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Future seminars
Seminar information archive ～11/03｜Today's seminar 11/04 | Future seminars 11/05～
2026/11/06
Algebraic Geometry Seminar
13:15-14:45 Room #118 (Graduate School of Math. Sci. Bldg.)
Shu Nimura (Nagoya University)
Moduli spaces of $G$-constellations on $\mathbb{C}^2$ and their birational geometry
Shu Nimura (Nagoya University)
Moduli spaces of $G$-constellations on $\mathbb{C}^2$ and their birational geometry
[ Abstract ]
Let $G$ be a finite subgroup of $GL_2(\mathbb{C})$. Ishii conjectured that a resolution of $\mathbb{C}^2/G$ is dominated by the maximal resolution of the quotient pair if and only if it is isomorphic to a moduli space of King-stable $G$-constellations. After reviewing the background, I will outline a proof of this conjecture. I will then present a complementary result on wall-crossing for moduli spaces of Bridgeland-stable $G$-constellations.
Let $G$ be a finite subgroup of $GL_2(\mathbb{C})$. Ishii conjectured that a resolution of $\mathbb{C}^2/G$ is dominated by the maximal resolution of the quotient pair if and only if it is isomorphic to a moduli space of King-stable $G$-constellations. After reviewing the background, I will outline a proof of this conjecture. I will then present a complementary result on wall-crossing for moduli spaces of Bridgeland-stable $G$-constellations.
2026/11/09
Operator Algebra Seminars
15:30-17:00 Room # 126 (Graduate School of Math. Sci. Bldg.)
Hikaru Sekiyama (Keio University)
KS-Groupoids and KS-Crossed Products associated with Inverse Semigroup Actions
Hikaru Sekiyama (Keio University)
KS-Groupoids and KS-Crossed Products associated with Inverse Semigroup Actions
Seminar on Geometric Complex Analysis
10:30-12:00 Room #126 (Graduate School of Math. Sci. Bldg.)
Junjiro Noguchi (The Univ. of Tokyo)
(Japanese)
https://forms.gle/8ERsVDLuKHwbVzm57
Junjiro Noguchi (The Univ. of Tokyo)
(Japanese)
[ Abstract ]
[ Reference URL ]https://forms.gle/8ERsVDLuKHwbVzm57
Tokyo Probability Seminar
16:00-17:30 Room #122 (Graduate School of Math. Sci. Bldg.)
We are having teatime from 15:20 in the common room on the second floor. Please join us.
Kenta Misumi (The University of Tokyo)
ディリクレ空間で、準至る所ベクトル束を作り、マルコフ過程に沿って準至る所ベクトルを平行移動する。
We are having teatime from 15:20 in the common room on the second floor. Please join us.
Kenta Misumi (The University of Tokyo)
ディリクレ空間で、準至る所ベクトル束を作り、マルコフ過程に沿って準至る所ベクトルを平行移動する。
[ Abstract ]
準正則 Dirichlet 空間上において準ベクトル束の概念を定式化することにより、その分類量として第一 Stiefel–Whitney 類が自然に現れる。さらに、この枠組みに適合する接続の理論を公理的に構築することで、対応する右過程の軌道に沿ったベクトルの準至る所での平行移動を構成することが可能となる。
この構成は、従来の Riemann 多様体あるいは sub-Riemann 多様体上の Brown 運動に沿う確率平行移動を包含するだけでなく、一般のジャンプ型過程に沿う確率平行移動も含む。さらには、RCD 空間など、特異空間上の Brown 運動に沿う確率平行移動さえも導く。
したがって本理論は、滑らかさに依存していた従来の平行移動の概念を、準正則 Dirichlet 形式と右過程を基礎とする非滑らかな対象へ拡張する一般的な枠組みを与えるものとなる。
準正則 Dirichlet 空間上において準ベクトル束の概念を定式化することにより、その分類量として第一 Stiefel–Whitney 類が自然に現れる。さらに、この枠組みに適合する接続の理論を公理的に構築することで、対応する右過程の軌道に沿ったベクトルの準至る所での平行移動を構成することが可能となる。
この構成は、従来の Riemann 多様体あるいは sub-Riemann 多様体上の Brown 運動に沿う確率平行移動を包含するだけでなく、一般のジャンプ型過程に沿う確率平行移動も含む。さらには、RCD 空間など、特異空間上の Brown 運動に沿う確率平行移動さえも導く。
したがって本理論は、滑らかさに依存していた従来の平行移動の概念を、準正則 Dirichlet 形式と右過程を基礎とする非滑らかな対象へ拡張する一般的な枠組みを与えるものとなる。
FJ-LMI Seminar
14:00-15:00 Room #123 (Graduate School of Math. Sci. Bldg.)
Joint with Probabilty Seminar
Ignacio MADRID (Institute of Industrial Science, The University of Tokyo)
Explaining exclusion-like current large deviations in a single-particle diffusion-renewal system (英語)
https://fj-lmi.cnrs.fr/seminars
Joint with Probabilty Seminar
Ignacio MADRID (Institute of Industrial Science, The University of Tokyo)
Explaining exclusion-like current large deviations in a single-particle diffusion-renewal system (英語)
[ Abstract ]
In recent experiments, Henri Vo Van Qui and Nicolas Clément (LIMMS, CNRS – IIS UTokyo) measure with astonishing precision the electrical current (net electron counts over time) created by molecules diffusing independently between two electrodes, where they can repeatedly receive or release single electrons. There, in a diffusion-limited regime, we found that the electrical current fluctuations recover a characteristic noise reduction: the electron count has an asymptotic variance-to-mean ratio of 1/3. Surprisingly, this same factor appears in boundary-driven symmetric simple exclusion processes, where it is attributed to multi-particle spatial exclusion. In our case however, no exclusion occurs between molecules. In this talk we will elucidate mathematically the conditions allowing a single diffusing particle to achieve these same current fluctuations.
I will develop a stochastic model of a diffusing particle carrying a two-state mark (charged or uncharged). Its position is a reflected Brownian motion on an interval, while its charge state jumps through electron-transfer reactions governed by boundary local time. The scaled cumulant generating function of the electron count can then be obtained from an associated eigenproblem with Robin boundary conditions, for which we give a complementary probabilistic renewal description. The analysis provides a rigorous explanation of the observed noise reduction and makes explicit how it arises from the timing of diffusive round trips between the boundaries. Moreover, in the diffusion-limited regime, the correspondence with exclusion systems extends beyond the 1/3 factor to the full asymptotic counting statistics, after normalization by the mean current. The model thus provides a minimal single-particle instance of this general result, identifying diffusion and boundary-driven jumps as sufficient ingredients.
[ Reference URL ]In recent experiments, Henri Vo Van Qui and Nicolas Clément (LIMMS, CNRS – IIS UTokyo) measure with astonishing precision the electrical current (net electron counts over time) created by molecules diffusing independently between two electrodes, where they can repeatedly receive or release single electrons. There, in a diffusion-limited regime, we found that the electrical current fluctuations recover a characteristic noise reduction: the electron count has an asymptotic variance-to-mean ratio of 1/3. Surprisingly, this same factor appears in boundary-driven symmetric simple exclusion processes, where it is attributed to multi-particle spatial exclusion. In our case however, no exclusion occurs between molecules. In this talk we will elucidate mathematically the conditions allowing a single diffusing particle to achieve these same current fluctuations.
I will develop a stochastic model of a diffusing particle carrying a two-state mark (charged or uncharged). Its position is a reflected Brownian motion on an interval, while its charge state jumps through electron-transfer reactions governed by boundary local time. The scaled cumulant generating function of the electron count can then be obtained from an associated eigenproblem with Robin boundary conditions, for which we give a complementary probabilistic renewal description. The analysis provides a rigorous explanation of the observed noise reduction and makes explicit how it arises from the timing of diffusive round trips between the boundaries. Moreover, in the diffusion-limited regime, the correspondence with exclusion systems extends beyond the 1/3 factor to the full asymptotic counting statistics, after normalization by the mean current. The model thus provides a minimal single-particle instance of this general result, identifying diffusion and boundary-driven jumps as sufficient ingredients.
https://fj-lmi.cnrs.fr/seminars
2026/11/10
Numerical Analysis Seminar
16:30-18:00 Room #122 (Graduate School of Math. Sci. Bldg.)
Takiko Sasaki (Musashino University)
Touchdown in Nonlocal MEMS Equations: Rescaling and the Construction of Higher-Order Profiles (Japanese)
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/
Takiko Sasaki (Musashino University)
Touchdown in Nonlocal MEMS Equations: Rescaling and the Construction of Higher-Order Profiles (Japanese)
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/
Tuesday Seminar on Topology
16:00-17:30 Room #hybrid/056 (Graduate School of Math. Sci. Bldg.)
Pre-registration required. See our seminar webpage.
Minkyu Kim (Korea Institute for Advanced Study)
Symmetric powers and double loop spaces of odd spheres (JAPANESE)
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Pre-registration required. See our seminar webpage.
Minkyu Kim (Korea Institute for Advanced Study)
Symmetric powers and double loop spaces of odd spheres (JAPANESE)
[ Abstract ]
Functors from free groups to abelian groups form a natural setting in algebra and topology, and Ext-groups between such functors govern, for instance, the stable homology of automorphism groups of free groups with twisted coefficients. Over the rationals, these Ext-groups are by now well understood for a wide range of polynomial functors, thanks to work of Christine Vespa. Gregory Arone recently extended Vespa's results to integral coefficients using a new homotopical approach. One case, however, has remained particularly delicate: the Ext-groups from the abelianization functor ab to its symmetric powers S^n. These vanish rationally, but in recent joint work with Vespa, we determined Ext^1 and Ext^2 and found that they contain substantial torsion.
In this talk, I will present an unexpected result obtained recently in joint work with Arone and Vespa: the integral Ext-groups Ext*(ab, S^n) can be expressed in terms of the homology of the double loop space of a sphere of odd dimension at least 3. This identification not only determines the groups in all degrees, but also connects them to the homology of braid groups via classical results in the topology of configuration spaces.
[ Reference URL ]Functors from free groups to abelian groups form a natural setting in algebra and topology, and Ext-groups between such functors govern, for instance, the stable homology of automorphism groups of free groups with twisted coefficients. Over the rationals, these Ext-groups are by now well understood for a wide range of polynomial functors, thanks to work of Christine Vespa. Gregory Arone recently extended Vespa's results to integral coefficients using a new homotopical approach. One case, however, has remained particularly delicate: the Ext-groups from the abelianization functor ab to its symmetric powers S^n. These vanish rationally, but in recent joint work with Vespa, we determined Ext^1 and Ext^2 and found that they contain substantial torsion.
In this talk, I will present an unexpected result obtained recently in joint work with Arone and Vespa: the integral Ext-groups Ext*(ab, S^n) can be expressed in terms of the homology of the double loop space of a sphere of odd dimension at least 3. This identification not only determines the groups in all degrees, but also connects them to the homology of braid groups via classical results in the topology of configuration spaces.
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.)
Jin Takahashi (Institute of Science Tokyo)
Existence of a separating norm for the semilinear heat equation (Japanese)
Jin Takahashi (Institute of Science Tokyo)
Existence of a separating norm for the semilinear heat equation (Japanese)
FJ-LMI Seminar
13:15-14:00 Room #056 (Graduate School of Math. Sci. Bldg.)
Timothée BÉNARD (CNRS - University Sorbonne Paris Nord)
Diophantine approximation and homogeneous dynamics
https://fj-lmi.cnrs.fr/seminars
Timothée BÉNARD (CNRS - University Sorbonne Paris Nord)
Diophantine approximation and homogeneous dynamics
[ Abstract ]
I will discuss the Diophantine properties of a typical point chosen by a fractal measure on $R^d$. We will see how this problem is connected to the effective equidistribution of certain random walks on the quotient $SL_{d+1}(\mathbb R)/SL_{d+1}(\mathbb Z)$. Joint work with Weikun He and Han Zhang.
[ Reference URL ]I will discuss the Diophantine properties of a typical point chosen by a fractal measure on $R^d$. We will see how this problem is connected to the effective equidistribution of certain random walks on the quotient $SL_{d+1}(\mathbb R)/SL_{d+1}(\mathbb Z)$. Joint work with Weikun He and Han Zhang.
https://fj-lmi.cnrs.fr/seminars
2026/11/11
Number Theory Seminar
17:00-18:00 Room #123 (Graduate School of Math. Sci. Bldg.)
Shuji Saito (University of Tokyo)
An integral Hyodo-Kato isomorphism
Shuji Saito (University of Tokyo)
An integral Hyodo-Kato isomorphism
[ Abstract ]
Let O_K be a complete mixed characteristic DVR with perfect residue field k and W=W(k) be the Witt ring and K_0=W[1/p]. Let X be a proper semistable scheme over O_K and X_0 and Y be its special and generic fibers respectively. The classical theorem of Hyodo and Kato provides a rational identification between the de Rham cohomology H^*_{DR}(Y/K) of Y over K and the rationalized log crystalline cohomology of X_0 relative to the log ring W^0 whose underlying ring is W and the log structure is associated to the map of monoids \mathbb{N} \to W sending 1 to 0.It gives a positive answer to a conjecture of Fontaine and Jannsen endowing H^*_{DR}(Y/K) with K_0[\phi,N]-module structure, where \phi is a Frobenius and N is a monodromy operator. A number of different proofs of the Hyodo-Kato theorem were given by Beilinson, Große-Kloenne and Ertl-Yamada.
The main result of this talk gives an integral version of the Hyodo-Kato theorem.
The proof is completely different from those given so far and relies on Yao’s log variant of the saturated de Rham complex of Bhatt, Lurie, and Mathew.
If time permits, I will explain a work in progress where the above result is upgraded by using derived log crystalline cohomology to a comparison theorem between categories of crystals on a log smooth scheme over the log point k^0 relative to W with different log structures lifting k^0 as well as its stacky reformulation via prismatization.
All results are joint works with Federico Binda (Milano).
Let O_K be a complete mixed characteristic DVR with perfect residue field k and W=W(k) be the Witt ring and K_0=W[1/p]. Let X be a proper semistable scheme over O_K and X_0 and Y be its special and generic fibers respectively. The classical theorem of Hyodo and Kato provides a rational identification between the de Rham cohomology H^*_{DR}(Y/K) of Y over K and the rationalized log crystalline cohomology of X_0 relative to the log ring W^0 whose underlying ring is W and the log structure is associated to the map of monoids \mathbb{N} \to W sending 1 to 0.It gives a positive answer to a conjecture of Fontaine and Jannsen endowing H^*_{DR}(Y/K) with K_0[\phi,N]-module structure, where \phi is a Frobenius and N is a monodromy operator. A number of different proofs of the Hyodo-Kato theorem were given by Beilinson, Große-Kloenne and Ertl-Yamada.
The main result of this talk gives an integral version of the Hyodo-Kato theorem.
The proof is completely different from those given so far and relies on Yao’s log variant of the saturated de Rham complex of Bhatt, Lurie, and Mathew.
If time permits, I will explain a work in progress where the above result is upgraded by using derived log crystalline cohomology to a comparison theorem between categories of crystals on a log smooth scheme over the log point k^0 relative to W with different log structures lifting k^0 as well as its stacky reformulation via prismatization.
All results are joint works with Federico Binda (Milano).
2026/11/12
Tokyo-Nagoya Algebra Seminar
15:00-16:30 Online
Yasuaki Hiraoka (Kyoto University)
パーシステントホモロジー：理論と応用 (Japanese)
[ Reference URL ]
https://www.math.nagoya-u.ac.jp/~aaron.chan/TNAseminar.html
Yasuaki Hiraoka (Kyoto University)
パーシステントホモロジー：理論と応用 (Japanese)
[ Reference URL ]
https://www.math.nagoya-u.ac.jp/~aaron.chan/TNAseminar.html
2026/11/16
Operator Algebra Seminars
15:30-17:00 Room # 126 (Graduate School of Math. Sci. Bldg.)
Kohki Sakamoto (The University of Tokyo)
Continuity results for random walks in hyperbolic percolation
Kohki Sakamoto (The University of Tokyo)
Continuity results for random walks in hyperbolic percolation
[ Abstract ]
Benjamini, Lyons, and Schramm proved that the simple random walk on the infinite cluster of supercritical Bernoulli percolation exhibits positive drift and entropy, provided the underlying graph is the Cayley graph of a non-amenable group. A natural question then arises: how do these asymptotic quantities vary with the percolation parameter? In this talk, I will provide some background on this topic and show that these quantities are continuous in the parameter when the group is Gromov hyperbolic. I will also discuss how this continuity contrasts with the conjectured behavior of
the Hausdorff dimension of the limit set.
Benjamini, Lyons, and Schramm proved that the simple random walk on the infinite cluster of supercritical Bernoulli percolation exhibits positive drift and entropy, provided the underlying graph is the Cayley graph of a non-amenable group. A natural question then arises: how do these asymptotic quantities vary with the percolation parameter? In this talk, I will provide some background on this topic and show that these quantities are continuous in the parameter when the group is Gromov hyperbolic. I will also discuss how this continuity contrasts with the conjectured behavior of
the Hausdorff dimension of the limit set.
Seminar on Geometric Complex Analysis
10:30-12:00 Room #126 (Graduate School of Math. Sci. Bldg.)
Kengo Hirachi (University of Tokyo)
Variation of characteristic numbers of strictly pseudoconvex domains (Japanese)
https://forms.gle/8ERsVDLuKHwbVzm57
Kengo Hirachi (University of Tokyo)
Variation of characteristic numbers of strictly pseudoconvex domains (Japanese)
[ Abstract ]
Renormalized characteristic numbers of strictly pseudoconvex domains give rise to global CR invariants of their boundaries. In this talk, I will present first-variation formulas for these invariants using the ambient metric. As an application, I will discuss a conjecture of Takeuchi expressing the Burns–Epstein invariant in terms of generalized total Q-prime curvatures, and establish it for boundaries obtained by smooth deformations of the sphere.
[ Reference URL ]Renormalized characteristic numbers of strictly pseudoconvex domains give rise to global CR invariants of their boundaries. In this talk, I will present first-variation formulas for these invariants using the ambient metric. As an application, I will discuss a conjecture of Takeuchi expressing the Burns–Epstein invariant in terms of generalized total Q-prime curvatures, and establish it for boundaries obtained by smooth deformations of the sphere.
https://forms.gle/8ERsVDLuKHwbVzm57
2026/11/17
Numerical Analysis Seminar
16:30-18:00 Room #122 (Graduate School of Math. Sci. Bldg.)
Kazunori Matsui (Tokyo University of Marine Science and Technology)
A projection method for a visco-elasto-plasticity model with linear kinematic hardening and its vanishing viscosity limit (Japanese)
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/
Kazunori Matsui (Tokyo University of Marine Science and Technology)
A projection method for a visco-elasto-plasticity model with linear kinematic hardening and its vanishing viscosity limit (Japanese)
[ Reference URL ]
https://sites.google.com/g.ecc.u-tokyo.ac.jp/utnas-bulletin-board/
Tuesday Seminar on Topology
16:00-17:30 Room #056 (Graduate School of Math. Sci. Bldg.)
Pre-registration required. See our seminar webpage.
Akihiro Takano (Osaka University)
On the cactus Thompson group (JAPANESE)
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
Pre-registration required. See our seminar webpage.
Akihiro Takano (Osaka University)
On the cactus Thompson group (JAPANESE)
[ Abstract ]
Inspired by Brin's construction of the braided Thompson group, Witzel and Zaremsky introduced cloning systems as a framework for constructing generalized Thompson groups. In particular, the cloning system on cactus groups gives rise to the cactus Thompson group. In this talk, I will introduce this group and discuss our results on its presentations, abelianization, and word length. This is joint work with Yuya Kodama (Kagoshima University).
[ Reference URL ]Inspired by Brin's construction of the braided Thompson group, Witzel and Zaremsky introduced cloning systems as a framework for constructing generalized Thompson groups. In particular, the cloning system on cactus groups gives rise to the cactus Thompson group. In this talk, I will introduce this group and discuss our results on its presentations, abelianization, and word length. This is joint work with Yuya Kodama (Kagoshima University).
https://park.itc.u-tokyo.ac.jp/MSF/topology/TuesdaySeminar/index_e.html
2026/11/20
Seminar on Mathematics for various disciplines
14:00-15:00 Room #056 (Graduate School of Math. Sci. Bldg.)
This seminar is held on Friday afternoon.
Dieter Bothe (Technische Universität Darmstadt)
Multicomponent Diffusion — Modelling and Analysis (English)
This seminar is held on Friday afternoon.
Dieter Bothe (Technische Universität Darmstadt)
Multicomponent Diffusion — Modelling and Analysis (English)
[ Abstract ]
Diffusion in fluid systems composed of several chemical species is a classical subject with a surprisingly rich mathematical structure. In contrast to scalar diffusion, the motion of the individual components is coupled through conservation laws, thermodynamic constraints, and interspecies interactions. This leads to strongly coupled systems of nonlinear partial differential equations, whose modelling and analysis are closely intertwined. The talk introduces the main ideas of multicomponent diffusion, from its physical and thermodynamic foundations to modern mathematical formulations. Particular emphasis is placed on the relation between physically meaningful constitutive laws and the qualitative properties of the resulting PDE systems, including the question of global regularity for large data.
Diffusion in fluid systems composed of several chemical species is a classical subject with a surprisingly rich mathematical structure. In contrast to scalar diffusion, the motion of the individual components is coupled through conservation laws, thermodynamic constraints, and interspecies interactions. This leads to strongly coupled systems of nonlinear partial differential equations, whose modelling and analysis are closely intertwined. The talk introduces the main ideas of multicomponent diffusion, from its physical and thermodynamic foundations to modern mathematical formulations. Particular emphasis is placed on the relation between physically meaningful constitutive laws and the qualitative properties of the resulting PDE systems, including the question of global regularity for large data.
2026/11/26
Discrete mathematical modelling seminar
17:00-18:30 Room #118 (Graduate School of Math. Sci. Bldg.)
Mariusz Białecki (Polish Academy of Sciences・Institute of Geophysics)
Catalan-like sequences as limits of a stochastic aggregation-disintegration process (English)
Mariusz Białecki (Polish Academy of Sciences・Institute of Geophysics)
Catalan-like sequences as limits of a stochastic aggregation-disintegration process (English)
[ Abstract ]
We define a stochastic process using simple, parameter-dependent rules for the aggregation of entities with the possibility of disintegration of the formed clusters and derive the corresponding Smoluchowski-type equations. We show that in the limit of infinite system size, the stationary distribution of cluster sizes is given by a solvable recurrence that, after rescaling, takes the form of a Catalan-like recurrence. This recurrence, for a specific choice of parameters, reproduces various integer sequences, including the Catalan, Motzkin, and Schröder numbers. We also solve the inverse problem, i.e., we show how to determine the values of the process's probability parameters to obtain a recurrence with predetermined coefficients.
We define a stochastic process using simple, parameter-dependent rules for the aggregation of entities with the possibility of disintegration of the formed clusters and derive the corresponding Smoluchowski-type equations. We show that in the limit of infinite system size, the stationary distribution of cluster sizes is given by a solvable recurrence that, after rescaling, takes the form of a Catalan-like recurrence. This recurrence, for a specific choice of parameters, reproduces various integer sequences, including the Catalan, Motzkin, and Schröder numbers. We also solve the inverse problem, i.e., we show how to determine the values of the process's probability parameters to obtain a recurrence with predetermined coefficients.
2026/11/27
Colloquium
15:30-16:30 Room #NISSAY Lecture Hall (Graduate School of Math. Sci. Bldg.)
Yuichiro Tanaka (Graduate School of Mathematical Sciences, The University of Tokyo)
Visible actions of real reductive groups and multiplicity-free representations (日本語)
Yuichiro Tanaka (Graduate School of Mathematical Sciences, The University of Tokyo)
Visible actions of real reductive groups and multiplicity-free representations (日本語)
[ Abstract ]
A unitary representation of a locally compact group is multiplicity-free if each irreducible representation appears at most once in its irreducible decomposition.
With the aim of a unified framework for multiplicity-free representations of Lie groups, T. Kobayashi introduced the notion of visible action for holomorphic group-actions. Kobayashi's propagation theorem for the multiplicity-freeness property provides a uniform explanation for the multiplicity-freeness of various kinds of representations that had previously been treated individually.
Furthermore, it has led to the discovery of new multiplicity-free theorems beyond those previously known. In this talk, I will present some basic results on visible actions and discuss several recent developments.
A unitary representation of a locally compact group is multiplicity-free if each irreducible representation appears at most once in its irreducible decomposition.
With the aim of a unified framework for multiplicity-free representations of Lie groups, T. Kobayashi introduced the notion of visible action for holomorphic group-actions. Kobayashi's propagation theorem for the multiplicity-freeness property provides a uniform explanation for the multiplicity-freeness of various kinds of representations that had previously been treated individually.
Furthermore, it has led to the discovery of new multiplicity-free theorems beyond those previously known. In this talk, I will present some basic results on visible actions and discuss several recent developments.
2026/11/30
Operator Algebra Seminars
15:30-17:00 Room #126 (Graduate School of Math. Sci. Bldg.)
Fuyuta Komura (Kyushu Institute of Technology)
Cartan-preserving *-automorphism groups: realization and obstructions for compact abelian groups
Fuyuta Komura (Kyushu Institute of Technology)
Cartan-preserving *-automorphism groups: realization and obstructions for compact abelian groups
[ Abstract ]
In this talk, we investigate automorphism groups preserving Cartan subalgebras of C*-algebras. First, we describe these groups in terms of automorphisms and 1-cocycles of twisted étale groupoids. As a consequence, we obtain a C*-algebraic analogue of a theorem of Feldman and Moore on Cartan-preserving automorphisms of von Neumann algebras. We then study Cartan-fixing automorphism groups. We show that every UCT Kirchberg algebra admits a Cartan subalgebra whose Cartan-fixing automorphism group contains every second countable compact abelian group. In contrast, for C*-algebras arising from expansive effective groupoids, we prove that compact Cartan-fixing automorphism groups must have finitely generated Pontryagin duals if the fixed point subalgebras are prime. As an application, we establish the existence of inequivalent Cartan subalgebras for Kirchberg algebras arising from expansive effective groupoids.
In this talk, we investigate automorphism groups preserving Cartan subalgebras of C*-algebras. First, we describe these groups in terms of automorphisms and 1-cocycles of twisted étale groupoids. As a consequence, we obtain a C*-algebraic analogue of a theorem of Feldman and Moore on Cartan-preserving automorphisms of von Neumann algebras. We then study Cartan-fixing automorphism groups. We show that every UCT Kirchberg algebra admits a Cartan subalgebra whose Cartan-fixing automorphism group contains every second countable compact abelian group. In contrast, for C*-algebras arising from expansive effective groupoids, we prove that compact Cartan-fixing automorphism groups must have finitely generated Pontryagin duals if the fixed point subalgebras are prime. As an application, we establish the existence of inequivalent Cartan subalgebras for Kirchberg algebras arising from expansive effective groupoids.
Seminar on Geometric Complex Analysis
10:30-12:00 Room #126 (Graduate School of Math. Sci. Bldg.)
Natsuo Miyatake (Tohoku Univ.)
Equilibrium Metrics and Complete Solutions (Japanese)
https://forms.gle/8ERsVDLuKHwbVzm57
Natsuo Miyatake (Tohoku Univ.)
Equilibrium Metrics and Complete Solutions (Japanese)
[ Abstract ]
Let $X$ be a compact Riemann surface. For a positive line bundle $L\to X$, a continuous Hermitian metric $e^{-\phi}h_\ast$ on $L$, and a nonpolar compact subset $K\subseteq X$, one can construct an equilibrium metric from the envelope of subharmonic weight functions bounded above by $\phi$ on $K$. The approximation of equilibrium metrics by sequences of semipositive singular Hermitian metrics has been studied in a variety of contexts, including complex geometry, potential theory, dynamical systems, and probability theory.
In this talk, I will introduce two functions, entropy and free energy, associated with a semipositive singular Hermitian metric on the canonical bundle, an integer $r\geq 2$, and a nonzero real number $\beta$. These quantities are defined by extending the construction, due to Li--Mochizuki, of complete solutions to the Hitchin equation for cyclic Higgs bundles associated with a holomorphic $r$-differential to the setting of complete solutions to a variant of the Hitchin equation associated with semipositive singular Hermitian metrics on the canonical bundle that are not necessarily induced by holomorphic $r$-differentials. Given an equilibrium metric and a sequence of semipositive singular Hermitian metrics approximating it and satisfying a natural normalization condition, I will introduce an appropriate notion of pullback of them to the canonical bundle of a Riemann surface. Within this framework, I will formulate the problem of determining whether the associated entropy increases and the associated free energy decreases. I will initiate the study of this problem and establish several quantitative results. I will also present several examples of approximations of equilibrium metrics to which my results apply, particularly those constructed from Bergman kernels. Furthermore, I will discuss the asymptotic expansion of a lower bound for the entropy.
[ Reference URL ]Let $X$ be a compact Riemann surface. For a positive line bundle $L\to X$, a continuous Hermitian metric $e^{-\phi}h_\ast$ on $L$, and a nonpolar compact subset $K\subseteq X$, one can construct an equilibrium metric from the envelope of subharmonic weight functions bounded above by $\phi$ on $K$. The approximation of equilibrium metrics by sequences of semipositive singular Hermitian metrics has been studied in a variety of contexts, including complex geometry, potential theory, dynamical systems, and probability theory.
In this talk, I will introduce two functions, entropy and free energy, associated with a semipositive singular Hermitian metric on the canonical bundle, an integer $r\geq 2$, and a nonzero real number $\beta$. These quantities are defined by extending the construction, due to Li--Mochizuki, of complete solutions to the Hitchin equation for cyclic Higgs bundles associated with a holomorphic $r$-differential to the setting of complete solutions to a variant of the Hitchin equation associated with semipositive singular Hermitian metrics on the canonical bundle that are not necessarily induced by holomorphic $r$-differentials. Given an equilibrium metric and a sequence of semipositive singular Hermitian metrics approximating it and satisfying a natural normalization condition, I will introduce an appropriate notion of pullback of them to the canonical bundle of a Riemann surface. Within this framework, I will formulate the problem of determining whether the associated entropy increases and the associated free energy decreases. I will initiate the study of this problem and establish several quantitative results. I will also present several examples of approximations of equilibrium metrics to which my results apply, particularly those constructed from Bergman kernels. Furthermore, I will discuss the asymptotic expansion of a lower bound for the entropy.
https://forms.gle/8ERsVDLuKHwbVzm57
2026/12/01
Tuesday Seminar of Analysis
16:00-17:30 Room # 002 (Graduate School of Math. Sci. Bldg.)
Keisuke Takasao (Kyoto University)
Convergence of an Allen-Cahn system to Brakke's multiphase mean curvature flow (Japanese)
Keisuke Takasao (Kyoto University)
Convergence of an Allen-Cahn system to Brakke's multiphase mean curvature flow (Japanese)
2026/12/07
Operator Algebra Seminars
15:30-17:00 Room # 126 (Graduate School of Math. Sci. Bldg.)
Akihiro Miyagawa (Kyoto University)
TBA
Akihiro Miyagawa (Kyoto University)
TBA
2026/12/08
Seminar on Mathematics for various disciplines
10:30-11:30 Room #056 (Graduate School of Math. Sci. Bldg.)
Prashant Athavale (Clarkson University)
Total Variation Methods for Denoising and Inpainting Crystallographic Orientation Maps (English)
Prashant Athavale (Clarkson University)
Total Variation Methods for Denoising and Inpainting Crystallographic Orientation Maps (English)
[ Abstract ]
Electron backscatter diffraction (EBSD) provides spatial maps of crystal orientations in polycrystalline materials. These measurements often contain noise and missing data, complicating the identification of grains and their boundaries. In this talk, I will introduce the mathematical challenges of restoring orientation maps and present our work on total variation methods for denoising and hybrid methods for filling missing regions. Particular attention will be given to preserving grain boundaries while recovering useful microstructural information. I will also discuss ongoing work on using shape tensors and statistical comparisons of grain geometry to guide stopping criteria for denoising flows. The presentation will emphasize connections between variational methods, geometric representations, and materials imaging.
Electron backscatter diffraction (EBSD) provides spatial maps of crystal orientations in polycrystalline materials. These measurements often contain noise and missing data, complicating the identification of grains and their boundaries. In this talk, I will introduce the mathematical challenges of restoring orientation maps and present our work on total variation methods for denoising and hybrid methods for filling missing regions. Particular attention will be given to preserving grain boundaries while recovering useful microstructural information. I will also discuss ongoing work on using shape tensors and statistical comparisons of grain geometry to guide stopping criteria for denoising flows. The presentation will emphasize connections between variational methods, geometric representations, and materials imaging.
2026/12/14
Operator Algebra Seminars
15:30-17:00 Room # 126 (Graduate School of Math. Sci. Bldg.)
Yuki Arano (Nagoya University)
Prime actions of compact groups on von Neumann algebras
Yuki Arano (Nagoya University)
Prime actions of compact groups on von Neumann algebras
2026/12/18
Algebraic Geometry Seminar
13:15-14:45 Room #118 (Graduate School of Math. Sci. Bldg.)
Yutaro Hiroi (University of Osaka)
TBA
Yutaro Hiroi (University of Osaka)
TBA
[ Abstract ]
TBA
TBA
