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 : 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 : 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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Date/Time : 14 Feb 2024 16:30
Venue : S17 #04-06 (SR1)
Speaker : Liu Xin, Chinese Academy of Sciences

We propose constraint dissolving approaches for optimization problems over a class of Riemannian manifolds. In these proposed approaches, solving a Riemannian optimization problem is transferred into the unconstrained minimization of a constraint dissolving function named CDF. Different from existing exact penalty functions, the exact gradient and Hessian of CDF are easy to compute. We study the theoretical properties of CDF and prove that the original problem and CDF have the same first-order and second-order stationary points, local minimizers, and Łojasiewicz exponents in a neighborhood of the feasible region. Remarkably, the convergence properties of our proposed constraint dissolving approaches can be directly inherited from the existing rich results in unconstrained optimization. Therefore, the proposed constraint dissolving approaches build up short cuts from unconstrained optimization to Riemannian optimization. Several illustrative examples further demonstrate the potential of the proposed approaches.

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Date/Time : 13 Feb 2024 15:00
Venue : S17 #04-05 (SR2)
Speaker : Lei Zhen, Fudan University

More than 20 years ago, Soffer-Weinstein considered a class of nonlinear Klein-Gordon equations with nice potentials. For the first time they proved that spatially localized and time-periodic solutions of the linear problem are destroyed by generic nonlinear Hamiltonian perturbations via slow radiation of energy to infinity, via energy transfer from the discrete to continuum modes, under the condition that the discrete modes are close to the continuous spectral modes. Since then, a long-standing open question is to study the the corresponding problem with small eigenvalues, which was also raised in the paper of Soffer-Weinstein 1999. In this talk, we will report our recent result on this problem which settles the above open question. This is a joint work with my students Jie Liu and Zhaojie Yang.

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Date/Time : 08 Feb 2024 16:00
Venue : S17 #05-12 (SR4)
Speaker : Rui Chen, IASM, Zhejiang University

Branching law is a classical topic in representation theory which concerns certain Hom functors. It is natural to consider Ext functors associated to those Hom functors. In this talk we will report some recent progress on the study of these Ext spaces, and show a conjecture of D. Prasad, which asserts that the Ext-analogue of GGP model vanishes for tempered representations. We will also discuss some applications.

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Date/Time : 08 Feb 2024 14:00
Venue : S17 #05-11 (SR5)
Speaker : Peter A Markowich, KAUST

We propose a mesoscopic modeling framework for optimal transportation networks with biological applications. The network is described in terms of a joint probability measure on the phase space of tensor-valued conductivity and position in physical space. The energy expenditure of the network is given by a functional consisting of a pumping (kinetic) and metabolic power-law term, constrained by a Poisson equation accounting for local mass conservation. We establish convexity and lower semicontinuity of the functional on appropriate sets. We then derive its gradient flow with respect to the 2-Wasserstein topology on the space of probability measures, which leads to a transport equation, coupled to the Poisson equation. To lessen the mathematical complexity of the problem, we derive a reduced Wasserstein gradient flow, taken with respect to the tensor-valued conductivity variable only. We then construct equilibrium measures of the resulting PDE system. Finally, we derive the gradient flow of the constrained energy functional with respect to the Fisher-Rao (or Hellinger-Kakutani) metric, which gives a reaction-type PDE. We calculate its equilibrium states, represented by measures concentrated on a hypersurface in the phase space.

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Date/Time : 08 Feb 2024 15:00
Venue : S17-05-11 (SR 5)
Speaker : Tan Bui-Thanh, The University of Texas at Austin

Digital twins (DTs) are digital replicas of systems and processes. At the core of a DT is a  physical/mathematical model which captures the behavior of the real system across temporal and spatial scales. One of the key roles of DTs is enabling “what if” scenario testing of hypothetical simulations to understand the implications at any point throughout the life cycle of the process, to monitor the process, and to calibrate parameters to match the actual process. In this talk we will present various (faster than) real time Scientific Deep Learning (SciDL) approaches for forward, inverse, and UQ problems. Both theoretical and numerical results for various problems including transport, heat, Burgers, Euler, and Navier-Stokes equations will be presented.

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