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Autumn Semester 2026

Date / Time Speaker Title Location
21 September 2026
15:15-16:45 		Ibrahim Trifa
ETHZ
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Symplectic Geometry Seminar

Title A multidimensional Birkhoff Theorem for some $C^0$ Lagrangians
Speaker, Affiliation Ibrahim Trifa, ETHZ
Date, Time 21 September 2026, 15:15-16:45
Location HG G 43
Abstract In 1922, Birkhoff proved that an essential embedded closed curve that is invariant under a symplectic twist map of the cylinder is a Lipschitz graph over the zero section. There has been several works extending this theorem in higher dimension (Herman, Katznelson–Ornstein, Siburg, Bialy–Polterovich...). In 2010, Arnaud proved that a closed exact Lagrangian in the cotangent bundle of a closed manifold that is invariant under the flow of a Tonelli Hamiltonian is a graph over the base. Some improvements on this theorem have been made on one hand by Bernard-dos Santos and Amorim-Oh-dos Santos, who showed a version of this theorem for Lipschitz Lagrangian submanifolds, and on the other hand by Charfi who replaced the invariance assumption with a recurrence assumption (for a topology he defined and called "reduced complexity convergence"). In this talk, I will explain how to define a new class of "$C^0$ Lagrangians" for which the multidimensional Birkhoff theorem holds, using a new variant of the reduced complexity convergence topology. In particular, this class contains all images of continuous graphs by $C^1$ exact symplectomorphisms, which can be less regular than Lipschitz. This is based on a work in progress with Skander Charfi.
A multidimensional Birkhoff Theorem for some $C^0$ Lagrangiansread_more
HG G 43
5 October 2026
15:15-16:45 		Yael Karshon
Tel Aviv University
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Symplectic Geometry Seminar

Title Classification of complexity one Hamiltonian torus actions
Speaker, Affiliation Yael Karshon, Tel Aviv University
Date, Time 5 October 2026, 15:15-16:45
Location HG G 43
Abstract I will report on our (ongoing; partially completed) classification, joint with Sue Tolman, of Hamiltonian torus actions of complexity one.
Classification of complexity one Hamiltonian torus actionsread_more
HG G 43
23 November 2026
15:15-16:45 		Jean Gutt
Université libre de Bruxelles
Details

Symplectic Geometry Seminar

Title Title T.B.A.
Speaker, Affiliation Jean Gutt, Université libre de Bruxelles
Date, Time 23 November 2026, 15:15-16:45
Location HG G 43
Abstract TBA
Title T.B.A.read_more
HG G 43
7 December 2026
15:15-16:45 		Grigory Mikhalkin
Université de Genève
Details

Symplectic Geometry Seminar

Title Title T.B.A.
Speaker, Affiliation Grigory Mikhalkin, Université de Genève
Date, Time 7 December 2026, 15:15-16:45
Location HG G 43
Abstract TBA
Title T.B.A.read_more
HG G 43
14 December 2026
15:15-16:45 		Alexandru Oancea
Université de Strasbourg
Details

Symplectic Geometry Seminar

Title Title T.B.A.
Speaker, Affiliation Alexandru Oancea, Université de Strasbourg
Date, Time 14 December 2026, 15:15-16:45
Location HG G 43
Abstract TBA
Title T.B.A.read_more
HG G 43

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