Date/Time : 22 Feb 2024 16:00
Venue : S17 #05-12 (SR4)
Speaker : Bin Xu, Tsinghua University

The irreducible admissible representations of Arthur class are the local components of automorphic representations. They are conjectured to be parametrized by the Arthur parameters, and the set of irreducible representations associated with a single Arthur parameter is called an Arthur packet. For quasisplit symplectic and orthogonal groups, the Arthur packets have been determined by Arthur, and their structure in the p-adic case can be understood in a very complicated way by the works of Moeglin, Atobe and myself. In this talk, we would like to introduce some conjectures on their structure in the real case, which are motivated by the results in the p-adic case. For simplicity we will only consider the symplectic groups. This is an ongoing project with Taiwang Deng and Chang Huang.

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Date/Time : 21 Feb 2024 14:00
Venue : S17 #06-11 (SR6)
Speaker : Chengyang Shao, University of Chicago

The speaker aims to report some advances in the study of irrotational oscillation of a water droplet under zero gravity. The governing physical laws, resembling the well-studied capillary water waves equation, are converted to a quasilinear dispersive para-differential system defined on the 2-sphere. Regarding this conversion, a coordinate-independent, global para-differential calculus defined on compact Lie groups and homogeneous spaces is developed as a toolbox. After discussing Cauchy theory under this novel formalism, the speaker will propose several problems concerning existence of periodic solutions, normal form reduction and generic lifespan estimates. It is pointed out that these problems are closely related to certain Diophantine equations.

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Date/Time : 21 Feb 2024 15:00
Venue : S17 #06-11 (SR6)
Speaker : Zhaojie Yang, Fudan University

In this talk, I will present a backward uniqueness result of bounded mild solutions of 3D Navier-Stokes Equations in the whole space with non-trivial final data. A direct consequence is that a solution must be axi-symmetric in [0, T] if it is so at time T. The proof is based on a new weighted estimate which enables to treat terms involving Calderon-Zygmund operators. The new weighted estimate is expected to have certain applications in control theory when classical Carleman-type inequality is not applicable. This is a joint work with Prof. Zhen Lei and Dr. Cheng Yuan.

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Date/Time : 21 Feb 2024 17:00
Venue : S17 #04-05 (SR2)
Speaker : Neil Barton, University of Oslo

A popular view in the philosophy of set theory is that of *potentialism*: the position that the set-theoretic universe unfolds as more sets come into existence or become accessible to us. This often gets formalised using *modal logic*, but there is always a question of how to move to *non-modal* theories. In this latter regard, a difficult question for the potentialist is to explain how *intensional entities* (entities individuated by an application condition rather than an extension) behave, and in particular what logic governs them. This talk will discuss some work in progress on this issue. We’ll see how to motivate acceptance of different propositional logics for different flavours of potentialism, and discuss the prospects for proving results about the kinds of first-order theories validated.

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Date/Time : 20 Feb 2024 15:00
Venue : S17 #04-06 (SR1)
Speaker : Yin Zhang, Chinese University of Hong Kong, Shenzhen

The Transformer architecture, in which the key innovation is a Self-Attention (SA) mechanism, has revolutionized AI fields since its introduction in 2017. However, it is difficult to extend the original architecture to deeper models with 10 or more layers. In this work, we show that this depth problem is caused by a phenomenon called token similarity escalation (TSE) which is driven by spectral properties of attention matrices in the SA mechanism. Based on the gained insights, we propose a simple de-escalation strategy to surgically removes excessive token similarity without discounting SA as a whole. Empirical evidence is presented to demonstrate the effectiveness of our scheme on small-scale models.

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Date/Time : 19 Feb 2024 17:00
Venue : S17 #04-06 (SR1)
Speaker : Geo Kam-Fai Tam, Radboud University Nijmegen

Let $G$ be a classical group (orthogonal, symplectic, unitary) over a p-adic field $F$, and $\pi$ be a supercuspidal representation of $G(F)$. We propose a general strategy to express the Langlands parameter of $\pi$, requiring only that the residual characteristic $p$ to be odd, with proven examples by the speaker (partly joint with Corinne Blondel) and others. To briefly explain the strategy, we apply M{\oe}glin’s theory of Jordan blocks and Bushnell-Kutzko’s theory of covering types to determine the endoscopic lift of $\pi$ into the general linear group whose dual expresses the dual group of $G$ as a complex matrix group. The cuspidal support of this lifted representation is given explicitly in terms of the inducing type of $\pi$ defined by Stevens’ construction of supercuspidal representations using skew semi-simple strata. This strategy is completely on the representation side. On the Galois side, we provide an example of describing the Langlands parameters of epipelagic representations constructed by Reeder-Yu, generalizing the results obtained by Oi on simple supercuspidal representations constructed by Gross-Reeder.

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Date/Time : 19 Feb 2024 16:00
Venue : S17 #04-06 (SR1)
Speaker : Max Gurevich, Technion - Israel Institute of Technology

Pursuing local foundations for Arthur theory, Adams-Barbasch-Vogan have introduced the concept of a weak Arthur packet. Those are sets of irreducible representations that share a common infinitesimal character specified by a choice of a dual unipotent conjugacy class, and have the smallest possible Gelfand-Kirillov dimension among those.

In a separate context, the p-adic setting of groups of classical type shows that Arthur theory behaves well with respect to ramification properties. Namely, an Arthur-type spherical representation determines the unique Arthur packet which contains it – a consequence of the extended Shahidi conjecture.

In a joint work with Emile Okada, we unveil a ‘weak’ analogy to this phenomenon: Roughly, a weak Arthur packet is characterized as the union of those Arthur packets that contain a weakly spherical representation.

Along the way we explicate how geometry of adjoint nilpotent orbits governs the composition of weak Arthur packets, while exhibiting Springer theory as an effective tool for studying weak sphericity in p-adic groups.

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Date/Time : 15 Feb 2024 16:00
Venue : S17 #05-12 (SR4)
Speaker : Yuanqing Cai, Kanazawa University

We discuss the trace formula for the n-fold covering group of SL_2. Along the way, we also revisit the work of Flicker on the trace formula for covering groups of GL(2).

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Date/Time : 14 Feb 2024 15:00
Venue : S17 #04-06 (SR1)
Speaker : Zhiming Chen, Chinese Academy of Sciences

We consider high-order unfitted finite element methods on Cartesian meshes with hanging nodes for elliptic interface problems, which release the work of body-fitted mesh generation and allow us to design adaptive finite element methods for solving curved geometric singularities. We introduce new concepts of the large element and interface deviation to solve the small cut cell problem of unfitted finite element methods. We construct reliable algorithms to merge small interface elements with their surrounding elements to automatically generate the finite element mesh whose elements are large with respect to both domains. Numerical examples are presented to illustrate the competitive performance of the method. This talk is based on joint works with Ke Li, Yong Liu and Xueshuang Xiang.

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