## Seminar information archive

Seminar information archive ～12/08｜Today's seminar 12/09 | Future seminars 12/10～

### 2016/12/22

#### Infinite Analysis Seminar Tokyo

14:00-17:30 Room #002 (Graduate School of Math. Sci. Bldg.)

Homology cobordisms over a surface of genus one (JAPANESE)

Generalization of Schur partition theorem (JAPANESE)

**Yuta Nozaki**(Graduate School of Mathematical Sciences, the University of Tokyo) 14:00-15:30Homology cobordisms over a surface of genus one (JAPANESE)

[ Abstract ]

Morimoto showed that some lens spaces have no genus one fibered knot,

and Baker completely determined such lens spaces.

In this talk, we introduce our results for the corresponding problem

formulated in terms of homology cobordisms.

The Chebotarev density theorem and binary quadratic forms play a key

role in the proof.

Morimoto showed that some lens spaces have no genus one fibered knot,

and Baker completely determined such lens spaces.

In this talk, we introduce our results for the corresponding problem

formulated in terms of homology cobordisms.

The Chebotarev density theorem and binary quadratic forms play a key

role in the proof.

**Shunsuke Tsuchioka**(Graduate School of Mathematical Sciences, the University of Tokyo) 16:00-17:30Generalization of Schur partition theorem (JAPANESE)

[ Abstract ]

The celebrated Rogers-Ramanujan partition theorem (RRPT) claims that

the number of partitions of n whose parts are ¥pm1 modulo 5

is equinumerous to the number of partitions of n whose successive

differences are

at least 2. Schur found a mod 6 analog of RRPT in 1926.

We will report a generalization for odd $p¥geq 3$ via representation

theory of quantum groups.

At p=3, it is Schur's theorem. The statement for p=5 was conjectured by

Andrews in 1970s

in a course of his 3 parameter generalization of RRPT and proved in 1994

by Andrews-Bessenrodt-Olsson with an aid of computer.

This is a joint work with Masaki Watanabe (arXiv:1609.01905).

The celebrated Rogers-Ramanujan partition theorem (RRPT) claims that

the number of partitions of n whose parts are ¥pm1 modulo 5

is equinumerous to the number of partitions of n whose successive

differences are

at least 2. Schur found a mod 6 analog of RRPT in 1926.

We will report a generalization for odd $p¥geq 3$ via representation

theory of quantum groups.

At p=3, it is Schur's theorem. The statement for p=5 was conjectured by

Andrews in 1970s

in a course of his 3 parameter generalization of RRPT and proved in 1994

by Andrews-Bessenrodt-Olsson with an aid of computer.

This is a joint work with Masaki Watanabe (arXiv:1609.01905).

### 2016/12/20

#### PDE Real Analysis Seminar

10:30-11:30 Room #056 (Graduate School of Math. Sci. Bldg.)

On the stability of the 3D Couette Flow (English)

**Nader Masmoudi**(Courant Institute, NYU)On the stability of the 3D Couette Flow (English)

[ Abstract ]

We will discuss the dynamics of small perturbations of the plane, periodic Couette flow in the 3D incompressible Navier-Stokes system at high Reynolds number. For sufficiently regular initial data, we determine the stability threshold for small perturbations and characterize the long time dynamics of solutions near this threshold. For rougher data, we obtain an estimate of the stability threshold which agrees closely with numerical experiments. The primary linear stability mechanism is an anisotropic enhanced dissipation resulting from the mixing caused by the large mean shear. There is also a linear inviscid damping similar to the one observed in 2D. The main linear instability is a non-normal instability known as the lift-up effect. There is clearly a competition between these linear effects. Understanding the variety of nonlinear resonances and devising the correct norms to estimate them form the core of the analysis we undertake. This is based on joint works with Jacob Bedrossian and Pierre Germain.

We will discuss the dynamics of small perturbations of the plane, periodic Couette flow in the 3D incompressible Navier-Stokes system at high Reynolds number. For sufficiently regular initial data, we determine the stability threshold for small perturbations and characterize the long time dynamics of solutions near this threshold. For rougher data, we obtain an estimate of the stability threshold which agrees closely with numerical experiments. The primary linear stability mechanism is an anisotropic enhanced dissipation resulting from the mixing caused by the large mean shear. There is also a linear inviscid damping similar to the one observed in 2D. The main linear instability is a non-normal instability known as the lift-up effect. There is clearly a competition between these linear effects. Understanding the variety of nonlinear resonances and devising the correct norms to estimate them form the core of the analysis we undertake. This is based on joint works with Jacob Bedrossian and Pierre Germain.

#### Tuesday Seminar on Topology

17:00-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Deligne-Mostow lattices and cone metrics on the sphere (ENGLISH)

**Irene Pasquinelli**(Durham University)Deligne-Mostow lattices and cone metrics on the sphere (ENGLISH)

[ Abstract ]

Finding lattices in PU(n,1) has been one of the major challenges of the last decades. One way of constructing lattices is to give a fundamental domain for its action on the complex hyperbolic space.

One approach, successful for some lattices, consists of seeing the complex hyperbolic space as the configuration space of cone metrics on the sphere and of studying the action of some maps exchanging the cone points with same cone angle.

In this talk we will see how this construction of fundamental polyhedra can be extended to almost all Deligne-Mostow lattices with three folding symmetry.

Finding lattices in PU(n,1) has been one of the major challenges of the last decades. One way of constructing lattices is to give a fundamental domain for its action on the complex hyperbolic space.

One approach, successful for some lattices, consists of seeing the complex hyperbolic space as the configuration space of cone metrics on the sphere and of studying the action of some maps exchanging the cone points with same cone angle.

In this talk we will see how this construction of fundamental polyhedra can be extended to almost all Deligne-Mostow lattices with three folding symmetry.

### 2016/12/19

#### Discrete mathematical modelling seminar

17:30-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Discrete Painlevé equations on the affine A3 surface (ENGLISH)

**Anton Dzhamay**(University of Northern Colorado)Discrete Painlevé equations on the affine A3 surface (ENGLISH)

[ Abstract ]

We explain how to construct the birational representation of the extended affine Weyl symmetry group D5 and consider examples of discrete Painlevé equations that correspond to certain translation elements in this group. One of the examples is the famous q-PV equation of Jimbo-Sakai. Some other examples are conjugated to it via explicit change of variables and we explain how representing translation elements as words in the group allows us to see the corresponding change of coordinates explicitly. We also show a new example of a discrete Painlevé equation that is elementary (short translation), but at the same time is different from the q-PVI equation.

We explain how to construct the birational representation of the extended affine Weyl symmetry group D5 and consider examples of discrete Painlevé equations that correspond to certain translation elements in this group. One of the examples is the famous q-PV equation of Jimbo-Sakai. Some other examples are conjugated to it via explicit change of variables and we explain how representing translation elements as words in the group allows us to see the corresponding change of coordinates explicitly. We also show a new example of a discrete Painlevé equation that is elementary (short translation), but at the same time is different from the q-PVI equation.

### 2016/12/17

#### Discrete mathematical modelling seminar

10:00-18:00 Room #056 (Graduate School of Math. Sci. Bldg.)

Factorization of Rational Mappings and Geometric Deautonomization (ENGLISH)

From the QRT maps to elliptic difference Painlevé equations (ENGLISH)

The complete degeneration scheme of four-dimensional Painlevé-type equations (ENGLISH)

Degeneration of the Painlevé divisors (ENGLISH)

Rational approximation and Schlesinger transformation (ENGLISH)

Spaces of initial conditions for nonautonomous mappings of the plane (ENGLISH)

**Anton Dzhamay**(University of Northern Colorado) 10:00-10:50Factorization of Rational Mappings and Geometric Deautonomization (ENGLISH)

[ Abstract ]

This talk is the first of two talks describing the joint project with Tomoyuki Takenawa and Stefan Carstea on geometric deautonomization.

The goal of this project is to develop a systematic approach for deautonomizing discrete integrable mappings, such as the QRT mappings, to non-automonous mappings in the discrete Painlevé family, based on the action of the mapping on the Picard lattice of the surface and a choice of an elliptic fiber. In this talk we will explain the main ideas behind this approach and describe the technique that allows us to recover explicit formulas defining the mapping from the known action on the divisor group (the factorization technique). We illustrate our approach by reconstructing the famous example of the q-PVI equation of Jimbo-Sakai from a simple QRT mapping.

This talk is the first of two talks describing the joint project with Tomoyuki Takenawa and Stefan Carstea on geometric deautonomization.

The goal of this project is to develop a systematic approach for deautonomizing discrete integrable mappings, such as the QRT mappings, to non-automonous mappings in the discrete Painlevé family, based on the action of the mapping on the Picard lattice of the surface and a choice of an elliptic fiber. In this talk we will explain the main ideas behind this approach and describe the technique that allows us to recover explicit formulas defining the mapping from the known action on the divisor group (the factorization technique). We illustrate our approach by reconstructing the famous example of the q-PVI equation of Jimbo-Sakai from a simple QRT mapping.

**Tomoyuki Takenawa**(Tokyo University of Marine Science and Technology) 11:00-11:50From the QRT maps to elliptic difference Painlevé equations (ENGLISH)

[ Abstract ]

This talk is the second part of the joint project with Anton Dzhamay and Stefan Carstea on geometric deautonomization and focuses on the elliptic case and the special symmetry groups. It is well known that two-dimensional mappings preserving a rational elliptic fibration, like the Quispel-Roberts-Thompson mappings, can be deautonomized to discrete Painlevé equations. However, the dependence of this procedure on the choice of a particular elliptic fiber has not been sufficiently investigated.

In this talk we establish a way of performing the deautonomization for a pair of an autonomous mapping and a fiber. Especially, in the case where the fiber is smooth elliptic, imposing certain restrictions on such non autonomous mappings, we obtain new and simple elliptic difference Painlevé equations, including examples whose symmetry groups do not appear explicitly in Sakai's classification.

This talk is the second part of the joint project with Anton Dzhamay and Stefan Carstea on geometric deautonomization and focuses on the elliptic case and the special symmetry groups. It is well known that two-dimensional mappings preserving a rational elliptic fibration, like the Quispel-Roberts-Thompson mappings, can be deautonomized to discrete Painlevé equations. However, the dependence of this procedure on the choice of a particular elliptic fiber has not been sufficiently investigated.

In this talk we establish a way of performing the deautonomization for a pair of an autonomous mapping and a fiber. Especially, in the case where the fiber is smooth elliptic, imposing certain restrictions on such non autonomous mappings, we obtain new and simple elliptic difference Painlevé equations, including examples whose symmetry groups do not appear explicitly in Sakai's classification.

**Hiroshi Kawakami**(Aoyama Gakuin University) 13:30-14:20The complete degeneration scheme of four-dimensional Painlevé-type equations (ENGLISH)

[ Abstract ]

In the joint work with H. Sakai and A. Nakamura, we constructed the degeneration scheme of four-dimensional Painlevé-type equations associated with unramified linear equations. In this talk I present the "complete" degeneration scheme of the four-dimensional Painlevé-type equations, which is constructed by means of the degeneration of HTL forms of associated linear equations.

In the joint work with H. Sakai and A. Nakamura, we constructed the degeneration scheme of four-dimensional Painlevé-type equations associated with unramified linear equations. In this talk I present the "complete" degeneration scheme of the four-dimensional Painlevé-type equations, which is constructed by means of the degeneration of HTL forms of associated linear equations.

**Akane Nakamura**(Josai University) 14:30-15:20Degeneration of the Painlevé divisors (ENGLISH)

[ Abstract ]

There are three types of curves associated with 4-dimensional algebraically completely integrable systems, namely the spectral curve, the Painlevé divisors, and the separation curve. I am going to explain these three curves of genus two taking examples derived from the isospectral limit of the 4-dimensional Painlevé-type equations and study the Namikawa-Ueno type degeneration.

There are three types of curves associated with 4-dimensional algebraically completely integrable systems, namely the spectral curve, the Painlevé divisors, and the separation curve. I am going to explain these three curves of genus two taking examples derived from the isospectral limit of the 4-dimensional Painlevé-type equations and study the Namikawa-Ueno type degeneration.

**Teruhisa Tsuda**(Hitotsubashi University) 16:00-16:50Rational approximation and Schlesinger transformation (ENGLISH)

[ Abstract ]

We show how rational approximation problems for functions are related to the construction of Schlesinger transformations. Also we discuss their applications to the theory of isomonodromic deformations or Painlevé equations. This talk is based on a joint work with Toshiyuki Mano.

We show how rational approximation problems for functions are related to the construction of Schlesinger transformations. Also we discuss their applications to the theory of isomonodromic deformations or Painlevé equations. This talk is based on a joint work with Toshiyuki Mano.

**Takafumi Mase**(the University of Tokyo) 17:00-17:50Spaces of initial conditions for nonautonomous mappings of the plane (ENGLISH)

[ Abstract ]

Spaces of initial conditions are one of the most important and powerful tools to analyze mappings of the plane. In this talk, we study the basic properties of general nonautonomous equations that have spaces of initial conditions. We will consider the minimization of spaces of initial conditions for nonautonomous systems and we shall discuss a classification of nonautonomous integrable mappings of the plane with a space of initial conditions.

Spaces of initial conditions are one of the most important and powerful tools to analyze mappings of the plane. In this talk, we study the basic properties of general nonautonomous equations that have spaces of initial conditions. We will consider the minimization of spaces of initial conditions for nonautonomous systems and we shall discuss a classification of nonautonomous integrable mappings of the plane with a space of initial conditions.

### 2016/12/14

#### Number Theory Seminar

18:00-19:00 Room #056 (Graduate School of Math. Sci. Bldg.)

On vanishing cycles and duality, after A. Beilinson (English)

**Luc Illusie**(Université Paris-Sud)On vanishing cycles and duality, after A. Beilinson (English)

[ Abstract ]

It was proved by Gabber in the early 1980's that $R\Psi$ commutes with duality, and that R\Phi preserves perversity up to shift. It had been in the folklore since then that this last result was in fact a consequence of a finer one, namely the compatibility of $R\Phi$ with duality. In this talk I'll give a proof of this, using a method explained to me by A. Beilinson.

It was proved by Gabber in the early 1980's that $R\Psi$ commutes with duality, and that R\Phi preserves perversity up to shift. It had been in the folklore since then that this last result was in fact a consequence of a finer one, namely the compatibility of $R\Phi$ with duality. In this talk I'll give a proof of this, using a method explained to me by A. Beilinson.

### 2016/12/13

#### Tuesday Seminar on Topology

17:00-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Plane fields on 3-manifolds and asymptotic linking of the tangential incompressible flows (JAPANESE)

**Yoshihiko Mitsumatsu**(Chuo University)Plane fields on 3-manifolds and asymptotic linking of the tangential incompressible flows (JAPANESE)

[ Abstract ]

This is a report on a project in (a very slow) progress which aims to prove the tightness of contact structures associated with algebraic Anosov flows without using Bennequin's nor Gromov's results.

After introducing an interpretation of asymptotic linking pairing in terms of differential forms, we attach a subspaces of exact 2-forms to each plane field. We analyze this space in the case where the plane field is an algebraic Anosov foliation and explain what can be done using results from foliated cohomology and frameworks for secondary characteristic classes. We also show some explicit computations.

To close the talk, a quantization phenomenon which happens when a foliation is deformed into a contact structure is explained and we state some perspectives on applying the results on foliations to the tightness.

This is a report on a project in (a very slow) progress which aims to prove the tightness of contact structures associated with algebraic Anosov flows without using Bennequin's nor Gromov's results.

After introducing an interpretation of asymptotic linking pairing in terms of differential forms, we attach a subspaces of exact 2-forms to each plane field. We analyze this space in the case where the plane field is an algebraic Anosov foliation and explain what can be done using results from foliated cohomology and frameworks for secondary characteristic classes. We also show some explicit computations.

To close the talk, a quantization phenomenon which happens when a foliation is deformed into a contact structure is explained and we state some perspectives on applying the results on foliations to the tightness.

#### Tuesday Seminar of Analysis

16:50-18:20 Room #126 (Graduate School of Math. Sci. Bldg.)

Distribution of eigenfunction mass on some really simple domains (English)

**Hans Christianson**(North Carolina State University)Distribution of eigenfunction mass on some really simple domains (English)

[ Abstract ]

Eigenfunctions are fundamental objects of study in spectral geometry and quantum chaos. On a domain or manifold, they determine the behaviour of solutions to many evolution type equations using, for example, separation of variables. Eigenfunctions are very sensitive to background geometry, so it is important to understand what the eigenfunctions look like: where are they large and where are they small? There are many different ways to measure what "large" and "small" mean. One can consider local $L^2$ distribution, local and global $L^p$ distribution, as well as restrictions and boundary values. I will give an overview of what is known, and then discuss some very recent works in progress demonstrating that complicated things can happen even in very simple geometric settings.

Eigenfunctions are fundamental objects of study in spectral geometry and quantum chaos. On a domain or manifold, they determine the behaviour of solutions to many evolution type equations using, for example, separation of variables. Eigenfunctions are very sensitive to background geometry, so it is important to understand what the eigenfunctions look like: where are they large and where are they small? There are many different ways to measure what "large" and "small" mean. One can consider local $L^2$ distribution, local and global $L^p$ distribution, as well as restrictions and boundary values. I will give an overview of what is known, and then discuss some very recent works in progress demonstrating that complicated things can happen even in very simple geometric settings.

### 2016/12/12

#### Tokyo Probability Seminar

16:50-18:20 Room #128 (Graduate School of Math. Sci. Bldg.)

**Takuma Akimoto**(Keio University)#### Seminar on Geometric Complex Analysis

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

(JAPANESE)

**Yu Kawakami**(Kanazawa University)(JAPANESE)

#### Operator Algebra Seminars

16:45-18:15 Room #126 (Graduate School of Math. Sci. Bldg.)

The Roe cocycle and an index theorem on partitioned manifolds, and toward generalizations

(Japanese)

**Tatsuki Seto**(Nagoya Univ.)The Roe cocycle and an index theorem on partitioned manifolds, and toward generalizations

(Japanese)

### 2016/12/07

#### Colloquium

15:30-16:30 Room #056 (Graduate School of Math. Sci. Bldg.)

On a conjecture of Bloch and Kato, and a local analogue.

**Uwe Jannsen**On a conjecture of Bloch and Kato, and a local analogue.

[ Abstract ]

In their seminal paper on Tamagawa Numbers of motives,

Bloch and Kato introduced a notion of motivic pairs, without

loss of generality over the rational numbers, which should

satisfy certain properties (P1) to (P4). The last property

postulates the existence of a Galois stable lattice T in the

associated adelic Galois representation V such that for each

prime p the fixed module of the inertia group of Q_p of

V/T is l-divisible for almost all primes l different from p.

I postulate an analogous local conjecture and show that it

implies the global conjecture.

In their seminal paper on Tamagawa Numbers of motives,

Bloch and Kato introduced a notion of motivic pairs, without

loss of generality over the rational numbers, which should

satisfy certain properties (P1) to (P4). The last property

postulates the existence of a Galois stable lattice T in the

associated adelic Galois representation V such that for each

prime p the fixed module of the inertia group of Q_p of

V/T is l-divisible for almost all primes l different from p.

I postulate an analogous local conjecture and show that it

implies the global conjecture.

### 2016/12/06

#### Tuesday Seminar of Analysis

16:50-18:20 Room #126 (Graduate School of Math. Sci. Bldg.)

On the trivialization of Bloch bundles and the construction of localized Wannier functions (English)

**Horia Cornean**(Aalborg University, Denmark)On the trivialization of Bloch bundles and the construction of localized Wannier functions (English)

[ Abstract ]

We shall present an introductory lecture on the trivialization of Bloch bundles and its physical implications. Simply stated, the main question we want to answer is the following: given a rank $N＼geq 1$ family of orthogonal projections $P(k)$ where $k＼in ＼mathbb{R}^d$, $P(＼cdot)$ is smooth and $＼mathbb{Z}^d$-periodic, is it possible to construct an orthonormal basis of its range which consists of vectors which are both smooth and periodic in $k$? We shall explain in detail the connection with solid state physics. This is joint work with I. Herbst and G. Nenciu.

We shall present an introductory lecture on the trivialization of Bloch bundles and its physical implications. Simply stated, the main question we want to answer is the following: given a rank $N＼geq 1$ family of orthogonal projections $P(k)$ where $k＼in ＼mathbb{R}^d$, $P(＼cdot)$ is smooth and $＼mathbb{Z}^d$-periodic, is it possible to construct an orthonormal basis of its range which consists of vectors which are both smooth and periodic in $k$? We shall explain in detail the connection with solid state physics. This is joint work with I. Herbst and G. Nenciu.

#### Tuesday Seminar on Topology

17:00-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Union of 3-punctured spheres in a hyperbolic 3-manifold (JAPANESE)

**Ken'ichi Yoshida**(The University of Tokyo)Union of 3-punctured spheres in a hyperbolic 3-manifold (JAPANESE)

[ Abstract ]

An essential 3-punctured sphere in a hyperbolic 3-manifold is isotopic to a totally geodesic one. We will classify the topological types for components of union of the totally geodesic 3-punctured spheres in an orientable hyperbolic 3-manifold. There are special types each of which appears in precisely one manifold.

An essential 3-punctured sphere in a hyperbolic 3-manifold is isotopic to a totally geodesic one. We will classify the topological types for components of union of the totally geodesic 3-punctured spheres in an orientable hyperbolic 3-manifold. There are special types each of which appears in precisely one manifold.

#### Classical Analysis

16:45-18:15 Room #154 (Graduate School of Math. Sci. Bldg.)

Introduction to resurgence on the example of saddle-node singularities (ENGLISH)

**David Sauzin**(CNRS)Introduction to resurgence on the example of saddle-node singularities (ENGLISH)

[ Abstract ]

Divergent power series naturally appear when solving such an elementary differential equation as x^2 dy = (x+y) dx, which is the simplest example of saddle-node singularity. I will discuss the formal classification of saddle-node singularities and illustrate on that case Ecalle's resurgence theory, which allows one to analyse the divergence of the formal solutions. One can also deal with resonant saddle-node singularities with one more dimension, a situation which covers the local study at infinity of some Painlevé equations.

Divergent power series naturally appear when solving such an elementary differential equation as x^2 dy = (x+y) dx, which is the simplest example of saddle-node singularity. I will discuss the formal classification of saddle-node singularities and illustrate on that case Ecalle's resurgence theory, which allows one to analyse the divergence of the formal solutions. One can also deal with resonant saddle-node singularities with one more dimension, a situation which covers the local study at infinity of some Painlevé equations.

### 2016/12/05

#### Seminar on Geometric Complex Analysis

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

(JAPANESE)

**Takahiro Oba**(Tokyo Institute of Technology )(JAPANESE)

#### Operator Algebra Seminars

16:45-18:15 Room #126 (Graduate School of Math. Sci. Bldg.)

On a generalized Fraïssé limit construction (English)

**Shuhei Masumoto**(Univ.Tokyo)On a generalized Fraïssé limit construction (English)

### 2016/12/01

#### Seminar on Probability and Statistics

16:00-18:00 Room #052 (Graduate School of Math. Sci. Bldg.)

On the determinant of the Malliavin matrix and density of random vector on Wiener chaos

**Ciprian Tudor**(Université Lille 1)On the determinant of the Malliavin matrix and density of random vector on Wiener chaos

[ Abstract ]

A well-known problem in Malliavin calculus concerns the relation between the determinant of the Malliavin matrix of a random vector and the determinant of its covariance matrix. We give an explicit relation between these two determinants for couples of random vectors of multiple integrals. In particular, if the multiple integrals are of the same order, we prove that two random variables in the same Wiener chaos either admit a joint density, either are proportional and that the result is not true for random variables in Wiener chaoses of different orders.

A well-known problem in Malliavin calculus concerns the relation between the determinant of the Malliavin matrix of a random vector and the determinant of its covariance matrix. We give an explicit relation between these two determinants for couples of random vectors of multiple integrals. In particular, if the multiple integrals are of the same order, we prove that two random variables in the same Wiener chaos either admit a joint density, either are proportional and that the result is not true for random variables in Wiener chaoses of different orders.

### 2016/11/29

#### Algebraic Geometry Seminar

15:30-17:00 Room #122 (Graduate School of Math. Sci. Bldg.)

Etale fundamental groups of F-regular schemes (English)

**Karl Schwede**(University of Utah)Etale fundamental groups of F-regular schemes (English)

[ Abstract ]

I will discuss recent work studying etale fundamental groups of the regular locus of F-regular schemes. I will describe how to use F-signature to bound the size of the fundamental group of an F-regular scheme, similar to a result of Xu. I will then discuss a recent extension showing that every F-regular scheme X has a finite cover Y, etale over the regular lcous of X, so that the etale fundamental groups of Y and the regular locus of Y agree. This is analogous to results of Greb-Kebekus-Peternell.

All the work discussed is joint with Carvajal-Rojas and Tucker or with with Bhatt, Carvajal-Rojas, Graf and Tucker.

I will discuss recent work studying etale fundamental groups of the regular locus of F-regular schemes. I will describe how to use F-signature to bound the size of the fundamental group of an F-regular scheme, similar to a result of Xu. I will then discuss a recent extension showing that every F-regular scheme X has a finite cover Y, etale over the regular lcous of X, so that the etale fundamental groups of Y and the regular locus of Y agree. This is analogous to results of Greb-Kebekus-Peternell.

All the work discussed is joint with Carvajal-Rojas and Tucker or with with Bhatt, Carvajal-Rojas, Graf and Tucker.

#### Tuesday Seminar on Topology

17:30-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Generalized spectral theory and its application to coupled oscillators on networks (JAPANESE)

**Hayato Chiba**(Kyushu University)Generalized spectral theory and its application to coupled oscillators on networks (JAPANESE)

[ Abstract ]

For a system of large coupled oscillators on networks, we show that the transition from the de-synchronous state to the synchronization occurs as the coupling strength increases. For the proof, the generalized spectral theory of linear operators is employed.

For a system of large coupled oscillators on networks, we show that the transition from the de-synchronous state to the synchronization occurs as the coupling strength increases. For the proof, the generalized spectral theory of linear operators is employed.

#### Tuesday Seminar of Analysis

16:50-18:20 Room #126 (Graduate School of Math. Sci. Bldg.)

Interior transmission eigenvalue problems on manifolds (Japanese)

**Naotaka Shouji**(Graduate School of Pure and Applied Sciences, University of Tsukuba)Interior transmission eigenvalue problems on manifolds (Japanese)

### 2016/11/28

#### Seminar on Geometric Complex Analysis

10:30-12:00 Room #128 (Graduate School of Math. Sci. Bldg.)

(JAPANESE)

**Satoshi Nakamura**(Tohoku University)(JAPANESE)

#### Operator Algebra Seminars

16:45-18:15 Room #126 (Graduate School of Math. Sci. Bldg.)

Fock space deformed by Coxeter groups (English)

**Takahiro Hasebe**(Hokkaido University)Fock space deformed by Coxeter groups (English)

#### Discrete mathematical modelling seminar

17:15-18:30 Room #056 (Graduate School of Math. Sci. Bldg.)

Who cares about integrability ? (ENGLISH)

**Alfred Ramani**(IMNC, Universite de Paris 7 et 11)Who cares about integrability ? (ENGLISH)

[ Abstract ]

I will start my talk with an introduction to integrability of continuous systems. Why is it important? Is it possible to give a definition of integrability which will satisfy everybody? (Short answer: No). I will then present the most salient discoveries of integrable systems, from Newton to Toda. Next I will address the question of discrete integrability. This will lead naturally to the question of discretisation (of continuous systems) and its importance in modelling. I will deal with the construction of integrable discretisations of continuous integrable systems and introduce the singularity confinement discrete integrability criterion. The final part of my talk will be devoted to discrete Painlevé equations. Due to obvious time constraints I will concentrate on one special class of these equations, namely those associated to the E8 affine Weyl group. I will present a succinct summary of our recent results as well as indications for future investigations.

I will start my talk with an introduction to integrability of continuous systems. Why is it important? Is it possible to give a definition of integrability which will satisfy everybody? (Short answer: No). I will then present the most salient discoveries of integrable systems, from Newton to Toda. Next I will address the question of discrete integrability. This will lead naturally to the question of discretisation (of continuous systems) and its importance in modelling. I will deal with the construction of integrable discretisations of continuous integrable systems and introduce the singularity confinement discrete integrability criterion. The final part of my talk will be devoted to discrete Painlevé equations. Due to obvious time constraints I will concentrate on one special class of these equations, namely those associated to the E8 affine Weyl group. I will present a succinct summary of our recent results as well as indications for future investigations.

### 2016/11/25

#### FMSP Lectures

10:25-12:10 Room #126 (Graduate School of Math. Sci. Bldg.)

Introduction to Logarithmic Geometry V (ENGLISH)

http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Ogus.pdf

**Arthur Ogus**(University of California, Berkeley)Introduction to Logarithmic Geometry V (ENGLISH)

[ Abstract ]

Logarithmic Geometry was invented (or discovered) in the 1980's, with crucial ideas contributed by Deligne, Faltings, Fontaine, Illusie, and especially K. Kato. It provides a systematic framework for the study of the related phenomena of compactification and degeneration in algebraic and arithmetic geometry, with applications to number theory. I will attempt to explain the main ideas and foundations of Kato's version of log geometry, with an emphasis on its geometric and topological aspects.

[ Reference URL ]Logarithmic Geometry was invented (or discovered) in the 1980's, with crucial ideas contributed by Deligne, Faltings, Fontaine, Illusie, and especially K. Kato. It provides a systematic framework for the study of the related phenomena of compactification and degeneration in algebraic and arithmetic geometry, with applications to number theory. I will attempt to explain the main ideas and foundations of Kato's version of log geometry, with an emphasis on its geometric and topological aspects.

http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Ogus.pdf

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