Sommer 2024
The talks start at 14.15 in Seminar Room IA 1/177 .
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11.07.2024 Manuel Stange "tba"
Abstract: tba
04.07.2024 Michael Vogel "tba"
Abstract: tba
16.05.2024 Paul Anton Wilke "Morse homology for Dirac equations"
Abstract: tba
25.04.2024 Johanna Bimmermann "tba"
Abstract: tba
18.04.2024 Lucas Dahinden "On the Causal Discontinuity of Morse Spacetimes"
Abstract: Morse spacetimes are generalisations of globally hyperbolic Lorentz
spacetimes: they feature singularities where time-level sets undergo a
topology change. Borde and Sorkin conjectured that such Morse spacetimes
are causally continuous if and only if neither the index nor the coindex
of the critical points is 1. This has been recently confirmed by Garcia
Heveling for the case of small anisotropy and Euclidean background
metric. Here, we provide a complementary counterexample: a four
dimensional Morse spacetime whose critical point has index 2 and large
enough anisotropy is causally discontinuous. Thus, the Borde-Sorkin
conjecture does not hold. The proof features a low regularity causal
structure and causal bubbling.
Winter 2023/24
The talks start at 14.15 in Seminar Room IA 1/177 .
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01.02.2024 Jan Eyll "Systolic Inequalities in Riemannian Surfaces"
Abstract: The systole of a nonsimply connected Riemannian manifold is the length of the shortest noncontractible closed curve. In the 1940s, it was discovered that for any Riemannian metric on the two-torus, its area is bounded from below by a constant multiple of the square of the systole. Inequalities of this kind are known as systolic inequalities, and in this talk we will present and prove sharp systolic inequalities for some explicit surfaces.
25.01.2024 Lucas Dahinden "C^0-stability of topological entropy for geodesic flows in dimension 3"
Abstract: Topological entropy is a numerical measure of how chaotic a map is. We are interested in how topological entropy depends on the map, especially how stable it is under perturbation. For example for diffeomorphisms homotopic to the identity on three dimensional manifolds it is well known that topological entropy can collapse to 0 after smooth perturbations, so it can be very unstable. We have managed to prove that for Reeb flows on three dimensional co-oriented compact contact manifolds topological entropy is surprisingly stable: a generic Reeb flow (with respect to C^\infty-topology) is a point of lower semi-continuity (with respect to C^0-topology) of the topological
entropy. The tool that allows us to conclude this is a mechanism discovered by Matthias Meiwes, by which one can approximate topological entropy using links of Reeb chords extracted from a horseshoe. In our work we show stability of these links under perturbation. This is joint work with Marcelo Alves, Matthias Meiwes and Abror Pirnapasov.
18.01.2024 Stefan Nemirovski "Sheaves, generating functions, and positive Legendrian isotopies"
Abstract: I will try to show in a simple example how the first two objects in the title are related and how they can be used to study the third.
11.01.2024 Lars Kelling "On the geometric construction of the Chas-Sullivan product"
Abstract: Given a closed oriented Riemannian manifold M, Chas and Sullivan introduced a product on the loop homology, i.e. the ordinary homology of the space of (free) loops in M. For two chains its product is given by the intersection of marked points in M followed by the
concatenation of appropriate loops.
I will discuss its geometric construction as a lift of the classical intersection product with focus on a direct description of tubular neighbourhoods to avoid the delicate setting of normal bundles on infinite dimensional Hilbert manifolds.
14.12.2023 Niklas Tuschinski "Der Satz von Arnold-Liouville"
23.11.2023 Patrick Henning "Computing ground states of Bose-Einstein condensates"
Abstract: In this talk we discuss the ground states of Bose-Einstein condensates which can be characterised as global minimizers of an energy functional on a Riemannian manifold. By formulating a suitable Riemannian gradient flow and by studying its decay properties, we infer a corresponding discrete gradient flow that can be used for practical computations. We will further discuss its global convergence and the asymptotic decay rates. For that we will show and exploit various links to a certain class of nonlinear eigenvalue problems.
16.11.2023 Pierre-Alexandre Arlove "Contact orderability and spectral selectors"
Abstract: Some contactomorphisms groups and isotopy classes of Legendrians are known to admit a natural partial order.
In this talk I will use this partial order to define functions on the latter spaces analogous to the spectral invariants in Symplectic Geometry coming from Lagrangian Floer homology. For
Legendrians, I will show that these functions are spectral selectors and, from there, derive dynamical applications such as the existence of translated points and Reeb chords. After discussing the non-degeneracy of the selectors, I will present applications to the geometric study of these spaces, namely : construction of time-functions and the study of various (bi-invariant) metrics. This is a joint work with Simon Allais.
09.11.2023 Souheib Allout "Compact Lorentz manifolds without closed geodesics"
Abstract: In the previous Retreat I gave a short talk about constructing
some examples of Lorentz manifolds without closed geodesics. In this
talk, we give more details, examples and questions in this direction.
26.10.2023 Alberto Abbondandolo "On Hamiltonian diffeomorphisms and domains"
Abstract: Suitably compactly supported Hamiltonian diffeomorphisms of the (2n-2)-dimensional ball can be lifted to smooth domains in the 2n-dimensional vector space.I will discuss this construction, its properties and how it can be used to construct counterexamples and symplectic embeddings.
19.10.2023 Alberto Abbondandolo "Symplectic capacities and geodesics in the space of contact forms"
Abstract: An old open question in symplectic geometry is whether all normalized symplectic capacities coincide on convex bodies. I will discuss this question, together with its relationship with Viterbo‘s conjecture and systolic inequalities, and the argument for giving it a positive answer in the case of smooth convex bodies which are C^2-close to a Euclidean ball. This argument involves understanding minimizing geodesics in the space of contact forms equipped with a Banach-Mazur pseudo-metric.
Sommer 2023
The talks start at 14.15 in Seminar Room IA 1/177 .
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22.06.2023 Simon Vialaret "Systolic inequalities for invariant contact forms on S^1-principal bundles"
Abstract: In Riemannian geometry, a systolic inequality aims to give a uniform bound on the length of the shortest closed geodesic for metrics with fixed volume on a given manifold. This notion generalizes to contact geometry, replacing the geodesic flow by the Reeb flow, and the length by the period. As opposed to the Riemannian case, it is known that there is no systolic inequality for general contact forms on a given contact manifold. In this talk, I will state and prove a systolic inequality for invariant contact forms on S^1-principal bundles over S^2.
15.06.2023 Liang Jin "Weak KAM theory for contact vector fields"
Abstract: Contact vector fields are natural generalizations of Hamiltonian vector fields on contact manifolds. Motivated by the classical weak KAM theory established for Tonelli Hamiltonian vector fields, a similar theory for certain "Tonelli" contact vector fields on the manifold of 1-jets was partially built. In this talk, we shall give a very brief introduction of such a theory without going to the details,
then offer simplest dynamical feedback from it.
04.05.2023 Johanna Bimmermann "Capacities of Hermitian Symmetric Spaces"
Abstract: We will introduce Hermitian symmetric spaces and focus on their
description as (co-)adjoint orbits.
From this description it will be easy to give a Hamiltonian that
generates a circle action. Finally this Hamiltonian circle action can be
used to determine the Gromov width and the Hofer-Zehnder capacity of any
Hermitian symmetric space of compact type.
27.04.2022 Michael Vogel "b-contact structures on tentacular hyperboloids"
Abstract: b-symplectic and b-contact structures were introduced as a model for symplectic and contact manifolds with boundary. However, in recent years they have been studied in their own right. I will give an introduction to these singular structures and some of the known theory about them. Then I will present a construction relating them to tentacular hyperboloids.
20.04.2023 Jonas Fritsch "The Arnold conjecture in the case of the symplectic torus"
Abstract: The Arnold conjecture states that any Hamiltonian diffeomorphism on a compact symplectic manifold, possesses at least
as many fixed points as a smooth function on that manifold must possess critical points. This conjecture was first proven
in the special case of the symplectic Torus by E. Zehnder and C. C. Conley. In this talk I will discuss a very similar proof
of this case, by first reframing the question into a variational problem on the Sobolev space of loops on the torus, and then
performing a saddle point reduction to restrict the problem to a finite dimensional subspace of the loop space to apply principles
similar to Morse-Theory.
Winter 2022/23
The talks start at 14.15 in Seminar Room IA 1/177 .
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26.01.2023 Stefan Suhr "Gromov-Hausdorff convergence and Lorentzian geometry, Part II"
Abstract: Following the discussion for the metric case I will propose a definition for Lorentzian-metric spaces and a Gromov-Hausdorff metric for these spaces. I will further discuss first results, especially stability of curvature and compactness. This is joint work with Ettore Minguzzi (Florence).
19.01.2023 Stefan Suhr "Gromov-Hausdorff convergence and Lorentzian geometry, Part I"
Abstract: I will introduce the Gromov-Hausdorff metric for compact metric spaces and discuss first properties which will lead to a proof of the Gromov precompactness Theorem for spaces of bounded Ricci curvature.
12.01.2023 Barney Bramham "Higher dimensional versions of Fathi’s formula for the Calabi invariant"
Abstract: For a Hamiltonian system on the 2-disc Fathi found a dynamical interpretation of the Calabi invariant as the average linking of pairs of trajectories. In this talk I will present formulae that extend this to arbitrary dimensions.
08.12.2022 Xian Dai "Teichmüller theory and Thurston's asymmetric metric, Part III"
01.12.2022 Xian Dai "Teichmüller theory and Thurston's asymmetric metric, Part II"
24.11.2022 Xian Dai "Teichmüller theory and Thurston's asymmetric metric, Part I"
Abstract: In these talks, we will give an overview of the classical Teichmüller theory which involves the interplay
of many mathematical subjects, including geometry, dynamics and topology. After the exposition, we will focus on an
asymmetric Finsler metric on the Teichmüller space proposed by Thurston which has natural relation to study of
extreme Lipschitz maps and length functions on hyperbolic surfaces.
17.11.2022 Luca Asselle "Exotic Zoll magnetic systems on the two-torus via the Nash-Moser IFT, Part II "
10.11.2022 Luca Asselle "Exotic Zoll magnetic systems on the two-torus via the Nash-Moser IFT, Part I"
Abstract: In this series of talks I will show how to construct examples of magnetic systems on the two-torus
(different from the trivial ones) with the property that every orbit with speed 1 is periodic. In the first talk we will
reformulate the question as a problem of determining the pre-image of zero under a suitable „action“ map, and explain
why the standard IFT fails in this setting. In the second talk we will instead show how such a map can be dealt
with by mean of a Nash-Moser type IFT.
03.11.2022 Lucas Dahinden "Robustness of Topological Entropy for Geodesic Flows"
Abstract: Topological entropy (=h) is a numerical invariant of maps that measures chaotic behaviour. It is well known, that h is not lower semi-continuous: There are examples where smooth perturbations lead to total collapse of h. However, sometimes a geometric feature prevents this collapse. In this talk we explore some of these features and discuss their consequences: On the 2-torus we investigate contractible closed geodesics, an intersection pattern that we call “ribbon” and on general manifolds we investigate a feature that we call “retractable neck”.
This is joint work with Marcelo Alves, Matthias Meiwes and Louis Merlin.
27.10.2022 Manuel Stange "Construction of geodesic foliations"
Abstract: In my talk i will discuss a theorem by H.Gluck & D.Singer adressing the
following problem: Given a Riemannian manifold N, which is seperated
into two components by a compact submanifold M of codimension 1, and two
geodesic fields G and G' transversal to M - When does there exist a new
metric on N, which conincides with the original metric outside an
arbitrary small neighbourhood U of M, and a field of geodesics (wrt.
this new metric), which connects G to G' within U (according to some
given map on M, which will describe exactly, what curves of G and G'
will be pieced together).