Date/Time : 17 Dec 2020 09:00
Venue : via Zoom
Speaker : Yau Wing Li, MIT

Join Zoom Meeting https://nus-sg.zoom.us/j/87836014196?pwd=MGdrTmVWN1NuWHFCbFg3YTBjMEZsUT09 Meeting ID: 878 3601 4196 Password: 339627 Abstract: Affine Hecke categories are categorifications of Iwahori-Hecke algebras, which are essential in the classification of irreducible representations of loop group LG with Iwahori-fixed vectors. The affine Hecke category has a monodromic counterpart, which contains sheaves with prescribed monodromy under the left and right actions of the maximal torus. We show that the neutral block of this monoidal category is equivalent to the neutral block of the affine Hecke category (with trivial torus monodromy) for the endoscopic group H. It is consistent with the Langlands functoriality conjecture.

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Date/Time : 17 Dec 2020 15:00
Venue : via Zoom
Speaker : Jue Liu , Tsinghua University

Join Zoom Meeting https://nus-sg.zoom.us/j/83066226180?pwd=NllOVE1TNXZxTXhqVGFyYVRxMlQ1UT09 Meeting ID: 830 6622 6180 Password: 886072 One of the fundamental questions in mathematical relativity is the weak cosmic censorship conjecture proposed by Penrose, which states that for generic asymptotically flat spacetime the singularities are invisible to the infinity observers. This is a global problem involved the whole future evolution of the asymptotically flat initial data. A suitable local version is the trapped surfaces conjecture proposed by Christodoulou, which states that given a first singularity, for generic initial data defined on an arbitrary domain containing the intersection of the past of the singularity and the initial hypersurface, the maximal future development must contain a trapped surface. In this talk, I will review the previous works on the formation of trapped surfaces and introduce the recent work by Junbin Li and me about the trapped surfaces conjecture when the first singularity is a spherical naked singularity constructed by Christodoulou.

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Date/Time : 16 Dec 2020 22:00
Venue : via Zoom
Speaker : Pengyu Le, University of Michigan

Join Zoom Meeting https://nus-sg.zoom.us/j/87148806876?pwd=VXlaNjlIaGNpK0c1SnY4YTQ1WUs1QT09 Meeting ID: 871 4880 6876 Password: 380377 Abstract: The Penrose inequality is a renowned inequality in GR proposed by Penrose in 1973. It conjectures a simple relation between the total mass of the spacetime and the mass of a black hole inside it. The original physical reasoning supporting this inequality by Penrose relates it to some most important and difficult problems in GR, i.e. the final state of the evolution of black holes and naked singularities (the weak cosmic censorship conjecture). A major breakthrough was achieved independently by Huisken, Ilmanen and by Bray, proving the Riemannian Penrose inequality. The Riemannian Penrose inequality is of course not the end of the story. There is another important case of the inequality on null hypersurfaces which is still open. The speaker will present his work confirming the null Penrose inequality on null hypersurfaces in a perturbed Schwarzschild spacetime. A major ingredient of the work is the study of perturbations of null hypersurfaces.

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Date/Time : 11 Dec 2020 14:00
Venue : via Zoom
Speaker : Zhang Lei, National University of Singapore

Join Zoom Meeting https://nus-sg.zoom.us/j/82676230389?pwd=UmJtUTN2dzhycTJ2UTFrWHpZVXpTZz09 Meeting ID: 826 7623 0389 Password: 654751 Abstract: Fourier coefficients of automorphic representations play a fundamental role in number theory and representation theory, the theory of automorphic forms in particular. For instance, they are used to count the number of representations of a natural number as the sum of four squares and now they play a crucial role in our approach to the celebrated Gan-Gross-Prasad Conjecture. One of my main goals is to justify the Local-Global principle of those Fourier coefficients. Motivated by this, I investigated various cases and connections with other topics in the past several years. I will report my results and my future research plan in this talk.

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Date/Time : 09 Dec 2020 15:00
Venue : via Zoom
Speaker : Tsviqa Lakrec , The Hebrew University of Jerusalem, Israel

Join Zoom Meeting https://nus-sg.zoom.us/j/81609880468?pwd=Wk5Dak1QK0ZpL1lkWVo5S0VOVVB5dz09 Meeting ID: 816 0988 0468 Password: 692705 Abstract: I will discuss a quantitative equidistribution result for the random walk on a torus arising from the action of the group of affine transformations. This is a joint work with Weikun He and Elon Lindenstrauss. Under the assumption that the Zariski closure of the group generated by the linear part acts strongly irreducibly on R^d, we give quantitative estimates (depending only on the linear part of the random walk) for how fast this random walk equidistributes, unless the initial point and the translation part of the affine transformations can be perturbed so that the random walk is trapped in a finite orbit of small cardinality. In particular, we prove that the random walk equidistributes in law to the Haar measure if and only if the random walk is not trapped in a finite orbit. This extends the results of Bourgain-Furman-Lindenstrauss-Mozes and He-de Saxcé on quantitative equidistribution of the random walk arising from the action of the automorphism group of the torus. I will also mention a new joint work with He and Lindenstrauss that extends these results to nilmanifolds.

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Date/Time : 08 Dec 2020 16:00
Venue : via Zoom
Speaker : Zhang Shuangjian Kelvin, Ecole Normale Superieure Paris

Join Zoom Meeting https://nus-sg.zoom.us/j/88422472817?pwd=UmFmckRDR2drSVdCZzExR0tGQVVkZz09 Meeting ID: 884 2247 2817 Password: 591367 Abstract:  A monopolist wishes to maximize her profits by finding an optimal price policy. After she announces a menu of products and prices, each agent will choose to buy that product which maximizes his own utility, if positive. The monopolist’s profits are the sum of the net earnings produced by each product sold. These are determined by the costs of production and the distribution of products sold, which in turn are based on the distribution of anonymous agents and the choices they make in response to the monopolist’s price menu. In this talk, I will first review the history of this bi-level maximization problem in microeconomics, including the celebrated work of two Nobel Laureates and one Fields Medalist as well as recent progress on existence, uniqueness, concavity, regularity, and numerical results. When present, concavity makes the problem more amenable to computational and theoretical analysis; it is key to obtaining uniqueness and stability results for the monopolist’s strategy in particular. The beautiful connection to the regularity theory in Optimal Transport will also be addressed. Then I will list a few open problems and discuss some current progress. Part of this talk represents joint work with Professor Robert J. McCann and Professor Guillaume Carlier.

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Date/Time : 07 Dec 2020 09:00
Venue : via Zoom
Speaker : Kwan Matthew, Stanford University

Join Zoom Meeting https://nus-sg.zoom.us/j/86360904887?pwd=MEFqS1MwcHRTQ0djTHhXUHFFaG43Zz09 Meeting ID: 863 6090 4887 Password: 557853 Abstract: Anti-concentration inequalities provide limits on the extent to which random variables can be concentrated: for example, they commonly give uniform upper bounds on the probability that a random variable takes any particular value. In this talk I’ll discuss some of the many connections between anti-concentration and combinatorics, initially focusing on applications to Ramsey graphs but also touching on a few other topics such as the polynomial Littlewood-Offord problem and permanents of random matrices.

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Date/Time : 02 Dec 2020 09:00
Venue : via Zoom
Speaker : Yu Zhao, MIT

Join Zoom Meeting https://nus-sg.zoom.us/j/85482725152?pwd=em9iM2ZlQk5EZEsyQ3ZIUm9ZVVN3Zz09 Meeting ID: 854 8272 5152 Password: 638381 The quantum toroidal algebra is the affinization of the quantum Heisenberg algebra. Schiffmann-Vasserot, Feigin-Tsymbaliuk and Negut studied the quantum toroidal algebra action on the Grothendieck group of Hilbert schemes of points on surfaces, which generalized the action by Nakajima and Grojnowski in cohomology. In this talk, we will categorify the above quantum toroidal algebra action. Our main technical tool is a detailed geometric study of certain nested Hilbert schemes of triples and quadruples, through the lens of the minimal model program, by showing that these nested Hilbert schemes are either canonical or semi-divisorial log terminal singularities.

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Date/Time : 01 Dec 2020 09:00
Venue : via Zoom
Speaker : Sun Zheng, Ohio State University

Join Zoom Meeting https://nus-sg.zoom.us/j/87275619715?pwd=WVJrYXVyVVFZbk5meXhna3kycTJEdz09 Meeting ID: 872 7561 9715 Password: 617596 In scientific and engineering computing, we usually encounter numerical discretizations of time-dependent equation systems associated with conserved or decreasing energy. The strong stability of a time stepping method refers to its ability to preserve the monotone energy-decay law at the discrete level, which may provide improved robustness for simulations. In this talk, we present a unified study on strong stability of explicit Runge-Kutta methods for linear semi-negative problems. In particular, we prove a both necessary and sufficient condition for strong stability of arbitrary odd-order methods.

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