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Seminar information archive
Seminar information archive ～01/13｜Today's seminar 01/14 | Future seminars 01/15～
2025/11/04
Tuesday Seminar on Topology
17:00-18:00 Online
Pre-registration required. See our seminar webpage.
Kazuto Takao (Tohoku University)
Diagrammatic criteria for strong irreducibility of Heegaard splittings and finiteness of Goeritz groups (JAPANESE)
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Pre-registration required. See our seminar webpage.
Kazuto Takao (Tohoku University)
Diagrammatic criteria for strong irreducibility of Heegaard splittings and finiteness of Goeritz groups (JAPANESE)
[ Abstract ]
Casson-Gordon gave a criterion for Heegaard splittings of 3-manifolds to be strongly irreducible. By strengthening it, Lustig-Moriah gave a criterion for Goeritz groups of Heegaard splittings to be finite. Their criteria are based on Heegaard diagrams formed by maximal disk systems of the handlebodies. We generalize them for arbitrary disk systems, including minimal ones. As an application, we give Heegaard splittings with non-minimal genera and finite Goeritz groups. This is based on joint work with Yuya Koda.
[ Reference URL ]Casson-Gordon gave a criterion for Heegaard splittings of 3-manifolds to be strongly irreducible. By strengthening it, Lustig-Moriah gave a criterion for Goeritz groups of Heegaard splittings to be finite. Their criteria are based on Heegaard diagrams formed by maximal disk systems of the handlebodies. We generalize them for arbitrary disk systems, including minimal ones. As an application, we give Heegaard splittings with non-minimal genera and finite Goeritz groups. This is based on joint work with Yuya Koda.
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Number Theory Seminar
17:00-18:00 Room #117 (Graduate School of Math. Sci. Bldg.)
Ryomei Iwasa (University of Copenhagen)
Descent and pro-excision
Ryomei Iwasa (University of Copenhagen)
Descent and pro-excision
[ Abstract ]
The theme of this talk is descent and excision of cohomology theories of schemes. We will start with a discussion of the canonical topology on spectral schemes. Unlike on classical schemes, this topology includes many other types of covers, such as h-covers. Then I will explain that THH and TC satisfy descent with respect to the canonical topology, which generalizes the flat descent by Bhatt—Morrow—Scholze. This in turn implies the cdh descent of K-theory on spectral schemes, despite its failure on classical schemes. Furthermore, this implies the cdh pro-excision of K-theory on spectral schemes, which generalizes the derived case by Kelly—Saito—Tamme (the original noetherian case is due to Kerz—Strunk—Tamme). Our proof of the cdh pro-excision is quite different from the previous ones and is more algebraic in nature. The results presented here are based on discussions with Antieau, Burklund, and Krause.
The theme of this talk is descent and excision of cohomology theories of schemes. We will start with a discussion of the canonical topology on spectral schemes. Unlike on classical schemes, this topology includes many other types of covers, such as h-covers. Then I will explain that THH and TC satisfy descent with respect to the canonical topology, which generalizes the flat descent by Bhatt—Morrow—Scholze. This in turn implies the cdh descent of K-theory on spectral schemes, despite its failure on classical schemes. Furthermore, this implies the cdh pro-excision of K-theory on spectral schemes, which generalizes the derived case by Kelly—Saito—Tamme (the original noetherian case is due to Kerz—Strunk—Tamme). Our proof of the cdh pro-excision is quite different from the previous ones and is more algebraic in nature. The results presented here are based on discussions with Antieau, Burklund, and Krause.
Operator Algebra Seminars
16:45-18:15 Room #126 (Graduate School of Math. Sci. Bldg.)
Jesse Reimann (TU Delft)
Split exact sequences and KK-equivalences of quantum flag manifolds
[ Reference URL ]
https://www.ms.u-tokyo.ac.jp/~yasuyuki/tokyo-seminar.htm
Jesse Reimann (TU Delft)
Split exact sequences and KK-equivalences of quantum flag manifolds
[ Reference URL ]
https://www.ms.u-tokyo.ac.jp/~yasuyuki/tokyo-seminar.htm
2025/10/31
Algebraic Geometry Seminar
13:00-14:30 Room #118 (Graduate School of Math. Sci. Bldg.)
Miguel Angel Barja (UPC-Barcelona)
Asymptotic and continuous constructions in the geography of fibred varieties
Miguel Angel Barja (UPC-Barcelona)
Asymptotic and continuous constructions in the geography of fibred varieties
[ Abstract ]
Given a fibred variety $X$ onto a smooth variety $T$ it is possible to consider different types of inequalities between birational invariants associated to a line bundle $L$, such as Noether, Slope or Severi inequalities. Most of these inequalities are closely related through asymptotic constructions and/or continuous functions that suggest the use of some new invariants. We will survey different constructions both in characteristic 0 and positive characteristic, and will focus in the case of varieties of maximal Albanese dimension, fibred over curves. If time permits, we will also give some ideas on fibrations over surfaces.
Given a fibred variety $X$ onto a smooth variety $T$ it is possible to consider different types of inequalities between birational invariants associated to a line bundle $L$, such as Noether, Slope or Severi inequalities. Most of these inequalities are closely related through asymptotic constructions and/or continuous functions that suggest the use of some new invariants. We will survey different constructions both in characteristic 0 and positive characteristic, and will focus in the case of varieties of maximal Albanese dimension, fibred over curves. If time permits, we will also give some ideas on fibrations over surfaces.
Algebraic Geometry Seminar
15:00-16:30 Room #118 (Graduate School of Math. Sci. Bldg.)
Masafumi Hattori (University of Nottingham)
Normal stable degeneration of Noether-Horikawa surfaces: Deformation Part
Masafumi Hattori (University of Nottingham)
Normal stable degeneration of Noether-Horikawa surfaces: Deformation Part
[ Abstract ]
Koll’ar and Shepherd-Barron constructed a general theory for a canonical geometric compactification of moduli of smooth surfaces with ample canonical class by adding degenerations with only semi log canonical singularities. Their moduli is now called the KSBA moduli and degenerations are called stable degenerations. It has been a long standing question to classify all stable degenerations for smooth canonically polarized surfaces. In this talk, we focus on Q-Gorenstein deformation theory on Horikawa surfaces, which are minimal surfaces of general type in the case where the Noether inequality $K^2\geq 2p_g-4$ is an equality. This talk is based on the joint work (arXiv:2507:17633) with Hiroto Akaike, Makoto Enokizono, and Yuki Koto.
Koll’ar and Shepherd-Barron constructed a general theory for a canonical geometric compactification of moduli of smooth surfaces with ample canonical class by adding degenerations with only semi log canonical singularities. Their moduli is now called the KSBA moduli and degenerations are called stable degenerations. It has been a long standing question to classify all stable degenerations for smooth canonically polarized surfaces. In this talk, we focus on Q-Gorenstein deformation theory on Horikawa surfaces, which are minimal surfaces of general type in the case where the Noether inequality $K^2\geq 2p_g-4$ is an equality. This talk is based on the joint work (arXiv:2507:17633) with Hiroto Akaike, Makoto Enokizono, and Yuki Koto.
2025/10/29
Geometric Analysis Seminar
13:30-14:30 Room #126 (Graduate School of Math. Sci. Bldg.)
Tommaso Rossi (Scuola Internazionale Superiore di Studi Avanzati)
On the rectifiability of metric measure spaces with lower Ricci curvature bounds (英語)
https://sites.google.com/view/tommasorossi/home-page
Tommaso Rossi (Scuola Internazionale Superiore di Studi Avanzati)
On the rectifiability of metric measure spaces with lower Ricci curvature bounds (英語)
[ Abstract ]
Given a metric measure space (X,d,m), the curvature-dimension condition CD(K,N), and the measure contraction property MCP(K,N), are synthetic notions of having Ricci curvature bounded below by K (and dimension bounded above by N). We prove some rectifiability results for CD(K,N) and MCP(K,N) metric measure spaces (X,d,m) with pointwise Ahlfors regular reference measure m and with m-almost everywhere unique metric tangents. Our strategy is based on the failure of the CD condition in sub-Finsler Carnot groups, on a new result on the failure of the non-collapsed MCP on sub-Finsler Carnot groups, and on a recent breakthrough by D. Bate. This is a joint work with M. Magnabosco and A. Mondino.
[ Reference URL ]Given a metric measure space (X,d,m), the curvature-dimension condition CD(K,N), and the measure contraction property MCP(K,N), are synthetic notions of having Ricci curvature bounded below by K (and dimension bounded above by N). We prove some rectifiability results for CD(K,N) and MCP(K,N) metric measure spaces (X,d,m) with pointwise Ahlfors regular reference measure m and with m-almost everywhere unique metric tangents. Our strategy is based on the failure of the CD condition in sub-Finsler Carnot groups, on a new result on the failure of the non-collapsed MCP on sub-Finsler Carnot groups, and on a recent breakthrough by D. Bate. This is a joint work with M. Magnabosco and A. Mondino.
https://sites.google.com/view/tommasorossi/home-page
2025/10/28
Tuesday Seminar on Topology
17:00-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)
Pre-registration required. See our seminar webpage.
Ayumu Inoue (Tsuda University)
On a relationship between quandle homology and relative group homology, from the view point of Seifert surfaces (JAPANESE)
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Pre-registration required. See our seminar webpage.
Ayumu Inoue (Tsuda University)
On a relationship between quandle homology and relative group homology, from the view point of Seifert surfaces (JAPANESE)
[ Abstract ]
Quandles and their homology are known to have good chemistry with knot theory. Associated with a triple of a group G, its automorphism, and its subgroup H satisfying a certain condition, we have a quandle. In this talk, we see that we have a chain map from the quandle chain complex of the quandle to the (Adamson/Hochschild) relative group chain complex of (G, H). We also see that this chain map has good chemistry with a triangulation of Seifert surface of a knot.
[ Reference URL ]Quandles and their homology are known to have good chemistry with knot theory. Associated with a triple of a group G, its automorphism, and its subgroup H satisfying a certain condition, we have a quandle. In this talk, we see that we have a chain map from the quandle chain complex of the quandle to the (Adamson/Hochschild) relative group chain complex of (G, H). We also see that this chain map has good chemistry with a triangulation of Seifert surface of a knot.
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Tuesday Seminar of Analysis
15:00-17:30 Room #002 (Graduate School of Math. Sci. Bldg.)
Lauri Särkiö (Aalto University) 15:00-16:00
Gradient higher integrability of parabolic double-phase equations (English)
Global well-posedness for 3D quadratic nonlinear Schrödinger equations (Japanese)
Lauri Särkiö (Aalto University) 15:00-16:00
Gradient higher integrability of parabolic double-phase equations (English)
[ Abstract ]
Elliptic double-phase problems have been studied extensively in the last decade since a series of results by Mingione and others. Recently several regularity results have been obtained also for parabolic double-phase equations, yet many questions remain unsolved. In this talk, we focus on gradient higher integrability, showing that solutions to parabolic double-phase equations belong to a slightly higher Sobolev class than assumed a priori. The talk is based on joint work with Wontae Kim, Juha Kinnunen and Kristian Moring.
Shinya Kinoshita (Nagoya University) 16:30-17:30Elliptic double-phase problems have been studied extensively in the last decade since a series of results by Mingione and others. Recently several regularity results have been obtained also for parabolic double-phase equations, yet many questions remain unsolved. In this talk, we focus on gradient higher integrability, showing that solutions to parabolic double-phase equations belong to a slightly higher Sobolev class than assumed a priori. The talk is based on joint work with Wontae Kim, Juha Kinnunen and Kristian Moring.
Global well-posedness for 3D quadratic nonlinear Schrödinger equations (Japanese)
[ Abstract ]
In this talk, we consider the Cauchy problem of the 3D nonlinear Schrödinger equations. It is known that if the nonlinearity is homogeneous of degree $p >2$, the general theory would provide the small data global existence of 3D NLS. In the quadratic case, which can be seen as a threshold of the small data global existence, the structure of nonlinearity plays a role and more sophisticated analysis is required. The aim in this talk is to show the global well-posedness in the scaling critical space with an additional angular regularity. The proof is based on the Fourier restriction norm method combined with several linear and bilinear estimates for the linear solutions.
In this talk, we consider the Cauchy problem of the 3D nonlinear Schrödinger equations. It is known that if the nonlinearity is homogeneous of degree $p >2$, the general theory would provide the small data global existence of 3D NLS. In the quadratic case, which can be seen as a threshold of the small data global existence, the structure of nonlinearity plays a role and more sophisticated analysis is required. The aim in this talk is to show the global well-posedness in the scaling critical space with an additional angular regularity. The proof is based on the Fourier restriction norm method combined with several linear and bilinear estimates for the linear solutions.
2025/10/27
Seminar on Geometric Complex Analysis
10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)
Shohei Ma (Institute of Science Tokyo)
. (Japanese)
[ Reference URL ]
https://forms.gle/gTP8qNZwPyQyxjTj8
Shohei Ma (Institute of Science Tokyo)
. (Japanese)
[ Reference URL ]
https://forms.gle/gTP8qNZwPyQyxjTj8
2025/10/21
FJ-LMI Seminar
16:00-16:40 Room #128 (Graduate School of Math. Sci. Bldg.)
Ramla ABDELLATIF (Université de Picardie)
Studying $p$-modular representations of $p$-adic groups in the setting of the Langlands programme (英語)
Ramla ABDELLATIF (Université de Picardie)
Studying $p$-modular representations of $p$-adic groups in the setting of the Langlands programme (英語)
[ Abstract ]
This talk aims to introduce the context of my primary research topic, namely $p$-modular representations of $p$-adic groups, as well as a current state of the art in the field, including some related questions I am currently exploring. After motivating the study of classical and modular Langlands correspondences for $p$-adic groups, I will explain why the $p$-modular setting (i.e. when representations of $p$-adic groups have coefficients in a field of positive characteristic equal to $p$) differs significantly from other settings (namely the complex and $\ell$-modular ones, with $\ell$ a prime distinct from $p$), then I will present the main results known so far about $p$-modular irreducible smooth representations of $p$-adic groups, with a particular focus on the special linear group $\mathrm{SL}_{2}$.
This talk aims to introduce the context of my primary research topic, namely $p$-modular representations of $p$-adic groups, as well as a current state of the art in the field, including some related questions I am currently exploring. After motivating the study of classical and modular Langlands correspondences for $p$-adic groups, I will explain why the $p$-modular setting (i.e. when representations of $p$-adic groups have coefficients in a field of positive characteristic equal to $p$) differs significantly from other settings (namely the complex and $\ell$-modular ones, with $\ell$ a prime distinct from $p$), then I will present the main results known so far about $p$-modular irreducible smooth representations of $p$-adic groups, with a particular focus on the special linear group $\mathrm{SL}_{2}$.
2025/10/20
Seminar on Geometric Complex Analysis
10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)
Hideyuki Ishi (Osaka Metropolitan Univ.)
A CR-Laplacian type operator for the Silov boundary of a homogeneous Siegel domain (Japanese)
https://forms.gle/gTP8qNZwPyQyxjTj8
Hideyuki Ishi (Osaka Metropolitan Univ.)
A CR-Laplacian type operator for the Silov boundary of a homogeneous Siegel domain (Japanese)
[ Abstract ]
Let $\Sigma$ be the Silov boundary of a homogeneous Siegel domain $D$ on which a Lie group $G$ acts transitively as affine transformations. The CR-structure on $\Sigma$ naturally induced from the ambient complex vector space is non-trivial if and only if $D$ is of non-tube type. In this case, $\Sigma$ is naturally identified with a two-step nilpotent Lie subgroup $N$ of $G$, called a generalized Heisenberg Lie group. Since the CR-structure is invariant under the action of $G$, the CR-cohomology space over $\Sigma$ can be regarded as a $G$-module. We consider unitarization of this presentation of $G$. The kernel of the CR-Laplacian does not give the solution because the natural Riemannian metric on $\Sigma$ is not $G$-invariant, so that the $G$-action does not preserve the space of CR-harmonic forms. Nevertheless, Nomura defined a unitary $G$-action on the space indirectly when $G$ is split solvable. In this talk, we introduce a space of CR-cochains with $G$-invariant inner product defined via the Fourier transform. Then the associated CR-operator is no longer a differential operator, while the kernel of the operator gives a unitarization of the representation of $G$ over the cohomology space.
[ Reference URL ]Let $\Sigma$ be the Silov boundary of a homogeneous Siegel domain $D$ on which a Lie group $G$ acts transitively as affine transformations. The CR-structure on $\Sigma$ naturally induced from the ambient complex vector space is non-trivial if and only if $D$ is of non-tube type. In this case, $\Sigma$ is naturally identified with a two-step nilpotent Lie subgroup $N$ of $G$, called a generalized Heisenberg Lie group. Since the CR-structure is invariant under the action of $G$, the CR-cohomology space over $\Sigma$ can be regarded as a $G$-module. We consider unitarization of this presentation of $G$. The kernel of the CR-Laplacian does not give the solution because the natural Riemannian metric on $\Sigma$ is not $G$-invariant, so that the $G$-action does not preserve the space of CR-harmonic forms. Nevertheless, Nomura defined a unitary $G$-action on the space indirectly when $G$ is split solvable. In this talk, we introduce a space of CR-cochains with $G$-invariant inner product defined via the Fourier transform. Then the associated CR-operator is no longer a differential operator, while the kernel of the operator gives a unitarization of the representation of $G$ over the cohomology space.
https://forms.gle/gTP8qNZwPyQyxjTj8
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.
Ryo Oizumi (National Institute of Population and Social Security Research)
Fredholm Integral Equations and Eigenstructure: Genealogical Expansions via Non–Hilbert–Schmidt Solutions
We are having teatime from 15:15 in the common room on the second floor. Please join us.
Ryo Oizumi (National Institute of Population and Social Security Research)
Fredholm Integral Equations and Eigenstructure: Genealogical Expansions via Non–Hilbert–Schmidt Solutions
[ Abstract ]
Fredholm integral equations play a central role in describing the long-term behavior of structured population models. In this talk, I present a determinant-free approach that constructs eigenfunctions through genealogical expansions, valid even beyond the Hilbert–Schmidt setting. The expansion is closely related to taboo probabilities in Markov chains, allowing eigenfunctions to be interpreted as cumulative ancestral contributions. As an application, I discuss age-structured branching processes and show how quantities such as expected generation counts and reproduction numbers naturally arise from the eigenvalue problem. This perspective highlights how eigenstructure encodes genealogical memory and opens connections between population dynamics, probability theory, and evolutionary processes.
Fredholm integral equations play a central role in describing the long-term behavior of structured population models. In this talk, I present a determinant-free approach that constructs eigenfunctions through genealogical expansions, valid even beyond the Hilbert–Schmidt setting. The expansion is closely related to taboo probabilities in Markov chains, allowing eigenfunctions to be interpreted as cumulative ancestral contributions. As an application, I discuss age-structured branching processes and show how quantities such as expected generation counts and reproduction numbers naturally arise from the eigenvalue problem. This perspective highlights how eigenstructure encodes genealogical memory and opens connections between population dynamics, probability theory, and evolutionary processes.
Tokyo-Nagoya Algebra Seminar
16:30-18:00 Online
Toshiya Yurikusa (Osaka Metropolitan University)
Finiteness and tameness of Jacobian algebras (Japanese)
https://www.math.nagoya-u.ac.jp/~aaron.chan/TNAseminar.html
Toshiya Yurikusa (Osaka Metropolitan University)
Finiteness and tameness of Jacobian algebras (Japanese)
[ Abstract ]
本講演では、有限次元ヤコビ代数をその表現型の観点から研究し、$E$不変量によって定義される$E$有限性および$E$-tame性と、$g$有限性、$\tau$傾有限性、表現有限性などの他の有限性・tame性の概念との対応について述べる。
まず、これらの性質がクイバーとポテンシャルの変異の下で不変であることを示す。その結果として、有限次元ヤコビ代数$\mathcal{J}(Q,W)$が$E$有限であることは、$g$有限、$\tau$傾有限、表現有限であることと同値であり、この場合には $Q$がDynkin型であることが分かる。この結果は、Demonetの「$E$有限なら$g$有限である」という予想を含む形で成立している。
また、$E$-tame性に関しては、例外的な3つの型を除いて、$g$-tame性および表現tame性と対応することが分かる。本講演は、Mohamad Haerizadeh氏との共同研究に基づくものである。
Zoom ID 829 2845 2592
Password 265160
[ Reference URL ]本講演では、有限次元ヤコビ代数をその表現型の観点から研究し、$E$不変量によって定義される$E$有限性および$E$-tame性と、$g$有限性、$\tau$傾有限性、表現有限性などの他の有限性・tame性の概念との対応について述べる。
まず、これらの性質がクイバーとポテンシャルの変異の下で不変であることを示す。その結果として、有限次元ヤコビ代数$\mathcal{J}(Q,W)$が$E$有限であることは、$g$有限、$\tau$傾有限、表現有限であることと同値であり、この場合には $Q$がDynkin型であることが分かる。この結果は、Demonetの「$E$有限なら$g$有限である」という予想を含む形で成立している。
また、$E$-tame性に関しては、例外的な3つの型を除いて、$g$-tame性および表現tame性と対応することが分かる。本講演は、Mohamad Haerizadeh氏との共同研究に基づくものである。
Zoom ID 829 2845 2592
Password 265160
https://www.math.nagoya-u.ac.jp/~aaron.chan/TNAseminar.html
2025/10/17
Colloquium
15:30-16:30 Room #NISSAY Lecture Hall (大講義室) (Graduate School of Math. Sci. Bldg.)
Kenichi Bannai (Keio University/RIKEN)
Building Inclusive Educational and Research Environments
—Current Situations and Responsibilities—
(日本語)
Kenichi Bannai (Keio University/RIKEN)
Building Inclusive Educational and Research Environments
—Current Situations and Responsibilities—
(日本語)
[ Abstract ]
The speaker has been engaged in artificial intelligence and machine learning research at the RIKEN Center for Advanced Intelligence Project for about ten years, collaborating with researchers from diverse fields. From 2020 to 2024, through serving as a member and later chair of the Gender Equality Committee of the Mathematical Society of Japan, the speaker also became more deeply aware of broader social issues in academia.
Creating inclusive educational and research environments where members from diverse backgrounds can thrive is essential for fostering a vibrant research community. This lecture, together with a panel discussion, aims to provide an opportunity to reflect on current challenges, explore the responsibilities of individuals and institutions, and discuss possible directions toward building better environments.
The speaker has been engaged in artificial intelligence and machine learning research at the RIKEN Center for Advanced Intelligence Project for about ten years, collaborating with researchers from diverse fields. From 2020 to 2024, through serving as a member and later chair of the Gender Equality Committee of the Mathematical Society of Japan, the speaker also became more deeply aware of broader social issues in academia.
Creating inclusive educational and research environments where members from diverse backgrounds can thrive is essential for fostering a vibrant research community. This lecture, together with a panel discussion, aims to provide an opportunity to reflect on current challenges, explore the responsibilities of individuals and institutions, and discuss possible directions toward building better environments.
2025/10/14
Operator Algebra Seminars
16:45-18:15 Room #126 (Graduate School of Math. Sci. Bldg.)
Frank Taipe (IMCA)
Compact quantum ergodic systems arising from planar algebras
[ Reference URL ]
https://www.ms.u-tokyo.ac.jp/~yasuyuki/tokyo-seminar.htm
Frank Taipe (IMCA)
Compact quantum ergodic systems arising from planar algebras
[ Reference URL ]
https://www.ms.u-tokyo.ac.jp/~yasuyuki/tokyo-seminar.htm
Tuesday Seminar on Topology
17:00-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)
Pre-registration required. See our seminar webpage.
Keiji Oguiso (The University of Tokyo)
On K3 surfaces with non-elementary hyperbolic automorphism group (JAPANESE)
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Pre-registration required. See our seminar webpage.
Keiji Oguiso (The University of Tokyo)
On K3 surfaces with non-elementary hyperbolic automorphism group (JAPANESE)
[ Abstract ]
This talk is based on my joint work with Professor Koji Fujiwara (Kyoto University) and Professor Xun Yu (Tianjin University).
Main result of this talk is the finiteness of the Néron-Severi lattices of complex projective K3 surfaces whose automorphism groups are non-elementary hyperbolic, under the assumption that the Picard number greater than or equal to 6 (which is optimal to ensure the finiteness). In this talk, after recalling basic facts and some special nice properties of K3 surfaces, the notion of hyperbolicity of group due to Gromov, and their importance and interest (in our view), I would like to explain first why the non-elementary hyperbolicity of K3 surface automorphism group is the problem of the Néron-Severi lattices and then how one can deduce the above-mentioned finiteness, via a recent important observation by Professors Kikuta and Takatsu (independently) on geometrically finiteness, with a new algebro-geometric study of genus one fibrations on K3 surfaces by us.
[ Reference URL ]This talk is based on my joint work with Professor Koji Fujiwara (Kyoto University) and Professor Xun Yu (Tianjin University).
Main result of this talk is the finiteness of the Néron-Severi lattices of complex projective K3 surfaces whose automorphism groups are non-elementary hyperbolic, under the assumption that the Picard number greater than or equal to 6 (which is optimal to ensure the finiteness). In this talk, after recalling basic facts and some special nice properties of K3 surfaces, the notion of hyperbolicity of group due to Gromov, and their importance and interest (in our view), I would like to explain first why the non-elementary hyperbolicity of K3 surface automorphism group is the problem of the Néron-Severi lattices and then how one can deduce the above-mentioned finiteness, via a recent important observation by Professors Kikuta and Takatsu (independently) on geometrically finiteness, with a new algebro-geometric study of genus one fibrations on K3 surfaces by us.
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Tuesday Seminar of Analysis
16:00-17:30 Room #002 (Graduate School of Math. Sci. Bldg.)
Takashi KAGAYA (Muroran Institute of Technology)
Inverse curvature flow of Legendre curves (Japanese)
Takashi KAGAYA (Muroran Institute of Technology)
Inverse curvature flow of Legendre curves (Japanese)
2025/10/10
Algebraic Geometry Seminar
10:00-11:30 Room #122 (Graduate School of Math. Sci. Bldg.)
Yuri Tschinkel (New York University)
Equivariant birational geometry
Yuri Tschinkel (New York University)
Equivariant birational geometry
[ Abstract ]
I will report on new results and constructions in higher-dimensional birational geometry in presence of actions of finite groups.
I will report on new results and constructions in higher-dimensional birational geometry in presence of actions of finite groups.
2025/10/08
Geometric Analysis Seminar
10:30-11:30 Room #126 (Graduate School of Math. Sci. Bldg.)
Jinpeng Lu (University of Helsinki)
Quantitative stability of Gel'fand's inverse problem (英語)
https://www.mv.helsinki.fi/home/jinpeng/
Jinpeng Lu (University of Helsinki)
Quantitative stability of Gel'fand's inverse problem (英語)
[ Abstract ]
Inverse problems study the determination of the global structure of a space or coefficients of a system from local measurements of solutions to the system. The problems are originally motivated from imaging sciences, where the goal is to deduce the structure of the inaccessible interior of a body from measurements at the exterior. A fundamental inverse problem, Gel'fand's inverse problem, asks to determine the geometry of a Riemannian manifold from local measurements of the heat kernel. In this talk, I will explain how the unique solvability of the classical Gel'fand's inverse problem can be established on manifolds via Tataru's optimal unique continuation theorem for the wave operator. Next, I will discuss our recent works on the uniqueness and stability of the inverse problem for the Gromov-Hausdorff limits of Riemannian manifolds with bounded sectional curvature. This talk is based on joint works with Y. Kurylev, M. Lassas, and T. Yamaguchi.
[ Reference URL ]Inverse problems study the determination of the global structure of a space or coefficients of a system from local measurements of solutions to the system. The problems are originally motivated from imaging sciences, where the goal is to deduce the structure of the inaccessible interior of a body from measurements at the exterior. A fundamental inverse problem, Gel'fand's inverse problem, asks to determine the geometry of a Riemannian manifold from local measurements of the heat kernel. In this talk, I will explain how the unique solvability of the classical Gel'fand's inverse problem can be established on manifolds via Tataru's optimal unique continuation theorem for the wave operator. Next, I will discuss our recent works on the uniqueness and stability of the inverse problem for the Gromov-Hausdorff limits of Riemannian manifolds with bounded sectional curvature. This talk is based on joint works with Y. Kurylev, M. Lassas, and T. Yamaguchi.
https://www.mv.helsinki.fi/home/jinpeng/
FJ-LMI Seminar
15:00-16:00 Room #056 (Graduate School of Math. Sci. Bldg.)
Sourav Ghosh (Ashoka University)
Proper actions on group manifolds (英語)
Sourav Ghosh (Ashoka University)
Proper actions on group manifolds (英語)
[ Abstract ]
In this talk, I will show how to use known examples of flat affine manifolds to obtain new examples of proper actions of discrete groups on group manifolds. This is a joint work with Toshiyuki Kobayashi.
In this talk, I will show how to use known examples of flat affine manifolds to obtain new examples of proper actions of discrete groups on group manifolds. This is a joint work with Toshiyuki Kobayashi.
2025/10/07
Tuesday Seminar on Topology
17:00-18:00 Online
Pre-registration required. See our seminar webpage.
Sakumi Sugawara (Hokkaido University)
Topology of hyperplane arrangements and related 3-manifolds (JAPANESE)
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Pre-registration required. See our seminar webpage.
Sakumi Sugawara (Hokkaido University)
Topology of hyperplane arrangements and related 3-manifolds (JAPANESE)
[ Abstract ]
One of the central questions in the topology of hyperplane arrangements is whether several topological invariants are combinatorially determined. While the cohomology ring of the complement has a combinatorial description, it remains open whether even the first Betti number of the Milnor fiber is. In contrast, the homeomorphism types of 3-manifolds appearing as the boundary manifold of projective line arrangements and the Milnor fiber boundary of arrangements in a 3-dimensional space are combinatorially determined. In this talk, we focus on these 3-manifolds. In particular, we will present the cohomology ring structure for the boundary manifold, originally due to Cohen-Suciu, and an explicit formula for the homology group of the Milnor fiber boundary of generic arrangements.
[ Reference URL ]One of the central questions in the topology of hyperplane arrangements is whether several topological invariants are combinatorially determined. While the cohomology ring of the complement has a combinatorial description, it remains open whether even the first Betti number of the Milnor fiber is. In contrast, the homeomorphism types of 3-manifolds appearing as the boundary manifold of projective line arrangements and the Milnor fiber boundary of arrangements in a 3-dimensional space are combinatorially determined. In this talk, we focus on these 3-manifolds. In particular, we will present the cohomology ring structure for the boundary manifold, originally due to Cohen-Suciu, and an explicit formula for the homology group of the Milnor fiber boundary of generic arrangements.
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
2025/10/06
Seminar on Geometric Complex Analysis
10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)
Yuya Takeuchi (Univ. of Tsukuba)
CR Paneitz operator on non-embeddable CR manifolds (Japanese)
https://forms.gle/gTP8qNZwPyQyxjTj8
Yuya Takeuchi (Univ. of Tsukuba)
CR Paneitz operator on non-embeddable CR manifolds (Japanese)
[ Abstract ]
The CR Paneitz operator, a CR invariant fourth-order linear differential operator, plays a crucial role in three-dimensional CR geometry. It is closely related to global embeddability, the CR positive mass theorem, and the logarithmic singularity of the Szegő kernel. In this talk, I will discuss the spectrum of the CR Paneitz operator on non-embeddable CR manifolds, with particular emphasis on how it differs from the embeddable case.
[ Reference URL ]The CR Paneitz operator, a CR invariant fourth-order linear differential operator, plays a crucial role in three-dimensional CR geometry. It is closely related to global embeddability, the CR positive mass theorem, and the logarithmic singularity of the Szegő kernel. In this talk, I will discuss the spectrum of the CR Paneitz operator on non-embeddable CR manifolds, with particular emphasis on how it differs from the embeddable case.
https://forms.gle/gTP8qNZwPyQyxjTj8
Tokyo Probability Seminar
16:30-18:00 Room #126 (Graduate School of Math. Sci. Bldg.)
The lecture is starting late. No Tea Time today.
Hiroshi Kawabi (Keio University)
Riemann多様体上の排他過程に対する流体力学極限
The lecture is starting late. No Tea Time today.
Hiroshi Kawabi (Keio University)
Riemann多様体上の排他過程に対する流体力学極限
[ Abstract ]
(コンパクトとは限らない)完備なRiemann多様体をグラフで離散化し, その上の排他過程に対するスケール極限を考察する。
本講演では, 石渡 聡 氏 (山形大学), 角田 謙吉 氏 (九州大学)と現在進行中の共同研究に基づき, 流体力学極限について得られた成果を報告する。
(コンパクトとは限らない)完備なRiemann多様体をグラフで離散化し, その上の排他過程に対するスケール極限を考察する。
本講演では, 石渡 聡 氏 (山形大学), 角田 謙吉 氏 (九州大学)と現在進行中の共同研究に基づき, 流体力学極限について得られた成果を報告する。
2025/10/03
Seminar on Probability and Statistics
16:00-17:10 Room #123 (Graduate School of Math. Sci. Bldg.)
Freddy Delbaen (ETH Zurich)
Writing Uncorrelated Random Variables as a sum of Independent Random Variables (English)
https://u-tokyo-ac-jp.zoom.us/meeting/register/-kK0DZB6SbeMyAye6ujPeA
Freddy Delbaen (ETH Zurich)
Writing Uncorrelated Random Variables as a sum of Independent Random Variables (English)
[ Abstract ]
With Majumdar I proved that for a random variable $X$ that is uncorrelated to a sigma algebra, there exists a best approximation by a random variable that is independent of the sigma algebra. Inductively we get a series of random variables whose terms are independent of the sigma algebra. We show that this series converge to $X$ in $L^2$. The proof uses the Knott-Smith theorem from transport theory. In an earlier version we could show that convergence took place in $L^1$.
[ Reference URL ]With Majumdar I proved that for a random variable $X$ that is uncorrelated to a sigma algebra, there exists a best approximation by a random variable that is independent of the sigma algebra. Inductively we get a series of random variables whose terms are independent of the sigma algebra. We show that this series converge to $X$ in $L^2$. The proof uses the Knott-Smith theorem from transport theory. In an earlier version we could show that convergence took place in $L^1$.
https://u-tokyo-ac-jp.zoom.us/meeting/register/-kK0DZB6SbeMyAye6ujPeA
2025/09/25
Applied Analysis
16:00-17:30 Room #128 (Graduate School of Math. Sci. Bldg.)
Fumihiko Onoue (Technische Universität München)
On the shape of fractional minimal surfaces (Japanese)
Fumihiko Onoue (Technische Universität München)
On the shape of fractional minimal surfaces (Japanese)
[ Abstract ]
Fractional perimeter (or fractional area) has been studied for more than a decade since Caffarelli, Roquejofffre, and Savin introduced its notion in 2010; however, there are still a lot of things unknown. In this talk, we discuss the shape of the boundary of sets minimizing their fractional perimeter under several boundary conditions, reviewing several interesting examples distinct from sets minimizing their classical perimeter. Moreover, if time permits, we present another notion of fractional area for smooth hypersurfaces with boundary, which was introduced by Paroni, Podio-Guidugli, and Seguin in 2018. Then we discuss the shape of critical points of their fractional area in several simple situations. This talk is partially based on a joint work with S. Dipierro and E. Valdinoci.
Fractional perimeter (or fractional area) has been studied for more than a decade since Caffarelli, Roquejofffre, and Savin introduced its notion in 2010; however, there are still a lot of things unknown. In this talk, we discuss the shape of the boundary of sets minimizing their fractional perimeter under several boundary conditions, reviewing several interesting examples distinct from sets minimizing their classical perimeter. Moreover, if time permits, we present another notion of fractional area for smooth hypersurfaces with boundary, which was introduced by Paroni, Podio-Guidugli, and Seguin in 2018. Then we discuss the shape of critical points of their fractional area in several simple situations. This talk is partially based on a joint work with S. Dipierro and E. Valdinoci.
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