## 過去の記録

### 2014年04月22日(火)

#### 解析学火曜セミナー

16:30-18:00   数理科学研究科棟(駒場) 128号室

[ 講演概要 ]
We consider a chemotaxis system with a logarithmic
sensitivity and a non-diffusive chemical substance. For some chemotactic
sensitivity constants, Ahn and Kang proved the existence of bounded
global solutions to the system. An entropy functional was used in their
argument to control the cell density by the density of the chemical
substance. Our purpose is to show the existence of bounded global
solutions for all the chemotactic sensitivity constants. Assuming the
smallness on the initial data in some sense, we can get uniform
estimates for time. These estimates are used to extend local solutions.
This talk is partially based on joint work with Yoshie Sugiyama (Kyusyu
Univ.) and Juan J. L. Vel\\'azquez (Univ. of Bonn).

### 2014年04月21日(月)

#### 数値解析セミナー

16:30-18:00   数理科学研究科棟(駒場) 056号室

[ 講演概要 ]
Stokes方程式やNavier-Stokes方程式の定常解に対する形状最適化問題は,これまで多く行われてきた.しかし, Navier-Stokes方程式の周期解に対しては十分に行われていない.本講演では,安定性理論を活用することで,Navier-Stokes方程式の周期解に対する形状最適化問題を人工血管の最適設計という現実の問題を通して考察する.
[ 講演参考URL ]
http://www.infsup.jp/utnas/

#### 複素解析幾何セミナー

10:30-12:00   数理科学研究科棟(駒場) 126号室

ラグランジュ平均曲率流とその具体例について (JAPANESE)

### 2014年04月19日(土)

#### 調和解析駒場セミナー

13:30-17:00   数理科学研究科棟(駒場) 128号室
このセミナーは,月に1度程度,不定期に開催されます.

Strichartz estimates for incompressible rotating fluids (JAPANESE)
[ 講演概要 ]
3次元全空間において,回転による Coriolis 力の影響を考慮した

Coriolis 力から生成される時間発展作用素に対して,
その線形時空積分評価の成立する最適な許容範囲を与える.
またその応用として,Euler 方程式の長時間可解性を考察する.

Youngwoo Koh 氏と Sanghyuk Lee 氏との共同研究に基づくものである.

[ 講演概要 ]

1次元の整数点(規則格子)全体で観測された値から、

しかし、多次元で、しかも、不規則配置の点集合上で観測された値から、

(参考:H. Wendland, Scattered Data Approximation,
Cambridge U.P., 2005)
ただし、特に無限の点集合の場合を扱うには、

もとの関数がベゾフ空間に属するときに、

### 2014年04月16日(水)

#### 代数学コロキウム

17:30-18:30   数理科学研究科棟(駒場) 056号室
Olivier Wittenberg 氏 (ENS and CNRS)
On the cycle class map for zero-cycles over local fields (ENGLISH)
[ 講演概要 ]
The Chow group of zero-cycles of a smooth and projective variety defined over a field k is an invariant of an arithmetic and geometric nature which is well understood only when k is a finite field (by higher-dimensional class field theory). In this talk, we will discuss the case of local and strictly local fields. We prove in particular the injectivity of the cycle class map to integral l-adic cohomology for a large class of surfaces with positive geometric genus over p-adic fields. The same statement holds for semistable K3 surfaces over C((t)), but does not hold in general for surfaces over C((t)) or over the maximal unramified extension of a p-adic field. This is a joint work with Hélène Esnault.

(本講演は「東京北京パリ数論幾何セミナー」として, インターネットによる東大数理, Morningside Center of MathematicsとIHESの双方向同時中継で行います.)

### 2014年04月15日(火)

#### トポロジー火曜セミナー

16:30-18:00   数理科学研究科棟(駒場) 056号室
Tea: 16:00 - 16:30 コモンルーム

On the rational string operations of classifying spaces and the
Hochschild cohomology (JAPANESE)
[ 講演概要 ]
Chataur and Menichi initiated the theory of string topology of
classifying spaces.
In particular, the cohomology of the free loop space of a classifying
space is endowed with a product
called the dual loop coproduct. In this talk, I will discuss the
algebraic structure and relate the rational dual loop coproduct to the
cup product on the Hochschild cohomology via the Van den Bergh isomorphism.

#### PDE実解析研究会

10:30-11:30   数理科学研究科棟(駒場) 056号室

An application of weighted Hardy spaces to the Navier-Stokes equations (JAPANESE)
[ 講演概要 ]
The purpose of this talk is to investigate decay orders of the L^2 energy of solutions to the incompressible homogeneous Navier-Stokes equations on the whole spaces by the aid of the theory of weighted Hardy spaces. The main estimates are two weighted inequalities for heat semigroup on weighted Hardy spaces and a weighted version of the div-curl lemma due to Coifman-Lions-Meyer-Semmes. It turns out that because of the use of weighted Hardy spaces, our decay orders of the energy can be close to the critical one of Wiegner.

#### Lie群論・表現論セミナー

16:30-18:00   数理科学研究科棟(駒場) 126号室

Toward the graded Cartan invariants of the symmetric groups (JAPANESE)
[ 講演概要 ]
We propose a graded analog of Hill's conjecture which is equivalent to K\\"ulshammer-Olsson-Robinson's conjecture on the generalized Cartan invariants of the symmetric groups.
We give justifications for it and discuss implications between the variants.
Some materials are based on the joint work with Anton Evseev.

### 2014年04月14日(月)

#### 複素解析幾何セミナー

10:30-12:00   数理科学研究科棟(駒場) 126号室

### 2014年04月10日(木)

#### 幾何コロキウム

10:00-11:30   数理科学研究科棟(駒場) 122号室

[ 講演概要 ]
Cayley 部分多様体は、Spin(7) 多様体内の極小部分多様体で Harvey, Lawson により導入された calibrated submanifold の一種である。calibrated submanifold の変形は、最初 Mclean により研究された。彼はコンパクトな場合を調べ、現在では錐、漸近的に錐な場合など非コンパクトな場合への拡張が試みられている。Cayley 錐の変形のモジュライ空間は、一般には滑らかでないことが知られているが、本講演では、R^8の等質 Cayley 錐の場合に着目し、具体的に変形の様子を調べることを考える。

### 2014年04月09日(水)

#### 作用素環セミナー

16:30-18:00   数理科学研究科棟(駒場) 122号室
Ryszard Nest 氏 (Univ. Copenhagen)
Index and determnant of n-tuples of commuting operators (ENGLISH)

### 2014年04月08日(火)

#### トポロジー火曜セミナー

16:30-18:00   数理科学研究科棟(駒場) 056号室
Tea: 16:00 - 16:30 コモンルーム

On the number of commensurable fibrations on a hyperbolic 3-manifold. (JAPANESE)
[ 講演概要 ]
By work of Thurston, it is known that if a hyperbolic fibred
$3$-manifold $M$ has Betti number greater than 1, then
$M$ admits infinitely many distinct fibrations.
For any fibration $\\omega$ on a hyperbolic $3$-manifold $M$,
the number of fibrations on $M$ that are commensurable in the sense of
Calegari-Sun-Wang to $\\omega$ is known to be finite.
In this talk, we prove that the number can be arbitrarily large.

#### 統計数学セミナー

13:00-14:10   数理科学研究科棟(駒場) 052号室
Alexandre Brouste 氏 (Universite du Maine, France)
Parametric estimation in fractional Ornstein-Uhlenbeck process (ENGLISH)
[ 講演概要 ]
Several statistical models that imply the fractional Ornstein-Uhlenbeck (fOU) process will be presented: direct observations of the process or partial observations in an additive independent noise, continuous observations or discrete observations. In this different settings, we exhibit large sample (or high-frequency) asymptotic properties of the estimators (maximum likelihood estimator, quadratic variation based estimator, moment estimator, …) for all parameters of interest of the fOU. We also illustrate our results with the R package yuima.
[ 講演参考URL ]
http://www.sigmath.es.osaka-u.ac.jp/~kamatani/statseminar/2014/00.html

### 2014年03月19日(水)

#### 古典解析セミナー

16:00-17:00   数理科学研究科棟(駒場) 128号室
Anton Dzhamay 氏 (University of Northern Colorado)
Discrete Schlesinger Equations and Difference Painlevé Equations (ENGLISH)
[ 講演概要 ]
The theory of Schlesinger equations describing isomonodromic
dynamic on the space of matrix coefficients of a Fuchsian system
w.r.t.~continuous deformations is well-know. In this talk we consider
a discrete version of this theory. Discrete analogues of Schlesinger
deformations are Schlesinger transformations that shift the eigenvalues
of the coefficient matrices by integers. By discrete Schlesinger equations
we mean the evolution equations on the matrix coefficients describing
such transformations. We derive these equations, show how they can be
split into the evolution equations on the space of eigenvectors of the
coefficient matrices, and explain how to write the latter equations in
the discrete Hamiltonian form. We also consider some reductions of those
equations to the difference Painlevé equations, again in complete parallel
to the differential case.

This is a joint work with H. Sakai (the University of Tokyo) and
T.Takenawa (Tokyo Institute of Marine Science and Technology).

### 2014年03月14日(金)

#### GCOEセミナー

16:00-16:50   数理科学研究科棟(駒場) 118号室
Kazufumi Ito 氏 (North Carolina State Univ.)
A new finite difference scheme based on staggered grids for Navier Stokes equations (ENGLISH)
[ 講演概要 ]
We develop a new method that uses the staggered grid only for the pressure node, i.e., the pressure gird is the center of the square cell and the velocities are at the node. The advantage of the proposed method compared to the standard staggered grid methods is that it is very straight forward to treat the boundary conditions for the velocity field, the fluid structure interaction, and to deal with the multiphase flow using the immersed interface methods. We present our analysis and numerical tests.

#### GCOEセミナー

17:00-17:50   数理科学研究科棟(駒場) 118号室
Jun Zou 氏 (The Chinese University of Hong Kong)
Efficient Domain Decomposition Methods for a Class of Linear and Nonlinear Inverse Problems (ENGLISH)
[ 講演概要 ]
In this talk we shall present several new domain decomposition methods for solving some linear and nonlinear inverse problems. The motivations and derivations of the methods will be discussed, and numerical experiments will be demonstrated.

### 2014年03月13日(木)

#### 講演会

10:15-11:45   数理科学研究科棟(駒場) 470号室
Michele Triestino 氏 (Ecole Normale Superieure de Lyon)
Almost sure triviality of the $C^1$-centralizer of random circle diffeomorphisms with periodic points (ENGLISH)
[ 講演概要 ]
By the end of the 80s, Malliavin and Shavgulidze introduced a measure on the space of C^1 circle diffeomorphisms which carries many interesting features. Perhaps the most interesting aspect is that it can be considered as an analog of the Haar measure for the group Diff^1_+(S^1).
The nature of this measure has been mostly investigated in connection to representation theory.
For people working in dynamical systems, the MS measure offers a way to quantify dynamical phenomena: for example, which is the probability that a random diffeomorphism is irrational? Even if this question have occupied my mind for a long time, it remains still unanswered, as many other interesting ones. However, it is possible to understand precisely what are the typical features of a diffeomorphism with periodic points.

#### GCOEセミナー

17:00-18:00   数理科学研究科棟(駒場) 122号室
STABILITY IN THE OBSTACLE PROBLEM FOR A SHALLOW SHELL (ENGLISH)
[ 講演参考URL ]
http://fmsp.ms.u-tokyo.ac.jp/bm02.pdf

### 2014年03月12日(水)

#### 講演会

10:15-11:45   数理科学研究科棟(駒場) 470号室
Michele Triestino 氏 (Ecole Normale Superieure de Lyon)
Invariant distributions for circle diffeomorphisms of
irrational rotation number and low regularity (ENGLISH)
[ 講演概要 ]
The main inspiration of this joint work with Andrés Navas is the beautiful result of Ávila and Kocsard: if f is a C^\\infty circle diffeomorphism of irrational rotation number, then the unique invariant probability measure is also the unique (up to rescaling) invariant distribution.
Using conceptual geometric arguments (Hahn-Banach...), we investigate the uniqueness of invariant distributions for C^1 circle diffeomorphisms of irrational rotation number, with particular attention to sharp regularity.
We prove that If the diffeomorphism is C^{1+bv}, then there is a unique invariant distribution of order 1. On the other side, examples by Douady and Yoccoz, and by Kodama and Matsumoto exhibit differentiable dynamical systems for which the uniqueness does not hold.

#### 数理人口学・数理生物学セミナー

15:00-17:00   数理科学研究科棟(駒場) 126号室
Andre M. de Roos 氏 (University of Amsterdam)
When size does matter: Ontogenetic symmetry and asymmetry in energetics
(ENGLISH)
[ 講演概要 ]
Body size (≡ biomass) is the dominant determinant of population dynamical processes such as giving birth or dying in almost all species, with often drastically different behaviour occurring in different parts of the growth trajectory, while the latter is largely determined by food availability at the different life stages. This leads to the question under what conditions unstructured population models, formulated in terms of total population biomass, still do a fair job. To contribute to answering this question we first analyze the conditions under which a size-structured model collapses to a dynamically equivalent unstructured one in terms of total biomass. The only biologically meaningful case where this occurs is when body size does not affect any of the population dynamic processes, this is the case if and only if the mass-specific ingestion rate, the mass-specific biomass production and the mortality rate of the individuals are independent of size, a condition to which we refer as “ontogenetic symmetry”. Intriguingly, under ontogenetic symmetry the equilibrium biomass-body size spectrum is proportional to 1/size, a form that has been conjectured for marine size spectra and subsequently has been used as prior assumption in theoretical papers dealing with the latter. As a next step we consider an archetypical class of models in which reproduction takes over from growth upon reaching an adult body size, in order to determine how quickly discrepancies from ontogenetic symmetry lead to relevant novel population dynamical phenomena. The phenomena considered are biomass overcompensation, when additional imposed mortality leads, rather unexpectedly, to an increase in the equilibrium biomass of either the juveniles or the adults (a phenomenon with potentially big consequences for predators of the species), and the occurrence of two types of size-structure driven oscillations, juvenile-driven cycles with separated extended cohorts, and adult-driven cycles in which periodically a front of relatively steeply decreasing frequencies moves up the size distribution. A small discrepancy from symmetry can already lead to biomass overcompensation; size-structure driven cycles only occur for somewhat larger discrepancies.
[ 講演参考URL ]
http://staff.science.uva.nl/~aroos/

#### GCOEセミナー

15:00-17:00   数理科学研究科棟(駒場) 126号室
Andre M. de Roos 氏 (University of Amsterdam)
When size does matter: Ontogenetic symmetry and asymmetry in energetics (ENGLISH)
[ 講演概要 ]
Body size (≡ biomass) is the dominant determinant of population dynamical processes such as giving birth or dying in almost all species, with often drastically different behaviour occurring in different parts of the growth trajectory, while the latter is largely determined by food availability at the different life stages. This leads to the question under what conditions unstructured population models, formulated in terms of total population biomass, still do a fair job. To contribute to answering this question we first analyze the conditions under which a size-structured model collapses to a dynamically equivalent unstructured one in terms of total biomass. The only biologically meaningful case where this occurs is when body size does not affect any of the population dynamic processes, this is the case if and only if the mass-specific ingestion rate, the mass-specific biomass production and the mortality rate of the individuals are independent of size, a condition to which we refer as “ontogenetic symmetry”. Intriguingly, under ontogenetic symmetry the equilibrium biomass-body size spectrum is proportional to 1/size, a form that has been conjectured for marine size spectra and subsequently has been used as prior assumption in theoretical papers dealing with the latter. As a next step we consider an archetypical class of models in which reproduction takes over from growth upon reaching an adult body size, in order to determine how quickly discrepancies from ontogenetic symmetry lead to relevant novel population dynamical phenomena. The phenomena considered are biomass overcompensation, when additional imposed mortality leads, rather unexpectedly, to an increase in the equilibrium biomass of either the juveniles or the adults (a phenomenon with potentially big consequences for predators of the species), and the occurrence of two types of size-structure driven oscillations, juvenile-driven cycles with separated extended cohorts, and adult-driven cycles in which periodically a front of relatively steeply decreasing frequencies moves up the size distribution. A small discrepancy from symmetry can already lead to biomass overcompensation; size-structure driven cycles only occur for somewhat larger discrepancies.
[ 講演参考URL ]
http://staff.science.uva.nl/~aroos/

### 2014年03月11日(火)

#### GCOEセミナー

17:00-18:00   数理科学研究科棟(駒場) 118号室
Lucie Baudouin 氏 (LAAS-CNRS, equipe MAC)
Inverse problem for the waves : stability and convergence matters (ENGLISH)
[ 講演概要 ]
This talk aims to present some recent works in collaboration with Maya de Buhan, Sylvain Ervedoza and Axel Osses regarding an inverse problem for the wave equation. More specifically, we study the determination of the potential in a wave equation with given Dirichlet boundary data from a measurement of the flux of the solution on a part of the boundary. On the one hand, we will focus on the question of convergence of the space semi-discrete inverse problems toward their continuous counterpart. Several uniqueness and stability results are available in the literature about the continuous setting of the inverse problem of determination of a potential in the wave equation. In particular, we can mention a Lipschitz stability result under a classical geometric condition obtained by Imanuvilov and Yamamoto, and a logarithmic stability result obtained by Bellassoued when the observation measurement is made on an arbitrary part of the boundary. In both situations, we can design a numerical process for which convergence results are proved. The analysis we conduct is based on discrete Carleman estimates, either for the hyperbolic or for the elliptic operator, in which case we shall use a result of Boyer, Hubert and Le Rousseau. On the other hand, still considering the same inverse problem, we will present a new reconstruction algorithm of the potential. The design and convergence of the algorithm are based on the Carleman estimates for the waves previously used to prove the Lipschitz stability. We will finally give some simple illustrative numerical simulations for 1-d problems.

### 2014年03月10日(月)

#### GCOEセミナー

13:30-15:00   数理科学研究科棟(駒場) 123号室
Erwin Bolthausen 氏 (University of Zurich)
The two-dimensional random walk in an isotropic random environment (ENGLISH)

#### 講演会

15:15-16:45   数理科学研究科棟(駒場) 123号室
Marielle Simon 氏 (ENS Lyon, UMPA)
Energy fluctuations in the disordered harmonic chain (ENGLISH)
[ 講演概要 ]
We study the energy diffusion in the disordered harmonic chain of oscillators: the usual Hamiltonian dynamics is provided with random masses and perturbed by a degenerate energy conserving noise. After rescaling space and time diffusively, we prove that energy fluctuations evolve following an infinite dimensional linear stochastic differential equation driven by the linearized heat equation. We also give variational expressions for the thermal diffusivity and an equivalent definition through the Green-Kubo formula. Since the model is non gradient, and the perturbation is very degenerate, the standard Varadhan's approach is reviewed under new perspectives.

#### GCOEセミナー

17:00-18:00   数理科学研究科棟(駒場) 118号室