Seminar information archive

Seminar information archive ~04/18Today's seminar 04/19 | Future seminars 04/20~

2024/04/17

Number Theory Seminar

17:00-18:00   Room #117 (Graduate School of Math. Sci. Bldg.)
Ahmed Abbes (Institut des Hautes Études Scientifiques, University of Tokyo)
Functoriality of the p-adic Simpson correspondence by proper push forward (English)
[ Abstract ]
Faltings initiated in 2005 a p-adic analogue of the (complex) Simpson correspondence whose construction has been taken up by various authors, according to several approaches. After recalling the one initiated by myself with Michel Gros, I will present our initial result on the functoriality of the p-adic Simpson correspondence by proper push forward, leading to a generalization of the relative Hodge-Tate spectral sequence. If time permits, I will give a brief overview of an ongoing project with Michel Gros and Takeshi Tsuji, aimed at establishing a more robust framework for achieving broader functoriality results of the p-adic Simpson correspondence, by both proper push forward and pullback.

Discrete mathematical modelling seminar

13:30-15:00   Room #126 (Graduate School of Math. Sci. Bldg.)
Jaume Alonso (Technische Universität Berlin)
Discrete Painlevé equations and pencils of quadrics in 3D (English)
[ Abstract ]
In this talk we propose a new geometric interpretation of discrete Painlevé equations. From this point of view, the equations are birational transformations of $\mathbb{P}^3$ that preserve a pencil of quadrics and map each quadric of the pencil to a different one, according to a Möbius transformation of the pencil parameter. This allows for a classification of discrete Painlevé equations based on the classification of pencils of quadrics in $\mathbb{P}^3$. In this scheme, discrete Painlevé equations are obtained as deformations of the 3D QRT maps introduced in the previous talk, which consist of the composition of two involutions along the generators of the quadrics of a pencil of quadrics until they meet a second pencil. The deformation is then a birational (often linear) transformation in $\mathbb{P}^3$ under which the pencil remains invariant, but the individual quadrics do not.

This is a joint work with Yuri Suris and Kangning Wei.

2024/04/16

Tuesday Seminar on Topology

17:00-18:00   Online
Pre-registration required. See our seminar webpage.
Hiroaki Karuo (Gakushuin University)
Skein algebras and quantum tori in view of pants decompositions (JAPANESE)
[ Abstract ]
To understand the algebraic structures of skein algebras and their generalizations, we usually try to embed these algebras into quantum tori using ideal triangulations of a surface and the splitting map. However, such a construction does not work for the skein algebras of closed surfaces and the Roger--Yang skein algebras of punctured surfaces.
In the talk, we define filtrations on these algebras using pants decompositions and embed the associated graded algebras into quantum tori. As a consequence, Roger--Yang skein algebras are quantizations of decorated Teichmuller spaces. This talk is based on a joint work with Wade Bloomquist (Morningside University) and Thang Le (Georgia Institute of Technology).
[ Reference URL ]
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html

2024/04/15

Tokyo Probability Seminar

16:00-17:30   Room #126 (Graduate School of Math. Sci. Bldg.)
We are having teatime from 15:15 in the common room on the second floor. Please join us.
Tomohiro Aya (Kyoto University)
Quantitative stochastic homogenization of elliptic equations with unbounded coefficients (日本語)

Seminar on Geometric Complex Analysis

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Yuya Takeuchi (Tsukuba Univ.)
Kohn-Rossi cohomology of spherical CR manifolds (Japanese)
[ Reference URL ]
https://forms.gle/gTP8qNZwPyQyxjTj8

2024/04/11

Applied Analysis

16:00-17:30   Room #126 (Graduate School of Math. Sci. Bldg.)
Jan Haskovec (KAUST, Saudi Arabia)
Non-Markovian models of collective motion (English)
[ Abstract ]
I will give an overview of recent results for models of collective behavior governed by functional differential equations with non-Markovian structure. The talk will focus on models of interacting agents with applications in biology (flocking, swarming), social sciences (opinion formation) and engineering (swarm robotics), where latency (delay) plays a significant role. I will characterize two main sources of delay - inter-agent communications ("transmission delay") and information processing ("reaction delay") - and discuss their impacts on the group dynamics. I will give an overview of analytical methods for studying the asymptotic behavior of the models in question and their mean-field limits. In particular, I will show that the transmission vs. reaction delay leads to fundamentally different mathematical structures and requires appropriate choice of analytical tools. Finally, motivated by situations where finite speed of information propagation is significant, I will introduce an interesting class of problems where the delay depends nontrivially and nonlinearly on the state of the system, and discuss the available analytical results and open problems here.
[ Reference URL ]
https://forms.gle/5cZ4WzqBjhsXrxgU6

2024/04/10

FJ-LMI Seminar

16:00-17:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Séverin PHILIP (京都大学 数理解析研究所, RIMS, Kyoto University)
Galois outer representation and the problem of Oda
(英語)
[ Abstract ]
Oda’s problem stems from considering the pro-l outer Galois actions on the moduli spaces of hyperbolic curves. These actions come from a generalization by Oda of the standard étale homotopy exact sequence for algebraic varieties over the rationals. We will introduce these geometric Galois actions and present some of the mathematics that they have stimulated over the past 30 years along with the classical problem of Oda. In the second and last part of this talk, we will see how a cyclic special loci version of this problem can be formulated and resolved in the case of simple cyclic groups using the maximal degeneration method of Ihara and Nakamura adapted to this setting.
[ Reference URL ]
https://fj-lmi.cnrs.fr/seminars/

Seminar on Probability and Statistics

13:30-14:40   Room #126 (Graduate School of Math. Sci. Bldg.)
Ivan Nourdin (University of Luxembourg)
Limit theorems for additive functionals of stationary Gaussian fields (English)
[ Abstract ]
In this talk, we will investigate central and non-central limit theorems for additive functionals of stationary Gaussian fields. Our main tool will be the Malliavin-Stein approach. Based on joint works with Nikolai Leonenko, Leonardo Maini and Francesca Pistolato.
[ Reference URL ]
https://forms.gle/uMKm3gVquLpYaVdc6

Discrete mathematical modelling seminar

13:30-15:00   Room #056 (Graduate School of Math. Sci. Bldg.)
Room change: Room 470 → Room 056
Jaume Alonso (Technische Universität Berlin)
Integrable birational maps and a generalisation of QRT to 3D (English)
[ Abstract ]
When completely integrable Hamiltonian systems are discretised, the resulting discrete-time systems are often no longer integrable themselves. This is the so-called problem of integrable discretisation. Two known exceptions to this situation in 3D are the Kahan-Hirota-Kimura discretisations of the Euler top and the Zhukovski-Volterra gyrostat with one non-zero linear parameter β, both birational maps of degree 3. The integrals of these systems define pencils of quadrics. By analysing the geometry of these pencils, we develop a framework that generalises QRT maps and QRT roots to 3D, which allows us to create new integrable maps as a composition of two involutions. We show that under certain geometric conditions, the new maps become of degree 3. We use these results to create new families of discrete integrable maps and we solve the problem of integrability of the Zhukovski-Volterra gyrostat with two β’s.

This is a joint work with Yuri Suris and Kangning Wei.

2024/04/09

Tuesday Seminar on Topology

17:00-18:30   Room #ハイブリッド開催/056 (Graduate School of Math. Sci. Bldg.)
Pre-registration required. See our seminar webpage.
Shouhei Honda (The University of Tokyo)
Topological stability theorem and Gromov-Hausdorff convergence (JAPANESE)
[ Abstract ]
Gromov-Hausdorff distance defines a distance on the set of all isometry classes of compact metric spaces. It is natural to ask about topological relationships between two compact metric spaces whose Gromov-Hausdorff distance is small. Cheeger-Colding provided a striking result about this question, under a (lower) curvature bound on Ricci curvature. In this talk we will improve this result sharply. This is a joint work with Yuanlin Peng (Tohoku University). If time permits, along this direction, we will also discuss a recent work about a topological stability result to flat tori via harmonic maps, where this is a joint work with Christian Ketterer (University of Freiburg), Ilaria Mondello (Université de Paris Est Créteil), Chiara Rigoni (University of Vienna) and Raquel Perales (CIMAT).
[ Reference URL ]
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html

2024/03/21

Applied Analysis

16:00-17:30   Room #126 (Graduate School of Math. Sci. Bldg.)
Mostafa Fazly (University of Texas at San Antonio)
Symmetry Results for Nonlinear PDEs (English)
[ Abstract ]
The study of qualitative behavior of solutions of Partial Differential Equations (PDEs) started roughly in mid-18th century. Since then scientists and mathematicians from different fields have put in a great effort to expand the theory of nonlinear PDEs. PDEs can be divided into two kinds: (a) the linear ones, which are relatively easy to analyze and can often be solved completely, and (b) the nonlinear ones, which are much harder to analyze and can almost never be solved completely.
We begin this talk by an introduction on foundational ideas behind the De Giorgi’s conjecture (1978) for the Allen-Cahn equation that is inspired by the Bernstein’s problem (1910). This conjecture brings together three groups of mathematicians: (a) a group specializing in nonlinear partial differential equations, (b) a group in differential geometry, and more specially on minimal surfaces and constant mean curvature surfaces, and (c) a group in mathematical physics on phase transitions. We then present natural generalizations and counterparts of the problem. These generalizations lead us to introduce certain novel concepts, and we illustrate why these novel concepts seem to be the right concepts in the context and how they can be used to study particular systems and models arising in Sciences. We give a survey of recent results.

2024/03/14

Colloquium

14:30-17:00   Room #大講義室(auditorium) (Graduate School of Math. Sci. Bldg.)
If you do not belong to Graduate School of Mathematical Sciences, the University of Tokyo, please apply from the form at [Reference URL].
Toshiyasu Arai (Graduate School of Mathematical Sciences, The University of Tokyo) 14:30-15:30
Many years from now (JAPANESE)
[ Abstract ]
I have been studying proof theory since the 1980's. In this talk I will talk about what happened to me in these 40 years, and let me report the latest result on ordinal analysis.
[ Reference URL ]
https://forms.gle/m38f1KRi67ECuA7MA
Masahiro Yamamoto (Graduate School of Mathematical Sciences, The University of Tokyo) 16:00-17:00
Mathematics, which I eventually found that I like: from the viewpoint of some marginal areas (JAPANESE)
[ Abstract ]
Looking back on my experiences over 40 years, I have been convinced that I have been loving my own mathematics among others.

After all, I can sum up as that all my mathematics are concerned with the three topics: control theories, inverse problems and time-fractional partial differential equations. Some of these research fields has already developed to major topics, while others keep still minor interests.

When I started studies on inverse problems in 1980's, there were very few population of mathematicians as specialists in Japan. In particular, inverse problems did not call great attention of mathematicians and were understood as marginal mathematical topics in spite of practical significance and demands On the other hand, possibly available methodologies and ideas have been exploited and integrated gradually. As consequence, main research partners have been outside Japan.
I have been enjoying not only the research contents, but also such wider collaboration.

Aiming at non-meaningless reference for the youngers, and trying not to be too retrospective, I will describe how I have done in mathematics as well as my research contents.
[ Reference URL ]
https://forms.gle/m38f1KRi67ECuA7MA

Tokyo-Nagoya Algebra Seminar

10:30-12:00   Online
Arashi Sakai (Nagoya University)
Lattices of torsion classes in representation theory of finite groups (Japanese)
[ Reference URL ]
http://www.math.nagoya-u.ac.jp/~aaron.chan/TNAseminar.html

2024/03/13

Numerical Analysis Seminar

16:30-17:30   Online
David Sommer (Weierstrass Institute for Applied Analysis and Stochastics)
Approximating Langevin Monte Carlo with ResNet-like neural network architectures (English)
[ Abstract ]
We analyse a method to sample from a given target distribution by constructing a neural network which maps samples from a simple reference distribution, e.g. the standard normal, to samples from the target distribution. For this, we propose using a neural network architecture inspired by the Langevin Monte Carlo (LMC) algorithm. Based on LMC perturbation results, approximation rates of the proposed architecture for smooth, log-concave target distributions measured in the Wasserstein-2 distance are shown. The analysis heavily relies on the notion of sub-Gaussianity of the intermediate measures of the perturbed LMC process. In particular, we derive bounds on the growth of the intermediate variance proxies under different assumptions on the perturbations. Moreover, we propose an architecture similar to deep residual neural networks (ResNets) and derive expressivity results for approximating the sample to target distribution map.
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/

Numerical Analysis Seminar

17:30-18:30   Online
Andreas Rathsfeld (Weierstrass Institute for Applied Analysis and Stochastics)
Analysis of the Scattering Matrix Algorithm (RCWA) for Diffraction by Periodic Surface Structures (English)
[ Abstract ]
The scattering matrix algorithm is a popular numerical method for the diffraction of optical waves by periodic surfaces. The computational domain is divided into horizontal slices and, by a clever recursion, an approximated operator, mapping incoming into outgoing waves, is obtained. Combining this with numerical schemes inside the slices, methods like RCWA and FMM have been designed.
The key for the analysis is the scattering problem with special radiation conditions for inhomogeneous cover materials. If the numerical scheme inside the slices is the FEM, then the scattering matrix algorithm is nothing else than a clever version of a domain decomposition method.
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/

2024/03/12

Tuesday Seminar of Analysis

16:00-17:30   Room #123 (Graduate School of Math. Sci. Bldg.)
Kobe Marshall-Stevens (University College London)
On the generic regularity of min-max CMC hypersurfaces (English)
[ Abstract ]
Smooth constant mean curvature (CMC) hypersurfaces serve as effective tools to study the geometry and topology of Riemannian manifolds. In high dimensions however, one in general must account for their singular behaviour. I will discuss how such hypersurfaces are constructed via min-max techniques and some recent progress on their generic regularity, allowing for certain isolated singularities to be perturbed away.
[ Reference URL ]
https://forms.gle/7mqzgLqhtBuAovKB8

2024/03/11

FJ-LMI Seminar

13:30-14:30   Room #117 (Graduate School of Math. Sci. Bldg.)
Florian SALIN (Université de Lyon - 東北大学)
Fractional Nonlinear Diffusion Equation: Numerical Analysis and Large-time Behavior. (英語)
[ Abstract ]
This talk will discuss a fractional nonlinear diffusion equation on bounded domains. This equation arises by combining fractional (in space) diffusion, with a nonlinearity of porous medium or fast diffusion type. It is known that, in the porous medium case, the energy of the solutions to this equation decays algebraically, and in the fast diffusion case, solutions extinct in finite time. Based on these estimates, we will study the fine large-time asymptotic behavior of the solutions. In particular, we will show that the solutions approach separate variable solutions as the time converges to infinity in the porous medium case, or as it converges to the extinction time in the fast diffusion case. However, the extinction time is not known analytically, and to compute it, we will introduce a numerical scheme that satisfies the same decay estimates as the continuous equation.
[ Reference URL ]
https://fj-lmi.cnrs.fr/seminars/

2024/02/21

Number Theory Seminar

17:00-18:00   Room #117 (Graduate School of Math. Sci. Bldg.)
Jens Niklas Eberhardt (University of Bonn)
K-motives and Local Langlands (English)
[ Abstract ]
In this talk, we construct a geometric realisation of the category of representations of the affine Hecke algebra. For this, we introduce a formalism of K-theoretic sheaves (called K-motives) on stacks. The affine Hecke algebra arises from the K-theory of the Steinberg stack, and we explain how to “category” using K-motives.
Lastly, we briefly discuss the relation to the local Langlands program.

2024/02/13

Tuesday Seminar on Topology

17:00-18:30   Room #ハイブリッド開催/056 (Graduate School of Math. Sci. Bldg.)
Pre-registration required. See our seminar webpage.
Paul Norbury (The University of Melbourne)
Measures on the moduli space of curves and super volumes (ENGLISH)
[ Abstract ]
In this lecture I will define a family of finite measures on the moduli space of smooth curves with marked points. The measures are defined via a construction analogous to that of the Weil-Petersson metric using the extra data of a spin structure. In fact, the measures arise naturally out of the super Weil-Petersson metric defined over the moduli space of super curves. The total measure can be identified with the volume of the moduli space of super curves. It can be calculated in many examples, and conjecturally satisfies a recursion analogous to Mirzakhani's recursion relations between Weil-Petersson volumes of moduli spaces of hyperbolic surfaces. This conjecture has been verified in many cases, including the so-called Neveu-Schwarz case where it coincides with the recursion of Stanford and Witten. The general case produces deformations of the Neveu-Schwarz volume polynomials, satisfying the same Mirzakhani-like recursion relations.
[ Reference URL ]
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html

2024/02/08

Information Mathematics Seminar

16:50-18:35   Room #128 (Graduate School of Math. Sci. Bldg.)
Keita Kanno (QunaSys Inc.)
Current status of quantum computing and its applications (Japanese)
[ Abstract ]
In this seminar, current status and future prospects of quantum computing and its applications will be presented.

2024/02/05

Applied Analysis

16:00-17:30   Room #122 (Graduate School of Math. Sci. Bldg.)
Reinhard Farwig (Technische Universität Darmstadt)
Viscous Flow in Domains with Moving Boundaries - From Bounded to Unbounded Domains (English)
[ Abstract ]
https://drive.google.com/file/d/1dJJU1ybE-n8yn3LZTReTeH2UFX9wXQv9/view?usp=drive_link
[ Reference URL ]
https://forms.gle/xKPKu1uw9PeHEEck9

Tokyo Probability Seminar

17:00-18:30   Room #126 (Graduate School of Math. Sci. Bldg.)
Sunder Sethuraman (University of Arizona)
Atypical behaviors of a tagged particle in asymmetric simple exclusion (English)
[ Abstract ]
Informally, the one dimensional asymmetric simple exclusion process follows a collection of continuous time random walks on Z interacting as follows: When a clock rings, the particle jumps to the nearest right or left with probabilities p or q=1-p, if that location is unoccupied. If occupied, the jump is suppressed and clocks start again.

In this system, seen as a toy model of `traffic', the motion of a distinguished or `tagged' particle is of interest. Starting from a stationary state, we study the `typical' behavior of a tagged particle, conditioned to deviate to an `atypical' position at time Nt, for a t>0 fixed. In the course of results, an `upper tail' large deviation principle, in scale N, is established for the position of the tagged particle. Also, with respect to `lower tail' events, in the totally asymmetric version, a connection is made with a `nonentropy' solution of the associated hydrodynamic Burgers equation. This is work with S.R.S. Varadhan (arXiv:2311.0780).

2024/01/30

FJ-LMI Seminar

16:30-17:30   Room # (Graduate School of Math. Sci. Bldg.)
Danielle HILHORST (CNRS, Université de Paris-Saclay, France)
Convergence of solutions of a one-phase Stefan problem with Neumann boundary data to a self-similar profile (英語)
[ Abstract ]
We study a one-dimensional one-phase Stefan problem with a Neumann boundary condition on the fixed part of the boundary.
We construct a unique self-similar solution and show that for a large class of initial data, the solution of the time evolution problem
converges to this self-similar solution as time tends to infinity. Similar results were already obtained by Bouguezzi, Hilhorst,
Miyamoto, and Scheid in the case of Dirichlet data on the fixed boundary. However, they had to show that the space derivative
of the solution uniformly converges to its limit. Here, our proof requires less regularity, which should make our arguments easier
to adapt to different settings.

This is a joint work with Sabrina Roscani and Piotr Rybka.
[ Reference URL ]
https://fj-lmi.cnrs.fr/seminars/

Applied Analysis

16:30-17:30   Room #002 (Graduate School of Math. Sci. Bldg.)
Danielle Hilhorst (CNRS / Université de Paris-Saclay)
Convergence of solutions of a one-phase Stefan problem with Neumann boundary data to a self-similar profile. (English)
[ Abstract ]
We study a one-dimensional one-phase Stefan problem with a Neumann boundary condition on the fixed part of the boundary.
We construct a unique self-similar solution and show that for a large class of initial data, the solution of the time evolution problem converges to this self-similar solution as time tends to infinity. Similar results were already obtained by Bouguezzi, Hilhorst, Miyamoto, and Scheid in the case of Dirichlet data on the fixed boundary. However, they had to show that the space derivative of the solution uniformly converges to its limit. Here, our proof requires less regularity, which should make our arguments easier to adapt to different settings.
This is a joint work with Sabrina Roscani and Piotr Rybka.

2024/01/26

thesis presentations

9:15-10:30   Room #118 (Graduate School of Math. Sci. Bldg.)
YAMAGUCHI Tatsuki (Graduate School of Mathematical Sciences University of Tokyo)
Studies on F-singularities in equal characteristic zero via ultraproducts
(超積を用いた等標数0におけるF-特異点の研究)

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