Seminar über Komplexe Geometrie
Fakultäten der RUB » Fakultät für Mathematik » Lehrstühle » Arbeitsgruppe Transformationsgruppen

Judith Brinkschulte (Leipzig)

Mittwoch 31, Januar 2018, 12:15, NA 5/24

Titel des Vortrages:
The normal bundle of Levi-flat real hypersurfaces.

Abstract :
There has been quite some interest in the classification of compact Levi-flat real hypersurfaces in complex manifolds, motivated by the different nature of classical examples on the one hand and basic problems in dynamical systems on the other hand. I will discuss the following recent result: Let X be a complex manifold of dimension > 2. Then there does not exist a smooth compact Levi-flat real hypersurface in X such that the nromal bundle to the Levi foliation admits a Hermitian metric with positive curvature along the leaves.

PD Dr. Stéphanie Cupit-Foutou (RUB)

Mittwoch 17, Januar 2018, 12:15, NA 5/24

Titel des Vortrages:
On momentum-polytopes of spherical varieties.

Hendrik Herrmann (Universität Köln)

Mittwoch 6. Dezember 2017, 12:00, NA 5/24

Titel des Vortrages:
An equivariant embedding theorem for CR manifolds with circle action

Dr. Tomasz Maciazek (Polen)

Mittwoch 29. November 2017, 12:00, NA 5/24

Titel des Vortrages:
Momentum polytope at the highest weight

Prof. Dimitri Akhiezer (Moskau/Rusßland)

Mittwoch 22. November 2017, 12:15, NA 5/24

Titel des Vortrages:
On common zeros of eigenfunctions of the Laplace operator

Christian Schuster (RUB)

Mittwoch 15. November 2017, 12:00, NA 5/24

Titel des Vortrages:
The Kobayashi pseudometric on subvarieties of abelian varieties

Chrstian Zöller (RUB)

Mittwoch 25. Oktober 2017, 12:00, NA 5/24

Titel des Vortrages:
The local structure theorem for projective representations

Jorret Bley (RUB)

Mittwoch 18. Oktober 2017, 12:00, NA 5/24

Titel des Vortrages:
Invariance of moment image of fibers of open proper maps

Prof Jón Magnússon (University of Iceland)

Montag 25. September 2017, 10:14, NA 4/24

Titel des Vortrages:
Intersection theory for analytic cycles

Abstract :
A description of intersection theory for analytic cycles on complex manifolds will be given. Then a generalisation of this theory will be discussed, where complex manifolds are replaced by so-called "nearly smooth complex spaces". These spaces are locally the image of a complex manifold by a finite and proper map.

Prof. Dr. Dmitry Timashev (Moskau)

Mittwoch 26. Juli 2017, 10:15 Uhr, NA 2/24

Titel des Vortrages:
Real orbits on complex spherical homogeneous spaces: the split case

Prof. Dr. Dmitry Timashev (Moskau)

Dienstag 25. Juli 2017, 10:15 Uhr, NA 1/64

Titel des Vortrages:
Classifying real orbits on complex symmetric spaces via Galois cohomology

Elizaveta Vishnyakova (Belo Horizonte)

Dienstag 18. Juli 2017, 10:15 Uhr, NA 1/64

Titel des Vortrages:
About graded manifolds

Abstract :
Vector bundles and double vector bundles, or $2$-fold vector bundles, arise naturally for instance as base spaces for algebraic structures such as Lie algebroids, Courant algebroids and double Lie algebroids. It is known that all these structures possess a unified description using the language of super\-geometry and graded manifolds of degree $\leq 2$. Indeed, a link has been established between the super and classical pictures by the geometrization process, leading to an equivalence of the category of graded manifolds of degree $\leq 2$ and the category of (double) vector bundles with additional structures..

Andriy Regeta (Grenoble)

Mittwoch 28.Juni 2017, 10:15 Uhr, NA 2/24

Titel des Vortrages:
Automorphism groups of affine toric varieties and affine surfaces

Abstract:
We are going to discuss the following problem: to which extent the group of automorphisms of an affine algebraic variety determines the variety? In general the answer is negative. On the other hand, H. Kraft proved that the group of automorphisms of the affine n-space seen as an ind-group determines the affine n-space in the category of connected affine varieties. In this talk we are going to discuss a similar result for affine toric varieties. In case of dimension two, we characterise a big class of affine surfaces by their automorphism groups viewed as abstract groups.

Bart van Steirteghem (Erlangen-Nürnberg/New York)

Dienstag 27.Juni 2017, 10:15 Uhr, NA 3/64

Titel des Vortrages:
Momentum polytopes of multiplicity free Hamiltonian manifolds

Abstract:
Generalizing T. Delzant's classification of toric symplectic manifolds, F. Knop has shown that multiplicity free Hamiltonian manifolds are classified by their momentum polytope and generic isotropy group. In this talk I will explain how the theory of (smooth affine) spherical varieties can be used to give a combinatorial characterization of the momentum polytopes of these manifolds. This is joint work with G. Pezzini.

Chin-Yu Hsiao (Taipeh)

Mittwoch 21. Juni 2017, 10:15 Uhr, NA 2/24

Titel des Vortrages:
G-invariant Szego kernel asymptotics and CR reduction

Abstract :
Let X be a compact connected orientable CR manifold with non-degenerate Levi curvature. Assume that X admits a connected compact Lie group action G. Under certain natural assumptions about the group action G, we show that the G-invariant Szego kernel for (0,q) forms is a complex Fourier integral operator, smoothing away the reduced space Y and there is a precise description of the singularity near Y. We apply our result to the case when X admits a transversal CR circle action and deduce an asymptotic expansion for the m-th Fourier component of the G-invariant Szego kernel for (0,q)-forms as m goes to infinity. As an application, we show that if m large enough, quantization commutes with reduction. This is a joint work with Rung-Tzung Huang.

Chin-Yu Hsiao (Taipeh)

Dienstag 20. Juni 2017, 10:15 Uhr, NA 3/64

Titel des Vortrages:
Bergman kernel asymptotics in complex geometry

Abstract:
In a work jointly with Marinescu at 2014, we obtained a full asymptotic expansion of the spectral function corresponding to the lower part of the spectrum of the Kodaira Laplacian on high tensor powers of a holomorphic line bundle. From this result, we could deduce some classical results in complex geometry ( Demailly's Morse inequalities, Bergman kernel asymptotics for ample line bundles). In this talk, I will explain how to obtain these classical results from our result. I will also mention our result about Bergman kernel asymptotics for semi-positive line bundles.

Maxime Pelletier (Lyon)

Mittwoch 14. Juni 2017, 10:15 Uhr, NA 2/24

Titel des Vortrages:
Some geometric methods to get results on coefficients from representation theory.

Abstract :
The branching coefficients appear in representation theory when one looks at the following situation: let G and H be two complex reductive groups such that H is a subgroup of G, and consider an irreducible representation of G. Then this is also a representation of H and as such it decomposes into a direct sum of irreducible ones. The multiplicities of the irreducible representations of H in this decomposition are called the branching coefficients. They have a geometric interpretation when the groups G and H are connected: they correspond to dimensions of spaces of invariant sections of line bundles over flag varieties.
This talk will then be mainly focused on a specific example of branching coefficients (even if most of the techniques apply to other ones): the Kronecker coefficients, which are indexed by triples of partitions of the same integer. A notion of "stable triple" can then be defined relatively to these coefficients, and we will see that their geometric interpretation allows to obtain a new proof of a characterisation of stability proven by S. Sam and A. Snowden. Finally we will discuss about some ways to produce such stable triples.

Ramiro Lafuente (Münster)

Mittwoch 7.Juni 2017, 10:15 Uhr, NA 2/24

Titel des Vortrages:
Moment maps and homogeneous Riemannian manifolds

Abstract:
The idea of the talk is to explain how the moment map of a real reductive Lie group representation appears naturally in the study of the geometry of homogeneous Riemannian manifolds. After explaining how the properties of the moment map inspire and yield results in the Riemannian setting, if time permit we will also discuss ongoing projects and open questions.

Prof. Mikhail Borovoi (Tel-Aviv/Bielefeld)

Mittwoch, 31. Mai 2017, 10:15 Uhr, NA 2/24

Titel des Vortrages:
Cayley groups

Abstract:
I will start the talk with the classical "Cayley transform" for the special orthogonal group SO(n) defined by Arthur Cayley in 1846. A connected linear algebraic group G over the field of complex numbers C is called a *Cayley group* if it admits a *Cayley map*, that is, a G-equivariant birational isomorphism between the group variety G and its Lie algebra Lie(G). For example, SO(n) is a Cayley group. A linear algebraic group G is called *stably Cayley* if G x S is Cayley for some C-torus S. I will consider semisimple algebraic groups, in particular, simple algebraic groups. I will describe classification of Cayley simple groups and of stably Cayley semisimple groups. (Based on joint works with Boris Kunyavskii and others.)
See also this answer: http://mathoverflow.net/a/101343/4149

Benoit Dejoncheere, University of Lyon

Mittwoch 24. Mai 2017, 10:15 Uhr, NA 2/24

Titel des Vortrages:
Rings of differential operators on some wonderful varieties of small rank

Abstract:
Let $X$ be a complex smooth projective algebraic variety. The algebra of global differential operators $D_X$ is rather badly understood, except in the case of curves, toric varieties, and flag varieties. In this talk, we will investigate these algebras in the case of some wonderful varieties of small rank. More precisely, if $X$ is a wonderful $G$-variety, two questions naturally arise, namely the study of the infinitesimal action of the Lie algebra of $G$, and when ${\cal L}$ is an invertible sheaf on $X$, the study of the algebra of twisted global differential operators $D_{X,{\cal L}}$ on the cohomology groups $H^i(X,{\cal L})$. We will give an answer to these questions in some particular cases, keeping in mind that wonderful varieties can be seen as generalizations of flag varieties.