

Place:
WeierstrassInstitute for Applied Analysis and Stochastics
Mohrenstraße 39, 10117 Berlin
ErhardSchmidt Lecture Room (ESH)

Time:
Every second Monday of the month, 2:00PM  3:00PM

Organizers:
Moritz EbelingRump,
Derk FrerichsMihov

Upcoming talks 





08.11.2021 
Clemens Bartsch (genua GmbH)


Career Opportunities for WIAS Alumni in IT Security? A Personal Case Study.
Abstract:
TBA.

15.11.2021 
Swetlana Giere (felmo GmbH)


A Conversation about Transition From Science to Data Science
Abstract:
TBA.

13.12.2021 
Andrea Zafferi (WG 1)


TBA
Abstract:
TBA.

11.01.2022 
Anh Duc Vu
(LG 6)


TBA
Abstract:
TBA.

14.02.2022 
Simon Breneis
(RG 6)


TBA
Abstract:
TBA.

Previous talks 





11.10.2021 
Alexander Gerdes
(RG 2)


Synchronization patterns in globally coupled StuartLandau oscillators
Abstract:
We s tudy clusterized states in globally coupled StuartLandau
oscillators as a paradigmatic model for patterning processes
[Kemeth2019].
To study 2Cluster states we set up a reduced model using collective
variables, in which the cluster size ratio [Ott2015] is an additional
bifurcation parameter. In the reduced system one can only observe
longitudinal instabilities leading to complex 2Cluster behaviour. By
including test oscillators, we can also study instabilities
transversal to the 2Cluster manifold i.e. changes of the cluster
type. Using numerical bifurcation analysis, we then find stability
regions of cluster solutions of different types. In these, solitary
states serve as primary patterns and allow an analytical treatment.
The identified instabilities can be seen as building blocks of
pathways to complex behaviour such as chimeras [Set2014] and extensive
chaos [KM1994] as well as splay states [Politi2019] occuring for
varying parameters. With the analytical and numerical approach
presented here we identify different transition scenarios from
synchrony to complex behaviour by reducing the coupling strength. We
locate each of these scenarios in regions in the plane of shear
parameters.
[KM1994] N. Nakagawa and Y. Kuramoto, Physica D: Nonlinear Phenomena,
75, Issues 13 (1994).
[Set2014] G. Sethia and A. Sen, Phys. Rev. Lett. 112 144101 (2014).
[Ott2015] W. L. Ku, M. Girvan, E. Ott, Chaos 25 , 123122 (2015).
[Kemeth2019] Kemeth, Felix P., Sindre W. Haugland, and Katharina
Krischer. Chaos: An Interdisciplinary Journal of Nonlinear Science
29.2 (2019): 023107.
[Politi2019] P. Clusella and A. Politi, Phys. Rev. E 99, 062201
(2019).
[Kemeth2021] Kemeth, Felix P., et al. Journal of Physics: Complexity
2.2 (2021): 025005.

13.09.2021 
Alireza Selahi Moghaddam (RG 7)


Modelling electrolytes with the PoissonNernstPlanckequation
Abstract:
The capacity of batteries is one of their central quantities and hence
of major interest. In order to calculate it, modelling the behaviour of
electrolytes is necessary. We give a short introduction into some of the
most important concepts in thermodynamics and chemistry, and finally
talk about a stateoftheart model using the
PoissonNernstPlanckequation.

09.08.2021 
Mina Stöhr (RG 2)


Bifurcations and Instabilities of Temporal Dissipative Solitons in DDEsystems with large delay
Abstract:
We study different bifurcation scenarios and instabilities of Temporal
Dissipative Solitons in systems with timedelayed feedback and large
delay. As these solitons can be described as homoclinic orbits in the
profile equation under the reappearance map, we use homoclinic
bifurcation theory for our comprehension of their bifurcations and
instabilities. We demonstrate our results with the examples of the
FitzHughNagumo system and MorrisLecar model with timedelayed
feedback.

12.07.2021 
Lorenzo Scaglione (RG 3)


Three months with ParMooN
Abstract:
What does it mean for a research group to implement its own software
library? During my internship at WIAS I discovered the huge world of
ParMooN, a C++ finite element library of the institute. Even if
sometimes it is quite painful to surf through the hundreds of header
and source files of the library, it is somehow fascinating to have an
idea of its complex architecture and of how it can help us to solve
quantitatively problems from the real world. The application of my
code is the simulation of the mechanical behaviour of an elastic
material embedded with a thin vasculature.
In my presentation, I will introduce the physical problem and the
library; then I will show the numerical results I have obtain until
now.
Sophie Luisa Plato (RG 4)


Biological pest control  Analysis and numerics for a spatialtemporal predatorprey system.
Abstract:
In the production of ornamental plants, as for example roses, it is
desirable to reduce the use of chemical pesticides in order to protect
the environment and the people involved in the production process. This
can be achieved by releasing natural enemies of the pest involved, which
do not harm the plants. A typical example of such a predatorprey pair
is the twospotted spider mite and the predatory mite.
In this talk we consider a system of two coupled evolution equations
modelling this predatorprey interaction. The first part of the talk is
devoted to the proof of the existence of weak solutions to this model
and in the second part we present our numerical approximations of these
solutions.

14.06.2021 
Jacob Gorenflos (FMP / Leibniz PhD Network)


A glimps into doing a Ph.D. in the Leibniz Association
Abstract:
Data is the best way to start a discussion. We, in the Leibniz PhD
Network, regularly discuss with Leibniz policy and politicians on how to
improve the situation of doctoral researchers. Surprisingly, there is
a significant lack of knowledge on how their situation really is.
Therefore, we started the working group survey in 2017. This kicked of
the biennial cycle in which we now survey you: 2017, 2019 and we are
currently finishing the development of the 2021 questionnaire. Since
2017, the survey and its results have been the basis of our work.
Here, you will be presented with the data of the 2019 survey and
a perspective on what to expect of the 2021 survey.
Lasse Ermoneit (RG 2)


Semianalytical approach to determine the timing jitter of a modelocked laser with optoelectronic feedback
Abstract:
Passively modelocked lasers are an important device among semiconductor lasers and are used to generate highfrequency regular pulses. Their mathematical description can be done via delay differential equations to avoid the more complex consideration with partial differential equations. Since any laser system is usually subject to noise due to quantum effects, this is incorporated into the system of equations and a nonlinear differential equation with delay and stochastic white noise is obtained.. The timing jitter is the measure for the irregularity of the pulses of the laser: It corresponds to the standard deviation of the noise influenced pulses to an external clock.
Here, an approach is presented that makes it possible to shortcut a large part of the fully numerical nonparallelizable computations in order to get to this key quantity, the timing jitter.

10.05.2021 
Dilara Abdel (LG 5)


Modelling charge transport in perovskite solar cells: Potentialbased and limiting ion depletion
Abstract:
Perovskite solar cells (PSCs) have become one
of the fastest growing photovoltaic technologies within the last
few years. However, their commercialization is still in its early
stages and several challenges need to be overcome. For this reason it
is paramount to understand the charge transport in perovskites better
via improved modelling and simulation. Unfortunately, there is a
discrepancy in the adequate modeling of the additional ionic transport
within the perovskite material with driftdiffusion equations. Thus,
we present a new charge transport model which is, unlike other models
in the literature, based on quasi Fermi potentials instead of
densities. This allows to easily include nonlinear diffusion (based on
for example FermiDirac, GaussFermi or Blakemore statistics) as well
as limit the ion depletion (via the FermiDirac integral of
order 1). We present numerical finite volume simulations to underline
the importance of limiting ion depletion.

03.05.2021 
Artur Stephan (RG 1)

PhD defense rehearsal 
Coarsegraining for gradient systems and Markov processes
Abstract:
Coarsegraining is a wellestablished tool in mathematical and natural
sciences for reducing the complexity ofa physical system and for
deriving effective models. In the talk, we consider several examples
that originate from interacting particle systems and describe reaction
and reaction diffusion systems. The aim is twofold: first,provide
mathematically rigorous results for physical coarsegraining. Secondly,
the so derived systems can be formulated in a mathematically equivalent
way, which provides new modelling insights.

12.04.2021 
Leonie Schmeller (RG 7)


Phasefield model with nonlinear elasticity (Modelling and Numerical
aspects)
Abstract:
In this talk, I will present a phase field model for a NeoHookian
(nonlinear) elastic material, which is then coupled to formulate
a CahnHillard type dynamic system. After introducing the individual
components of the problem, a coupling is discussed. We set up a weak
formulation and show a strategy to implement and solve the problem
numerically.

15.03.2021 
Heide Langhammer (RG 5)

Instead of 08.03.2021 
Inhomogeneous Random Graphs: A large deviations result for their cluster sizes and its implications.
Abstract:
An inhomogeneous random graph consists of a fixed number of vertices
that are connected via random edges. The edge probabilities depend on an
additional parameter that we call the vertex type. We want to study how
such a random graph decomposes into its connected components. In
particular, we want to understand conditions for the existence of
macroscopic components, whose size is proportional to the total number
of vertices. Once the model parameters surpass a certain threshold,
a (unique) macroscopic component appears with high probability. I will
explain how this phase transition can be studied via large deviations
theory which reformulates the probabilistic calculus of the model into
an optimization problem for a certain function.
I will also discuss in which ways the inhomogeneous random graph model
can be linked to models of coagulation that I will only briefly sketch.

08.02.2021 
David Sommer (RG 4)


Dynamic Programming Approach for Robust Receding Horizon Control in Continuous Systems
Abstract:
There is still little connection in the literature between the field of
Modelbased Reinforcement Learning (MBRL) and the field of Continuous
Optimal Control. In Continuous Optimal Control, the ODE model,
describing a real physical process, is usually regarded as ground truth.
This may lead to catastrophic failures when the resulting control is
applied to the real system, due to errors in the model. In MBRL, this
issue is often addressed by keeping and continuously updating
a posterior over model parameters, but successful applications so far
are mostly limited to Markov Decision Processes which are discrete in
time. We propose a modelbased decisiontime planning agent for
continuous optimal control problems of arbitrary horizon length.
Continuous updates of the model parameters during the online phase
enable handling of complex unknown dynamics even with simple linear
models. During planning, robust feedbackcontrol laws are computed in
a Dynamic Programming sense by utilizing Bellman's principle.

11.01.2021 
Alexandra Quitmann (RG 5)


Spin systems and random loops
Abstract:
Random loop models are systems of statistical mechanics whose
configurations can be viewed as collections of closed loops living in
higher dimensional space. They are interesting objects on its own and
further have a close connection to other important statistical mechanics
models such as spin systems. In this talk, I will introduce random loop
models, discuss a conjecture about the occurrence of macroscopic loops
and explain its role as alternative formulation of spin systems.

07.12.2020 
Moritz EbelingRump
(RG 4)


Topology Optimization subject to a Local Volume Constraint
Abstract:
The industry sector of additive manufacturing has shown remarkable
growth in previous years and is predicted to continue growing at a rate
of 15% in the coming years. It progressed from prototyping to actual
production. Topology Optimization and Additive Manufacturing have been
called a "match made in heaven", because Topology Optimization
can aid engineers to take advantage of the newfound design freedom.
Commonly a perimeter term is incorporated which avoids checkerboarding,
but also counteracts the desired creation of infill structures. By
incorporating a local volume constraint mesoscale holes are introduced.
Analytically, the existence of unique solutions is shown. Apart from
better cooling properties and a larger resilience to local material
damage, these structures demonstrate an improved nonlinear material
behavior. One observes an increased critical buckling load
 a potentially catastrophic failure mode that would not be taken into
account if only considering linear elasticity.

11.11.2020 
Derk Frerichs
(RG 3)


The very basics of numerical analysis  or what am I doing here when I'm
not drinking coffee?
Abstract:
When we drink coffee, caffeine spreads in our body through our blood.
The flow of particles inside a media, e.g. the caffeine inside the
blood, can be described with the so called convectiondiffusionreaction
equations that are often approximated using numerical algorithms. In
this talk the basic concepts of numerical analysis are explained with
the help of a conforming Courant finite element discretization of the
convectiondiffusionreaction equations. Afterwards a short outlook is
given that explains my current research activities.
Numerical examples round up the presentation.

22.11.2018 
Markus Mittnenweig (RG 1)


Entropy methods for quantum and classical evolution equations

27.08.2018 
Clemens Bartsch (RG 3)


Postquantum cryptography and the first quantumsafe digital signature scheme
Abstract:
In May 2018 news spread far beyond the cryptologist community:
a group of German, Dutch and American computer scientists had published
the first quantumresilient digital signature scheme as an internet standard (RFC 8391),
thus taking a major step towards arming digital signature against future attacks
with quantum computers.
The proposed XMSS scheme (eXtended Merkle Signature Scheme) makes use of
cryptographic hash functions, which are considered quantumsafe.
In this talk we want to lead the audience towards an understanding of the importance
and mode of operation of digital signature schemes, the threat that quantum computers
might in the near future pose to them, and how the newly standardized scheme offers
resilience against quantum computer attacks.
We will start with a general introduction of digital signature and an explanation
of a basic version of the widespread RSA algorithm and its major weaknesses,
focusing on factorization attacks. Then we will introduce the basics of quantum computing,
show how Shor's algorithm enables them to very efficiently perform factorization attacks,
thus breaking RSA, and finally introduce XMSS and give an explanation for why it is
supposed to be safe against quantumaided attacks.
Code examples and examples of quantum computations performed with a
prototypical 5qubit processor (IBM Q Experience) will be included in the talk.

19.02.2018 
Thomas Frenzel (RG 1)


Working with Wasserstein gradient flows
Abstract:
This talk explains what the Wasserstein distance is, how it generates a gradient flow for the heat equation and how to pass to the limit in a sandwich model with thin plates.

19.06.2017 
Artur Stephan (Guest of RG 1)

starts at 1:00 PM 
On approximations of solutions of evolution equations using semigroups
Abstract:
In the talk, some results of my master thesis will be discussed. We approximate the solution of a nonautonomous linear evolution equation in the operatornorm topology. The approximation is derived using the Trotter product formula and can be estimated. As an example, we consider the diffusion equation perturbed by a time dependent potential.

12.06.2017 
Clemens Bartsch (RG 3)

starts at 11:00 AM 
A mixed stochasticnumeric algorithm for transported interacting particles
Abstract:
A coupled system of population balance and convectiondiffusion equations is solved numerically, employing stochastic and finite element techniques in combination. While the evolution of the particle population is modelled as a Markov jump process and solved with a stochastic simulation algorithm, transport of temperature and species concentration are subject to a finite element approximation. We want to briefly introduce both the stochastic and the deterministic approach and discuss some difficulties to overcome when combining them. A proof of concept simulation of a flow crystallizer in 2D is presented.

08.05.2017 
Sibylle Bergmann (RG 7)


An atomistically informed phasefield model for describing the solidliquid interface kinetics in silicon
Abstract:
An atomistically informed parametrization of a phasefield model for describing the anisotropic mobility of liquidsolid interfaces in silicon is presented. The model is derived from a consistent set of atomistic data and thus allows to directly link molecular dynamics and phase field simulations. Expressions for the free energy density, the interfacial energy and the temperature and orientation dependent interface mobility are systematically fitted to data from molecular dynamics simulations based on the StillingerWeber interatomic potential. The temperaturedependent interface velocity follows a VogelFulcher type behavior and allows to properly account for the dynamics in the undercooled melt.
Our three dimensional simulations reproduce the expected physical behavior of a silicon crystal in a melt, e.g. the critical nucleation radius and the experimentally observed equilibrium shape.

24.10.2016 
Johannes Neumann (RG 4)


The phase field approach for topology optimization
Abstract:
In this talk I will present an approach on topology optimization
based on the phase field model from [Blank et. al., 2014] which utilizes
the AllanCahn gradient flow. This method natively includes changes to the
topology during the optimization and replaces sharp interfaces with
boundary layers for smoothness. Instead of the primedual active set method a Lagrangian
approach is considered.

10.10.2016 
Swetlana Giere (RG 3)


A Walk to a Random Forest

05.09.2016 
Alexander Weiß (GetYourGuide: Head of Data Science)


Talk on professional experience in the field of data science

22.08.2016 
Michael Hofmann (RG 2)


Einfluss dynamischer Resonanzen auf die Wechselwirkung optischer FemtosekundenPulse mit transparenten Dielektrika

27.06.2016 
Florian Eichenauer (RG 1)

starts at 4:00 PM 
Analysis for Dissipative MaxwellBloch Type Models

20.06.2016 
Paul Helly (Guest of RG 1)


A structurepreserving finite difference scheme for the CahnHilliard equation

25.01.2016 
Alena Moriakova (Guest of RG 2)


Analysis of periodic solutions of the MackeyGlass equation
Abstract:
The MackeyGlass equation is the nonlinear time delay differential
equation, which describes the formation of white blood cells.
We study the possibility of simultaneous existence of several stable
attractors (periodic solutions) in this equation.
As a research method we use method of uniform normalization.

11.01.2016 
Thomas Frenzel (RG 1)


(Evolutionary) GammaConvergence and micromacro limits

23.11.2015 
Sina Reichelt (RG 1)


Twoscale homogenization of systems of nonlinear parabolic equations
Abstract:
We consider two different classes of systems of nonlinear parabolic equations, namely, reactiondiffusion systems and CahnHilliardtype equations. While the latter class admits a gradient structure, the former does in general not admit one. The equation's coefficients are periodically oscillating with a period which is proportional to the characteristic microscopic length scale. Using the method of twoscale convergence, we rigorously derive effective (upscaled or homogenized) equations for the limit of smaller and smaller periods. Therefore, depending on the class of systems under consideration, we use either suitable Gronwalltype estimates (for Lipschitz continuous reaction terms) or Gammaconvergence (for energy functionals).

09.11.2015 
Mayya Zhilova (RG 6)


Bootstrap confidence sets under model misspecification

26.10.2015 
Dmitry Puzyrev (RG 1)

starts at 10:00 AM (ErhardSchmidt lecture room) 
Delay Induced Multistability and Zigzagging of Laser Cavity Solitons

21.09.2015 
Clemens Bartsch (RG 3)

starts at 2:30 PM 
An Assessment of Solvers for Saddle Point Problems Emerging from the Incompressible NavierStokes equations
