Tuesday, July 17th 2018, Karl Oeljeklaus (Marseille)
“Schottky group action on complex manifolds”
The talk starts at 12:15 in NA 4/24.

Abstract:
In this talk we consider Schottky group actions on complex 
manifolds. As an abstract group a Schottky group is isomorphic to a 
non-abelian free group F, and a Schottky group action on the complex 
manifold X yields an invariant connected open subset U of X on which the 
action of F is free and properly discontinuous. Our motivation for the 
study of Schottky group actions comes from the fact that the quotient 
manifolds U/F have interesting properties.
In the main case when the initial complex manifold is 
homogeneous-rational of dimension grater than one, i.e. a flag manifold, 
the quotients are non-Kähler compact complex manifolds. We investigate 
their analytic and geometric invariants such as the Kodaira and 
algebraic dimension, the Picard group and the deformation theory, thus 
extending results due to Lárusson and to Seade-Verjovsky. We recover 
examples of equivariant compactifications of Schottky quotients of 
SL(2,C). This is a joint work with Christian Miebach (Calais).




Tuesday, May 15th 2018, Felix Schlenk (Neuchâtel)
“Hanner products K x K^\circ are symplectic balls”
The talk starts at 16:15 in NA 3/99.

Abstract:

Hanner polytopes are constructed recursively by starting from a line segment and taking 
Cartesian products and polars.
We show that for every Hanner polytope K in R^n the product K x K^\circ with its polar
is symplectomorphic to the ball in R^{2n} of the same volume.

Hanner polytopes are the conjectured minimizers in Mahler's conjecture from convex geometry,
and balls are the conjectured minimizers in Viterbo's systolic conjecture from symplectic geometry.
The above result, that was conjectured by Yaron Ostrover, thus symplectically identifies
the minimizers of these two conjectures.

This is work joint with Roman Karasev.



Wednesday, April 26th 2017, Vladimir Chernov (Dartmouth)
"Intersection and self-intersection points of loops on a surface, brackets and cobrackets."
The talk starts at 16:15 in NA 01/99.

Abstract:

Given two free homotopy classes of loops on an oriented surface, it is natural to ask how
to compute the minimal number of intersection points of loops from these two classes.
We show that this number is equal to the number of terms in the Andersen-Mattes-Reshetikhin
Poisson bracket of these two classes. Chas found examples showing that a similar statement
does not, in general, hold for the Goldman Lie bracket.

Turaev conjectured that his cobracket of a loop is zero if and only if the loop is homotopic
to a simple loop. Counterexamples to this conjecture were found by Chas. Cahn has modified
Turaev's operation in the spirit of the Andersen-Mattes-Reshetikhin bracket so that the
conjecture becomes true for the modified operation.

The talk is based on joint work with Patricia Cahn.

Monday, July 13th 2015, Dominik Ostermayr (Universität zu Köln)
"Automorphism Supergroups Of Supermanifolds."
The talk starts at 14.15 in NA 4/64.

Thursday, February 26th 2015, Sheila Sandon (Strasbourg)
"Bi-invariant metrics on the group of contact transformations."
The talk starts at 11.15 in NA 01/99.

Abstract:

Contact manifolds are odd-dimensional smooth manifolds endowed with a maximally non-integrable field of hyperplanes. They are intimately related to symplectic manifolds, i.e. even dimensional smooth manifolds endowed with a closed non-degenerate 2-form. Although in symplectic topology a famous bi-invariant metric, the Hofer metric, has been studied since more than 20 years ago, it is only recently that some somehow analogue bi-invariant metrics have been discovered on the group of diffeomorphism that preserve the contact structure. In my talk I will review for non-experts these constructions and discuss how they are related to some other rigidity phenomena in contact topology that have been discovered in the last few years, in particular the notion of orderability (due to Eliashberg and Polterovich) and an analogue in contact topology of Gromov's famous non-squeezing theorem.

Thursday, October 30th, 2014 Sergey Ivashkovich (Lille)
"From the Jacobian Conjecture to Gromov theory and Levi-flat hypersurfaces."
The talk starts at 17.15 in NA 01/99.

Thursday, October 30th 2014, Alan Hucklebery (RUB and Jacobs University, Bremen)
"Fond memories of collaborations with Moscow Math."
The talk starts at 16.15 in NA 01/99.

Thursday, June 26th 2014,Hermann Schulz-Baldes (FAU Erlangen-Nürnberg)
"Topological insulators and index theory".

Abstract:

According to a widely accepted terminology, a topological insulator is a (independent) Fermion system which has surface modes that are not exposed to Anderson localization. This stability results from topological constraints given by non-trivial invariants like non-commutative Chern 
numbers and higher winding numbers, but sometimes also more subtle $Z_2$ invariants associated to adequate Fredholm operators with symmetries. Prime examples are quantum Hall systems, but the talk also considers chiral and BdG systems as well as time-reversal symmetric systems with $Z_2$ invariants.

Tuesday, May 13th 2014, Hans-Werner Henn (University of Strasbourg) 
"Hopkins K(2)-local Picard groups."
The talk starts at 16.15 in NA 1/64.

Tuesday, July 16th, 2013 Christian Becker (University of Potsdam)
"Differential cohomology, stratifolds and the Cheeger-Chern-Simons construction."
The talk starts at 16 o'clock c.t. in NA 1/58.

Monday, July 15th 2013, E. Vinberg (Moscow State University)
"Chevalley-type theorems."
The talk starts at 14 o'clock c.t. in NA 2/64. 

Thursday, June 6th 2013, Rob Kirby (University of California)
"Are all manifolds triangulable?"
The talk starts at 16 o'clock c.t. in NA 01/99. 

Monday, April 15th 2013, Gustav Lehrer (University of Sydney)
"The cohomology of the Milnor fibre of a hyperplane arrangement."
The talk starts at 16 o'clock c.t. in NA 2/64. 

Tuesday, January 29th 2013, Markus Szymik (Universität Kopenhagen)
"Universelle Operationen in Algebra und Geometrie"
The talk starts at 16 o'clock c.t. in NA 01/99. 

Monday, January 14th 2013, Ingrid Bauer-Catanese (University of Bayreuth)
"Produkt-Quotienten Flächen: Ergebnisse und Probleme."
The talk starts at 16 o'clock c.t. in NA 2/64.

Monday, January 23rd 2012, Dmitriy Rumynin (University of Warwick)
"Finite G-sets and finite W-algebras."
The talk starts at 14 o'clock c.t. in NA 2/24. 

Friday, January 20th 2012, Florin Dumitrescu
"Superconnections and supergeometry."
The talk starts at 10 o'clock c.t. in NA 4/64. 

Monday, December 19th 2011, Markus Reineke (University Wuppertal)
"GW/QM correspondence."
The talk starts at 14 o'clock c.t. in NA 2/24.

Thursday, December 15 th 2011, Hansjörg Geiges (University of Cologne)
"How to draw 5-dimensional manifolds"
The talk starts at 17:30 s.t. in NA 5/64.

Thursday, December 15th 2011, Roger Bielawski (University of Leeds)
"Poisson structures on quiver varieties"
The talk starts at 16 o'clock c.t. in NA 5/64.

Monday, December 12th 2011, Stefan Maubach (Jacobs University Bremen)
"Affine Algebraic Geometry over finite fields."
The talk starts at 16 o'clock c.t. in NA 2/64.

Monday, December 5th, 2011 Tarig Abdelgadir (MPI Bonn)
"Quivers of sections on toric orbifolds."
The talk starts at 12 o'clock c.t. in NA 2/64.

Monday, November 21st, 2011 Peter Heinzner (Ruhr University Bochum)
"Über eine Idee von Kempf-Ness."
The talk starts at 12 o'clock c.t. in NA 1/58.

Monday, November 14th 2011, De-Qui Zhang (U Singapore)
"Lifting problems and transgression for non-abelian gerbes."
The talk starts at 16 o'clock c.t. in NA 2/64. 

Tuesday, November 8th 2011, Konrad Waldorf (University of Regensburg)
"Lifting problems and transgression for non-abelian gerbes."
The talk starts at 16 o'clock c.t. in NA 01/99.

Monday, October 31st 2011, Sebastian Herpel (Ruhr-University Bochum)
"Finite subgroups of algebraic groups"
The talk starts at 16 o'clock c.t. in NA 2/64. 

Wednesday, October 12th 2011, William G. Dwyer (Notre Dame)
"Morita Theory and its Generalizations"
The talk starts at 16 o'clock c.t. in NA 01/99.