University of Pittsburgh (U.S.A.))
Thu, 13 Jun 2024 • 10:30-12:00h • Pontdriesch 14, room 008 SeMath (host: Heiko von der Mosel)

Abstract

In this minicourse, we will delve into various aspects of topological obstructions within the framework of Sobolev spaces. To illustrate fundamental principles, we will initially explore a Sobolev adaptation of the Brouwer Fixed Point theorem. This exploration will naturally lead us to considerations regarding the definition of degree for Sobolev maps between manifolds. Subsequently, we will examine Sobolev maps with restricted rank, alongside several examples illustrating topological obstructions encountered in the approximation or extension of Sobolev maps. It is assumed that participants are acquainted with the theory of Sobolev spaces in Euclidean contexts. Any topological concept will be carefully defined throughout the course.

https://en.wikipedia.org/wiki/Hopf_fibration#/media/File:Hopf_Fibration.png

Short Course
Topological Obstructions for Sobolev Spaces (2/3)
Armin Schikorra (University of Pittsburgh (U.S.A.))
Wed, 12 Jun 2024 • 10:30-12:00h • Pontdriesch 14, room 008 SeMath (host: Heiko von der Mosel)

Abstract

In this minicourse, we will delve into various aspects of topological obstructions within the framework of Sobolev spaces. To illustrate fundamental principles, we will initially explore a Sobolev adaptation of the Brouwer Fixed Point theorem. This exploration will naturally lead us to considerations regarding the definition of degree for Sobolev maps between manifolds. Subsequently, we will examine Sobolev maps with restricted rank, alongside several examples illustrating topological obstructions encountered in the approximation or extension of Sobolev maps. It is assumed that participants are acquainted with the theory of Sobolev spaces in Euclidean contexts. Any topological concept will be carefully defined throughout the course.

https://en.wikipedia.org/wiki/Hopf_fibration#/media/File:Hopf_Fibration.png

Short Course
Topological Obstructions for Sobolev Spaces (1/3)
Armin Schikorra (University of Pittsburgh (U.S.A.))
Tue, 11 Jun 2024 • 10:30-12:00h • Pontdriesch 14, room 008 SeMath (host: Heiko von der Mosel)

Abstract

In this minicourse, we will delve into various aspects of topological obstructions within the framework of Sobolev spaces. To illustrate fundamental principles, we will initially explore a Sobolev adaptation of the Brouwer Fixed Point theorem. This exploration will naturally lead us to considerations regarding the definition of degree for Sobolev maps between manifolds. Subsequently, we will examine Sobolev maps with restricted rank, alongside several examples illustrating topological obstructions encountered in the approximation or extension of Sobolev maps. It is assumed that participants are acquainted with the theory of Sobolev spaces in Euclidean contexts. Any topological concept will be carefully defined throughout the course.

https://en.wikipedia.org/wiki/Hopf_fibration#/media/File:Hopf_Fibration.png

Basic Notions Seminar
What is a Markov decision process?
Johannes Müller (RWTH Aachen University)
Tue, 28 May 2024 • 16:30-17:30 • Pontdriesch 14, bldg 1953, EDDy's seminar room 256

Abstract

We will introduce Markov decision processes, which were introduced in the 1950s in the seminal works of Bellman and Blackwell and form the mathematical model underlying reinforcement learning. We will discuss the central objects including value functions and state-action visitation distributions, which can be interpreted as generalized stationary distributions of the Markov process. Depending on the time, we will cover classic solution strategies like value iteration, policy iteration and linear programming principles and touch on the problem of partial observability.

Weekly Seminar
A proof of the Toponogov conjecture
Wilhelm Klingenberg (Durham University (UK))
Thu, 16 May 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath (host: Umberto Hryniewicz)

Abstract

In 1995 Toponogov authored the following conjecture: “Every smooth strictly convex and complete surface of the type of a plane has an umbilic point, possibly at infinity”. In our talk, we will outline a proof, in collaboration with Brendan Guilfoyle, namely that (i) the Fredholm index of an Riemann Hilbert boundary problem for holomorphic discs associated to a putative counterexample is negative. Thereby, (ii) no solutions may exist for a generic perturbation of the boundary condition (iii) however, the geometrization by a neutral metric gives rise to barriers for the continuity method to prove existence of a holomorphic disc.

Basic Notions Seminar
The ABCs of Boltzmann's equation
Giacomo Borghi (RWTH Aachen University)
Tue, 07 May 2024 • 16:30-17:30 • Pontdriesch 14, bldg 1953, EDDy's seminar room 256

Abstract

Introduced to model rarefied gas, the Boltzmann equation is a flexible tool to model particles (or, more generally, entities) evolving via binary interaction. In this seminar, we review the basic elements of this model, the entropy dissipation theorem, and discuss its approximation via particle systems.

Weekly Seminar
Long-time behavior of the Stokes-transport system in a channel
Antoine Leblond (Max Planck Institute for Meteorology, Hamburg)
Thu, 02 May 2024 • 10:00 coffee/tea in 001 // 10:30-11:30 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Juliette Dubois)

Abstract

We consider here a 2d incompressible fluid in a periodic channel, whose density is advected by pure transport, and whose velocity is given by the Stokes equation with gravity source term. Dirichlet boundary conditions are taken for the velocity field on the bottom and top of the channel, and periodic conditions in the horizontal variable. We prove that the affine stratified density profile is stable under small perturbations in Sobolev spaces and show convergence of the density to another limiting stratified density profile for large time with an explicit algebraic decay rate. Moreover, we are able to precisely identify the limiting profile as the decreasing vertical rearrangement of the initial density. Finally, we show that boundary layers are formed for large times in the vicinity of the upper and lower boundaries. These boundary layers, which had not been identified in previous works, are given by a self-similar Ansatz and driven by a linear mechanism. This allows us to precisely characterize the long-time behavior beyond the constant limiting profile and enlighten the optimal decay rate.

This is a joint work with Anne-Laure Dalibard and Julien Guillod (Laboratoire Jacques-Louis Lions, Sorbonne Université, Paris, France), part of my PhD thesis.

Weekly Seminar
Two-scale finite element approximation of a homogenized plate model
Christoph Smoch (University of Bonn)
Thu, 18 Apr 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath (host: Sasa Lukic)

Abstract

We study the discretization of a homogenized and dimension reduced model for the elastic deformation of microstructured thin plates proposed by Hornung, Neukamm, and Velčić in 2014. Thereby, a nonlinear bending energy is based on a homogenized quadratic form which acts on the second fundamental form associated with the elastic deformation. Convergence is proven for a multi-affine finite element discretization of the involved three-dimensional microscopic cell problems and a discrete Kirchhoff triangle discretization of the two-dimensional isometry-constrained macroscopic problem. Finally, the convergence properties are numerically verified in selected test cases and qualitatively compared with deformation experiments for microstructured sheets of paper.

Social Event
Math Chat
for Bachelor's and Master's students
Wed, 17 Apr 2024 • 17:00-18:00h • Templergraben 55, seminar room 114

The doctoral researchers of the Research Training Group (EDDy) are pleased to announce the next Math Chat (everyone is invited).

Math Chats are informal gatherings hosted by PhD students for bachelor’s and master’s students who are interested in finding out more about what research is like, how to find a thesis advisor, which seminars & opportunities are available through EDDy at the RWTH, etc.

Basic Notions Seminar
Shallow Water Equation
Sophie Hörnschemeyer (RWTH Aachen University)
Mon, 15 Apr 2024 • 16:30-17:30 • Pontdriesch 14, bldg 1953, EDDy's seminar room 256

Abstract

The shallow water equations are a frequently used approximation for modelling currents in rivers, lakes or oceans. But what exactly are the shallow water equations? In this Basic Notion Seminar we will derive the shallow water equations, analyse their properties and briefly discuss their numerical implementation.

Weekly Seminar
A Variational model involving nonlocal interactions of Wasserstein type
Ihsan Topaloglu (Virginia Commonwealth University (U.S.A.))
Thu, 11 Apr 2024 • 10:00 coffee/tea in 001 // 10:30-11:30 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Maria Westdickenberg)

Abstract

In this talk I will consider a variational problem which appears in models of bilayer membranes. After introducing and deriving the model I will establish the existence of volume-constrained minimizers where the energy functional consists of two competing terms: a surface energy term penalizing transitions between sets and a nonlocal energy involving the Wasserstein distance between equal volume sets. In the second part of the talk I will consider the maximization of the minimum Wasserstein distance between two given sets, and show that this maximum is obtained by a micella. These results are drawn from joint works with Almut Burchard, Davide Carazzato, Michael Novack, and Raghavendra Venkatraman.

Weekly Seminar
Regularity of harmonic and n-harmonic maps into compact Riemannian manifolds: known results and open questions
Paweł Strzelecki (University of Warsaw (Poland))
Thu, 14 Mar 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath (host: Heiko von der Mosel)

Abstract

I plan to present a survey talk on regularity of harmonic and n-harmonic maps into compact Riemannian manifolds, putting the problem in a historical perspective and discussing known results, from the papers of R. Schoen and K. Uhlenbeck which appeared 40 years ago, through the work of F. Hélein on harmonic maps on planar domains, to the recent results of A. Schikorra and my Polish collaborators, M. Miśkiewicz and B. Petraszczuk.

I shall also present a new example by Petraszczuk who proved that a specific mildly nonlinear elliptic system in the plane (with a quadratic nonlinearity in the gradient) - considered already by J. Frehse in 1973 has the following property: given an arbitrary compact set K in the disc, there exists a solution which is discontinuous precisely on K, and smooth elsewhere. A few related open questions will be stated at the end.

Joint Seminar: EDDy+SFB special seminar in PDE/Analysis (special day/time/location)
The stochastic Klausmeier system or A stochastic activator-inhibitor system
Erika Hausenblas (Montanuniversität Leoben (Austria))
Tue, 27 Feb 2024 • 14:00-15:00h • Templergraben 55, 1st floor, lecture hall III (host: Chunxi Jiao)

Abstract

Nonlinear partial differential equations appear naturally in many biological or chemical systems. E.g., activator-inhibitor systems play a role in morphogenesis and may generate different patterns. Noisy random fluctuations are ubiquitous in the real world. The randomness leads to various new phenomena and may have a non-trivial impact on the behaviour of the solution. The presence of the stochastic term (or noise) in the model often leads to qualitatively new types of behaviour, which helps to understand the real processes and is also often more realistic. Due to the interplay of noise and nonlinearity, noise-induced transitions, stochastic resonance, metastability, or noise-induced chaos may appear. Noise in stochastic Turing patterns expands the range of parameters in which Turing patterns appears.

The topic of the talk is a nonlinear partial differential equation disturbed by stochastic noise. Here, we will present recent results about the existence of martingale solutions using a stochastic version of a Tychanoff-Schauder type Theorem. In particular, we will introduce the stochastic Klausmeier system, a system that is not monotone, nor does it satisfy a maximum principle. So, the existence of a solution can only be shown using compactness arguments.

In the talk, we first introduce stochastic (partial) differential equations, and then we will present the Klausmeier system. Secondly, we will introduce the notion of martingale solutions and present our main result. Finally, we will outline the proof of our main result, i.e., the proof of the existence of martingale solutions.

Ringvorlesung
The structure of a function near a critical point
Umberto Hryniewicz (RWTH Aachen University)
Thu, 22 Feb 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath

Abstract

The aim of this lecture is to revise two basic results on the structure of a $C^2$ function near a critical point. The first result is the classical Morse Lemma; it asserts that, in a special coordinate system, a function near a nondegenerate critical point coincides with the quadratic form given by its Hessian up to an additive constant. The nondegeneracy assumption here means that the Hessian has a trivial kernel. The second result is the Gromoll-Meyer splitting lemma; it handles the general critical point. Both results hold in infinite-dimensions. We believe these results would be useful for those whose research touches on the topic of gradient descent.

Ringvorlesung
A nonlinear Dirac equation expressed in the space algebra
Michael Westdickenberg (RWTH Aachen University)
Thu, 15 Feb 2024 • 10:00 coffee/tea in 001 // 10:30-12:00 talk in 008 • Pontdriesch 14, room 008 SeMath

Abstract

The Dirac equation was the first quantum wave equation that combined in a consistent way the principles of quantum mechanics and special relativity. It describes spin-$1/2$ massive particles such as electrons. Classically, the Dirac equation is written in terms of Dirac spinors, which are functions of time and space taking values in $\mathbb{C}^4$. In this talk we will present a reformulation of the Dirac equation in a more compact form using the space algebra $\mathcal{Cl}(3)$, which is a Clifford algebra that augments the vector calculus in $\mathbb{R}^3$ with a multiplicative structure. We consider a nonlinear extension of the Dirac equation introduced by French physicist Daviau. Unlike the well-known cubic Dirac equations, which is also known as Soler model and which has attracted considerable interest in the mathematical and physical community, this new model is not well-studied. Unlike the cubic model, it has the same homogeneity as the linear Dirac equation, but the nonlinearity is only Lipschitz continuous. This nonlinear Dirac equation admits a clean and symmetric split into the left and right-handed spinor components. We will discuss a global existence result for a regularized equation and derive a hydrodynamics formulation analogous to the Madelung transform for the Schrödinger equation.

Successful Ph.D. defense
Congratulations to Giacomo Borghi

On February 1, 2024 Giacomo Borghi successfully defended his Ph.D. thesis titled

Mean-field theory for consensus-based optimization and extensions to constrained and multi-objective problems.
Weekly Seminar
Homogenization of 2D Euler equations with impermeable inclusions
Mitia Duerinckx (Université Libre de Bruxelles (Belgium))
Thu, 25 Jan 2024 • 10:30-11:30h • Pontdriesch 14, room 008 SeMath (host: Richard Schubert)

Abstract

This work is devoted to the homogenization of perfect incompressible fluid flows described by the 2D Euler equations in several heterogeneous settings. Starting from the vorticity formulation, the question is naturally split into the homogenization of the div-curl problem defining the fluid velocity and the homogenization of the transport equation for the vorticity. Yet, due to heterogeneities, the fluid velocity typically has large small-scale oscillations, on top of its large-scale variations. In such a multiscale setting, the homogenization of the corresponding transport equation for the vorticity becomes a highly delicate question and a well-posed limit equation might not exist. As we shall see, this difficulty is related to the possibility of a localization phenomenon: part of the vorticity may in principle become trapped due to heterogeneities. We illustrate the topic by focussing on two model problems: the homogenization of the 2D Euler equations with impermeable inclusions and the homogenization of the 2D lake equations. In both cases, we shall see how vorticity localization can be ruled out in some situations.

Weekly Seminar
Impact of interaction forces in first order many-agent systems for swarm manufacturing
Mattia Zanella (University of Pavia (Italy))
Thu, 18 Jan 2024 • 10:00 coffee/tea in 001 // 10:30-11:30 talk in 008 • Pontdriesch 14, room 008 SeMath (host: Michael Herty)

Abstract

We study equilibration rates for nonlocal Fokker-Planck equations arising in swarm manufacturing. The PDEs of interest are characterised by a time-dependent nonlocal diffusion coefficient and a nonlocal drift, modeling the relaxation of a large swarms of agents, feeling each other in terms of their distance, towards the steady profile characterized by a uniform spreading over a domain. The result follows by combining entropy methods for quantifying the decay of the solution towards its quasi-stationary distribution, with the properties of the quasi-stationary profile.

Bibliography: [1] F. Auricchio, G. Toscani, M. Zanella. Trends to equilibrium for a nonlocal Fokker-Planck equation. Applied Mathematics Letters, 145: 108746, 2023. [2] F. Auricchio, G. Toscani, M. Zanella. Fokker-Planck modeling of many-agent systems in swarm manufacturing: asymptotic analysis and numerical results. Communications in Mathematical Sciences, 21(6):1655-1677, 2023.