Date/Time : 29 Sep 2025 15:00
Venue : S17 #04-05 (Seminar Room 2)
Speaker : Yifei Zhao, University of Muenster

Around 10 years ago, Weissman constructed the local Langlands correspondence (LLC) for covers of split tori defined by Brylinski-Deligne extensions. An intriguing phenomenon is that the LLC map is not surjective in general. The goal of my talk is to relate this phenomenon to the existence of “extended pure inner forms” of covers. This is based on joint work with Luozi Shi.

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Date/Time : 25 Sep 2025 16:00
Venue : S17 #04-05 (Seminar Room 2)
Speaker : Keiji Oguiso, The University of Tokyo

My talk is based on my joint work with Professor Koji Fujiwara (Kyoto University) and Professor Xun Yu (Tianjin University).

Main result of my talk is the finiteness of the Neron-Severi lattices of complex projective K3 surfaces whose automorphism groups are non-elementary hyperbolic under the assumption that the Picard number greater than or equal to 6 (which is optimal to ensure the finiteness). In this talk, after recalling the notion of hyperbolicity of group due to Gromov and its importance in mathematics, I would like to explain first why the non-elementary hyperbolicity of K3 surface automorphism group is the problem of the Neron-Severi lattices and next how one can deduce the above-mentioned finiteness, via the study of genus one fibrations on K3 surfaces and recent work of Kikuta and Takatsu on geometrically finiteness.

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Date/Time : 25 Sep 2025 15:00
Venue : S17 #05-12 (Seminar Room 4)
Speaker : Lars Nerger, Alfred Wegener Institute

Data assimilation is used to estimate the state in components of Earth system models, and to estimate parameters that control processes in these models. The estimate is computed by combining the model state with real observations, like the temperature of the ocean surface or winds in the atmosphere, accounting for the uncertainties in both the model and the observations. The observations are characterized by significant uncertainties and by extreme sparseness. For example, for the ocean satellites can only observe the surface while measurements inside the ocean can only be done by ships or autonomous instruments, of which only a few thousand are distributed over the world ocean. Skillful data assimilation methods have been developed to utilize the sparse observational information with complex models used on supercomputers. Here, ensemble simulations provide uncertainty estimates, but multiply the modeling cost. In addition, the model nonlinearity induces challenges because common methods like ensemble Kalman filters assume Gaussian error distributions. This presentation will discuss the computational, algorithmic, and practical challenges and recent developments in these complex applications of data assimilation.

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Date/Time : 24 Sep 2025 16:00
Venue : S17 #04-04 (Seminar Room 3)
Speaker : Shuichiro Takeda, The University of Osaka

Let V be a quadratic space over a nonarchimedean local field of characteristic 0. The orthogonal group O(V) and the special orthogonal group SO(V) have a unique nontrivial GL_1 -extension called GPin(V) and GSpin(V), respectively. Let W\subseteq V be a subspace of codimension 1. Then there are natural inclusions GPin(W)\subseteq
GPin(V) and GSpin(W)\subseteq GSpin(V). One can then consider the Gan-Gross-Prasad (GGP) periods for GPin and GSpin. In this talk, I will talk about the multiplicity-at-most-one theorem for the local GGP periods for GPin and GSpin. This is a joint work with Melissa Emory.

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Date/Time : 23 Sep 2025 16:00
Venue : S17 #05-11 (Seminar Room 5)
Speaker : Chong Jing Quan, National University of Singapore

For classical groups, the explicit construction of local Arthur packets in terms of enhanced Langlands parameter has been studied by a sequence of work of C. Mœglin, B. Xu and H. Atobe. In this talk, we will discuss Atobe’s algorithm for this construction. We will introduce the combinatorial object called extended multi-segments and their associated representations. If time permits, we will talk about several operators on extended multi-segments that preserves the associated representations.

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Date/Time : 19 Sep 2025 16:00
Venue : S17 #05-11 (Seminar Room 5)
Speaker : Giulio Ruzza, University of Lisbon

Random matrix theory is a central field in Mathematical Physics, linking probability, physics, number theory, and combinatorics. A main theme is Universality: the fact that, at large size, eigenvalue statistics look the same across many different models and even show up in distant areas such as random growth processes, random tilings, and the zeros of the Riemann zeta function. We will focus on unitary-invariant ensembles, where orthogonal polynomials play a key role and whose asymptotic properties can be studied using the Riemann–Hilbert method.

Topics: Unitary-invariant ensembles. Orthogonal polynomials. Determinantal Point Processes. Universality. Riemann-Hilbert problems. Nonlinear steepest descent asymptotic method applied to Orthogonal Polynomials.

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Date/Time : 18 Sep 2025 16:00
Venue : S17 #05-11 (Seminar Room 5)
Speaker : Giulio Ruzza, University of Lisbon

Random matrix theory is a central field in Mathematical Physics, linking probability, physics, number theory, and combinatorics. A main theme is Universality: the fact that, at large size, eigenvalue statistics look the same across many different models and even show up in distant areas such as random growth processes, random tilings, and the zeros of the Riemann zeta function. We will focus on unitary-invariant ensembles, where orthogonal polynomials play a key role and whose asymptotic properties can be studied using the Riemann–Hilbert method.

Topics: Unitary-invariant ensembles. Orthogonal polynomials. Determinantal Point Processes. Universality. Riemann-Hilbert problems. Nonlinear steepest descent asymptotic method applied to Orthogonal Polynomials.

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Date/Time : 17 Sep 2025 17:00
Venue : S17 #05-11 (Seminar Room 5)
Speaker : Andrea Volpi, University of Udine

This is joint work with Damir Dzhafarov and Reed Solomon. A relational structure is strongly indivisible if for every partition M = X0 ⊔ X1, the induced substructure on X0 or X1 is isomorphic to M. Cameron (1997) showed that a graph is strongly indivisible if and only if it is the complete graph, the completely disconnected graph, or the random graph. We analyze the strength of Cameron’s theorem using tools from computability theory and reverse mathematics. We show that Cameron’s theorem is is effective up to computable presentation, and give a partial result towards showing that the full theorem holds in the ω-model REC. We also establish that Cameron’s original proof makes essential use of the stronger induction scheme IΣ2.

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Date/Time : 17 Sep 2025 16:30
Venue : S17 #04-05 (Seminar Room 2)
Speaker : Li Yifei, University of Tübingen

In this talk, we present a numerical analysis of the Eyles-King-Styles tumour growth model, a free boundary problem with nontrivial bulk-surface coupling. Unlike existing evolving bulk problems or evolving surface problems, this bulk-surface coupling necessitates H^{1/2}-order analysis on the surface Γ. We establish a comprehensive H^{1/2}(Γ) framework that captures the coupling between the bulk and surface. It enables a rigorous convergence analysis for continuous finite elements of polynomial degree at least 3, yielding optimal-order H^1(Ω)/H^{1/2}(Γ) error bounds.

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