Seminar information archive

Seminar information archive ~05/28Today's seminar 05/29 | Future seminars 05/30~

2023/06/28

Algebraic Geometry Seminar

13:30-15:00   Room #ハイブリッド開催/056 (Graduate School of Math. Sci. Bldg.)
Yohsuke Matsuzawa (Osaka Metropolitan University)
Preimages question and dynamical cancellation
[ Abstract ]
Pulling back an invariant subvariety by a self-morphism on projective variety, you will get a tower of increasing closed subsets. Working over a number field, we expect that the set of rational points contained in this increasing subsets eventually stabilizes. I am planning to discuss several results on this problem, such as the case of etale morphisms, morphisms on the product of two P^1. I will also present some counter examples that occur when we drop some of the assumptions. This work is based on a joint work with Matt Satriano and Jason Bell, and recent work in progress with Kaoru Sano.

Number Theory Seminar

17:00-18:00   Room #117 (Graduate School of Math. Sci. Bldg.)
Yuta Nakayama (University of Tokyo)
The integral models of the RSZ Shimura varieties (日本語)
[ Abstract ]
We prove that the integral models of Shimura varieties by Rapoport, Smithling and Zhang proposed to describe variants of the arithmetic Gan–Gross–Prasad conjecture are isomorphic to the models by Pappas and Rapoport. This extends our previous work that compares the former models and the Kisin–Pappas models. We rely on the construction of the models of Pappas and Rapoport, not on their characterization.

2023/06/27

Numerical Analysis Seminar

16:30-18:00   Room #002 (Graduate School of Math. Sci. Bldg.)
Toshihiro Yamada (Hitotsubashi University)
Solving high-dimensional partial differential equations via deep learning and probabilistic methods (Japanese)
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/

2023/06/26

Tokyo Probability Seminar

17:00-18:30   Room #126 (Graduate School of Math. Sci. Bldg.)
簗島 瞬 (東京都立大学)
δ次元Bessel引越過程の構成方法,サンプルパス生成方法,および汎関数期待値の数値計算法について (日本語)

Seminar on Geometric Complex Analysis

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Masataka IWAI (Osaka Univeristy)
Miyaoka type inequality for terminal weak Fano varieties
[ Abstract ]
In this talk, we show that $c_2(X)c_1(X)^{n-2}$ is positive for any $n$-dimensional terminal weak Fano varieties $X$. As a corollary, we obtain some inequalities (Miyaoka type inequalities) with respect to $c_2(X)c_1(X)^{n-2}$ and $c_1(X)^{n}$. This is joint work with Chen Jiang and Haidong Liu (arXiv:2303.00268).
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/06/23

Algebraic Geometry Seminar

13:30-15:00   Room #ハイブリッド開催/117 (Graduate School of Math. Sci. Bldg.)
Kohsuke Shibata (Tokyo Denki University)
Minimal log discrepnacies for quotient singularities
[ Abstract ]
In this talk, I will discuss recent joint work with Yusuke Nakamura on minimal log discrepancies for quotient singularities. The minimal log discrepancy is an important invariant of singularities in birational geometry. The denominator of the minimal log discrepancy of a variety depends on the Gorenstein index. On the other hand, Shokurov conjectured that the Gorenstein index of a Q-Gorenstein germ can be bounded in terms of its dimension and minimal log discrepancy. In this talk, I will explain basic properties for quotient singularities and show Shokurov's index conjecture for quotient singularities.

2023/06/22

Applied Analysis

16:00-17:30   Room #126 (Graduate School of Math. Sci. Bldg.)
Jiwoong Jang (University of Wisconsin Madison)
Convergence rate of periodic homogenization of forced mean curvature flow of graphs in the laminar setting (English)
[ Abstract ]
Mean curvature flow with a forcing term models the motion of a phase boundary through media with defects and heterogeneities. When the environment shows periodic patterns with small oscillations, an averaged behavior is exhibited as we zoom out, and providing mathematical treatment for the behavior has received a great attention recently. In this talk, we discuss the periodic homogenization of forced mean curvature flows, and we give a quantitative analysis for the flow starting from an entire graph in a laminated environment.
[ Reference URL ]
https://forms.gle/BTuFtcmUVnvCLieX9

Information Mathematics Seminar

16:50-18:35   Room #128 (Graduate School of Math. Sci. Bldg.)
Tatsuaki Okamoto (NTT)
Lattice-based cryptography (Japanese)
[ Abstract ]
Explanation of lattice-based cryptography

2023/06/21

Number Theory Seminar

17:00-18:00   Room #117 (Graduate School of Math. Sci. Bldg.)
Stefan Reppen (Stockholm University)
On moduli of principal bundles under non-connected reductive groups (英語)
[ Abstract ]
Let $C$ be a smooth, connected projective curve over an algebraically closed field $k$ of characteristic 0, and let $G$ be a non-connected reductive group over $k$. I will explain how to decompose the stack of $G$-bundles $\text{Bun}_G$ into open and closed substacks $X_i$ which admits finite torsors $\text{Bun}_{\mathcal{G}_i} \to X_i$, for some connected reductive group schemes $\mathcal{G}_i$ over $C$. I explain how to use this to obtain a projective good moduli space of semistable $G$-bundles over $C$, for a suitable notion of semistability. Finally, after stating a result concerning finite subgroups of connected reductive groups over $k$, I explain how to see that essentially finite $H$-bundles are not dense in the moduli space of semistable degree 0 $H$-bundles, for any connected reductive group $H$ not equal to a torus.

2023/06/20

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.
Arnaud Maret (Sorbonne Université)
Moduli spaces of triangle chains (ENGLISH)
[ Abstract ]
In this talk, I will describe a moduli space of triangle chains in the hyperbolic plane with prescribed angles. We will relate this moduli space to a specific character variety of representations of surface groups into PSL(2,R). This identification provides action-angle coordinates for the Goldman symplectic form on the character variety. If time permits, I will explain why the mapping class group action on that particular character variety is ergodic.
[ Reference URL ]
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html

2023/06/19

Seminar on Geometric Complex Analysis

10:30-12:00   Room #128 (Graduate School of Math. Sci. Bldg.)
Nobuhiro Honda (Tokyo University of Technology)
A new construction method for 3-dimensional indefinite Zoll manifolds
[ Abstract ]
The Penrose correspondence gives correspondences between special geometric structures on manifolds and complex manifolds, one of which is between Einstein-Weyl structures on 3-manifolds and complex surfaces. The latter complex surfaces are called mini-Twister spaces. In this talk, I will show that compact mini-Zeister spaces can be constructed in a natural way from hyperelliptic curves of arbitrary species, and that the resulting 3-manifolds have a remarkable geometric property called the Zoll property, which means that all geodesics are closed. A typical example is a sphere. The three-dimensional Einstein-Weyl manifold obtained in this study is indefinite, and the geodesics considered are spatial. These Einstein-Weyl manifolds can be regarded as generalizations of those given in arXiv:2208.13567.

Translated with www.DeepL.com/Translator (free version)
[ Reference URL ]
https://u-tokyo-ac-jp.zoom.us/meeting/register/tZEqceqsrTIjEtRxenSMdPogvCxlWzAogj5A

2023/06/15

Information Mathematics Seminar

16:50-18:35   Room #128 (Graduate School of Math. Sci. Bldg.)
Tatsuaki Okamoto (NTT)
Cryptosystems based on elliptic curves (Japanese)
[ Abstract ]
Explanation of crypto-systems based on elliptic curves

2023/06/14

Algebraic Geometry Seminar

14:00-15:30   Room #ハイブリッド開催/056 (Graduate School of Math. Sci. Bldg.)
Wenliang Zhang (University of Illinois Chicago)
Vanishing of local cohomology modules
[ Abstract ]
Studying the vanishing of local cohomology modules has a long and rich history, and is still an active research area. In this talk, we will discuss classic theorems (due to Grothendieck, Hartshorne, Peskine-Szpiro, and Ogus), recent developments, and some open problems.

2023/06/13

Operator Algebra Seminars

16:45-18:15   Room #126 (Graduate School of Math. Sci. Bldg.)
Kan Kitamura (Univ. Tokyo)
Around homogeneous spaces of complex semisimple quantum groups
[ Reference URL ]
https://www.ms.u-tokyo.ac.jp/~yasuyuki/tokyo-seminar.htm

Tuesday Seminar on Topology

17:00-18:00   Online
Pre-registration required. See our seminar webpage.
Shunsuke Usuki (Kyoto University)
On a lower bound of the number of integers in Littlewood's conjecture (JAPANESE)
[ Abstract ]
Littlewood's conjecture is a famous and long-standing open problem on simultaneous Diophantine approximation. It is closely related to the action of diagonal matrices on ${\rm SL}(3,\mathbb{R})/{\rm SL}(3,\mathbb{Z})$, and M. Einsiedler, A. Katok and E. Lindenstrauss showed in 2000's that the exceptional set for Littlewood's conjecture has Hausdorff dimension zero by using some rigidity for invariant measures under the diagonal action. In this talk, I explain that we can obtain some quantitative result on the result of Einsiedler, Katok and Lindenstrauss by studying the empirical measures with respect to the diagonal action.
[ Reference URL ]
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html

Lie Groups and Representation Theory

17:00-18:00   Room #online (Graduate School of Math. Sci. Bldg.)
Yoshiki Oshima (The University of Tokyo)
Examples of discrete branching laws of derived functor modules (Japanese)
[ Abstract ]
We consider the restriction of Zuckerman's derived functor modules for symmetric pairs of real reductive groups assuming that it is discretely decomposable in the sense of Kobayashi. By using a classification result, it can be shown that the restriction decomposes as a direct sum of Zuckerman's derived functor modules for the subgroup. In the last talk, by using the realization of representations as D-modules, a decomposition of Zuckerman's modules corresponding to an orbit decomposition of flag varieties was explained. In this talk, we would like to see that such a decomposition can be written as a direct sum of Zuckerman's modules of the subgroup in some concrete examples.

2023/06/08

Information Mathematics Seminar

16:50-18:35   Room #128 (Graduate School of Math. Sci. Bldg.)
Tatsuaki Okamoto (NTT)
Security, construction and proof of digital signatures (Japanese)
[ Abstract ]
Explanation of security, construction and proof of digital signatures

Discrete mathematical modelling seminar

18:15-19:15   Online
This seminar will be held using Zoom. If you wish to participate, please contact R. Willox by email.
Andy Hone (University of Kent)
Deformations of Zamolodchikov periodicity, discrete integrability and the Laurent property (English)
[ Abstract ]
Zamolodchikov periodicity was a conjectured property of Y-systems observed in exactly solvable models of quantum field theory associated with simple Lie algebras. The advent of Fomin & Zelevinsky's theory of cluster algebras provided an ideal mathematical framework for proving and formulating extensions of this property. Recently we have found a family of birational maps which deforms the periodic dynamics observed by Zamolodchikov, and destroys the Laurent property that is an inherent feature of cluster dynamics, but still preserves integrability. In this talk we present a variety of examples of deformed integrable maps in types A, B & D, and show how to restore the Laurent property in higher dimensions. This is the combined result of joint work with Grabowski, Kouloukas, Kim and Mase.

2023/06/07

Algebraic Geometry Seminar

13:30-15:00   Room #ハイブリッド開催/056 (Graduate School of Math. Sci. Bldg.)
Fuetaro Yobuko (Nagoya University)
Quasi-F-splitting and Hodge-Witt
[ Abstract ]
Quasi-F-splitting is an extension of F-splitting, which is defined for schemes in positive characteristic. On the other hand, Hodge-Wittness is defined for smooth proper schemes over a perfect field using the de Rham-Witt complex and ordinarity implies Hodge-Wittness. In this talk, I will explain (unexpected) relations between F-split/quasi-F-split and ordinary/Hodge-Witt via examples and properties.

Number Theory Seminar

17:00-18:00   Room #117 (Graduate School of Math. Sci. Bldg.)
Hirofumi Yamamoto (The University of Tokyo)
On the dimension of spaces of p-ordinary half-integral weight Siegel modular forms
of degree 2 (日本語)
[ Abstract ]
Hecke eigenforms whose eigenvalues at $p$ are $p$-adic units are called $p$-ordinary. The dimension of the spaces spanned by $p$-ordinary Siegel modular eigenforms is known to be bounded regardless of the power of $p$ of levels and weights. Furthermore, Hida showed that the degree is bounded in the case of half-integral weight modular forms, using the Shimura correspondence. In this talk, I will explain that a similar result holds for half-integral weight $p$-ordinary Siegel modular forms of degree 2. Let $F$ be a $p$-ordinary Hecke eigen cuspform, and $\pi_F$ be the corresponding cuspidal representation of $Mp_4(\mathbb{A}_\mathbb{Q})$. Then the weight of $F$ determines the Hecke eigenvalue of $\pi_F$ at $p$. Therefore, we can relate $F$ to $p$-ordinary integral weight Siegel modular forms or elliptic modular forms by using the local theta correspondence and the method of Ishimoto(Ibukiyama conjecture).

2023/06/06

Operator Algebra Seminars

16:45-18:15   Room #126 (Graduate School of Math. Sci. Bldg.)
Maria Stella Adamo (Univ. Tokyo)
Wightman fields and their construction for a class of 2D CFTs
[ Reference URL ]
https://www.ms.u-tokyo.ac.jp/~yasuyuki/tokyo-seminar.htm

Numerical Analysis Seminar

16:30-18:00   Room #002 (Graduate School of Math. Sci. Bldg.)
Hideyuki Azegami (Nagoya Industrial Science Research Institute)
Relation between regularity and numerical solutions of shape optimization problems
(Japanese)
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/

Tuesday Seminar of Analysis

17:00-18:30   Room #128 (Graduate School of Math. Sci. Bldg.)
Erik Skibsted (Aarhus University)
Stationary completeness; the many-body short-range case (English)
[ Abstract ]
For a general class of many-body Schr\"odinger operators with short-range pair-potentials the wave and scattering matrices as well as the restricted wave operators are all defined at any non-threshold energy. In fact this holds without imposing any a priori decay condition on channel eigenstates and even for models including long-range potentials of Derezi\'nski-Enss type. For short-range models we improve on the known \emph{weak continuity} statements in that we show that all non-threshold energies are \emph{stationary complete}, resolving in this case a recent conjecture. A consequence is that the above scattering quantities depend \emph{strongly continuously} on the energy parameter at all non-threshold energies (whence not only almost everywhere as previously demonstrated). Another consequence is that the scattering matrix is unitary at any such energy. Our procedure yields (as a side result) a new and purely stationary proof of asymptotic completeness for many-body short-range systems.
[ Reference URL ]
https://forms.gle/kWHDfb6J6kcjfSah8

Lie Groups and Representation Theory

17:30-18:30   Room #056 (Graduate School of Math. Sci. Bldg.)
Joint with Tuesday Seminar on Topology
Atsumu Sasaki (Tokai University)
Visible actions on reductive spherical homogeneous spaces and their invariant measures
(Japanese)
[ Abstract ]
Toshiyuki Kobayashi has established propagation theorem of multiplicity-freeness property.
This theorem enables us to give an unified explanation of multiplicity-freeness of multiplicity-free representations which have been found so far, and also to find new examples of multiplicity-free representations systematically. Kobayashi further has introduced the notion of visible actions on complex manifolds as a basic condition for propagation theorem of multiplicity-freeness property. This notion plays an important role to this theorem and also brings us to find various decomposition theorems of Lie groups and homogeneous spaces.

In this talk, we explain visible actions on reductive spherical homogeneous spaces. In particular, we see that for a visible action on reductive spherical homogeneous space our construction of a submanifold which meets every orbit (called a slice) is given by an explicit description of a Cartan decomposition for this space. As a corollary of this study, we characterize the invariant measure on a reductive spherical homogeneous space by giving an integral formula for a Cartan decomposition explicitly.

Tuesday Seminar on Topology

17:30-18:30   Room #ハイブリッド開催/056 (Graduate School of Math. Sci. Bldg.)
Joint with Lie Groups and Representation Theory Seminar. See our seminar webpage.
Atsumu Sasaki (Tokai University)
Visible actions on reductive spherical homogeneous spaces and their invariant measures (JAPANESE)
[ Abstract ]
Toshiyuki Kobayashi has established propagation theorem of multiplicity-freeness property. This theorem enables us to give an unified explanation of multiplicity-freeness of multiplicity-free representations which have been found so far, and also to find new examples of multiplicity-free representations systematically. Kobayashi further has introduced the notion of visible actions on complex manifolds as a basic condition for propagation theorem of multiplicity-freeness property. This notion plays an important role to this theorem and also brings us to find various decomposition theorems of Lie groups and homogeneous spaces.

In this talk, we explain visible actions on reductive spherical homogeneous spaces. In particular, we see that for a visible action on reductive spherical homogeneous space our construction of a submanifold which meets every orbit (called a slice) is given by an explicit description of a Cartan decomposition for this space. As a corollary of this study, we characterize the invariant measure on a reductive spherical homogeneous space by giving an integral formula for a Cartan decomposition explicitly.
[ Reference URL ]
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html

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