過去の記録

過去の記録 ~05/21本日 05/22 | 今後の予定 05/23~

2015年12月08日(火)

トポロジー火曜セミナー

17:00-18:30   数理科学研究科棟(駒場) 056号室
Tea : Common Room 16:30 -- 17:00
山田 裕一 氏 (電気通信大学)
レンズ空間手術と4次元多様体の Kirby calculus (JAPANESE)
[ 講演概要 ]
「3次元球面内の結び目に沿うデーン手術でレンズ空間が生じるもの
を決定せよ」という問題は「レンズ空間手術」と呼ばれています。Berge のリス
ト(1990) が完全なリストと信じられており Heegaard Floer 理論によって進展
はしたものの、解決には至っていません。手法が4次元多様体論に近づいていま
す。その一方 Minimally twisted 5 chain link の例外的デーン手術が再確認さ
れて、レンズ空間からのレンズ空間手術や2成分絡み目に視野が広がったりして
います。
 講演では、Berge のリストの多様さと規則性を紹介しつつ、異なる結び目から
同じレンズ空間が生じる組で構成する4次元多様体(丹下基生氏(筑波大)との
共同研究)についてお話しします。

2015年12月07日(月)

東京確率論セミナー

16:50-18:20   数理科学研究科棟(駒場) 128号室
Jean-Dominique Deuschel 氏 (TU Berlin)
Quenched invariance principle for random walks in time-dependent balanced random environment
[ 講演概要 ]
We prove an almost sure functional limit theorem for a random walk in an space-time ergodic balanced environment under certain moment conditions. The proof is based on the maximal principle for parabolic difference operators. We also deal with the non-elliptic case, where the corresponding limiting diffusion matrix can be random in higher dimensions. This is a joint work with N. Berger, X. Guo and A. Ramirez.

複素解析幾何セミナー

10:30-12:00   数理科学研究科棟(駒場) 128号室
巴山 竜来 氏 (専修大学)
Cycle connectivity and pseudoconcavity of flag domains (Japanese)
[ 講演概要 ]
We consider an open real group orbit in a complex flag variety which has no non-constant function. We introduce Huckleberry's results on cycle connectivity and show that it is pseudoconcave if it satisfies a certain condition on the root system of the Lie algebra. In Hodge theory, we are mainly interested in the case where it is a Mumford-Tate domain. We also discuss Hodge theoretical meanings of this work.

代数幾何学セミナー

15:30-17:00   数理科学研究科棟(駒場) 122号室
Alexey Bondal 氏 (IPMU)
Flops and spherical functors (English)
[ 講演概要 ]
I will describe various functors on derived categories of coherent sheaves
related to flops and relations between these functors. A categorical
version of deformation theory of systems of objects in abelian categories
will be outlined and its relation to flop spherical functors will be
presented.

2015年12月04日(金)

談話会・数理科学講演会

16:50-17:50   数理科学研究科棟(駒場) 123号室
佐々田槙子 氏 (東京大学大学院数理科学研究科)
確率論の問題に現れる無限直積空間上の完全形式と閉形式について (JAPANESE)
[ 講演概要 ]
ミクロな系の時間発展を表す確率過程からマクロな系の時間発展を表す偏微分方程式を導出する流体力学極限は統計力学を基礎付ける重要なスケール極限の一種である。その証明において、無限直積空間上の微分1形式(もしくはそれに対応するもの)を考え、閉形式の空間を完全形式の空間とその直交空間に分解するという定理が重要な役割を果たしている。この定理は確率過程に対応する(微分)作用素ごとに証明されてきたが、既存の手法は代数的な内容と解析的な内容が混在した複雑なものであった。しかし、講演者らは最近、この問題を代数的な問題と解析的な問題に分け、代数的な部分については、モデルの詳細によらずに非常に一般的に成立することを明らかにした。また、解析的な問題についても、$L^2$空間が作用素とよい関係を持つ基底を持つ場合には、直感的でシンプルな証明が得られることを示した。しかし、この周辺の話題については問題の定式化も含めて未解明な部分が多く、代数や幾何、関数解析等の専門家からの助言が大きな進展をもたらす可能性があると考えている。そこで、講演では問題の背景にはあまり触れずに、無限直積空間上のある種の1形式に関する話題に絞って紹介したい。特に、{0,1}の無限直積に対する具体的な問題を中心に紹介する。
[ 講演参考URL ]
http://www.ms.u-tokyo.ac.jp/teacher/sasada.html

幾何コロキウム

10:00-11:30   数理科学研究科棟(駒場) 126号室
竹山 美宏 氏 (筑波大学)
q-ボゾン系とアフィンヘッケ代数の変形 (Japanese)
[ 講演概要 ]
q-ボゾン系は笹本-和達によって定義された一次元可積分確率過程である. その Q行列は量子逆散乱法により構成できて, 代数的ベーテ仮設法を適用することで固有ベクトルが得られる. 最近の研究で, q-ボゾン系はアフィンヘッケ代数のある変形の表現からも得られることがわかった. この枠組みでは, Q行列の転置に対する固有ベクトルが自然に得られ, 座標的ベーテ仮設法で構成されたものと一致する. この講演では, 以上の結果について概説し, これら二つの方法の関係について議論する.

2015年12月03日(木)

統計数学セミナー

16:40-18:00   数理科学研究科棟(駒場) 123号室
本講演は,数物フロンティア・リーディング大学院のFMSPレクチャーズとして行います.
Arnak Dalalyan 氏 (ENSAE ParisTech)
Learning theory and sparsity ~ Sparsity and low rank matrix learning ~
[ 講演概要 ]
In this third lecture, we will present extensions of the previously introduced sparse recovery techniques to the problems of machine learning and statistics in which a large matrix should be learned from data. The analogue of the sparsity, in this context, is the low-rankness of the matrix. We will show that such matrices can be effectively learned by minimizing the empirical risk penalized by the nuclear norm. The resulting problem is a problem of semi-definite programming and can be solved efficiently even when the dimension is large. Theoretical guarantees for this method will be established in the case of matrix completion with known sampling distribution.

FMSPレクチャーズ

16:40-18:00   数理科学研究科棟(駒場) 123号室
"Learning theory and sparsity" 全3回講演の(3)
Arnak Dalalyan 氏 (ENSAE ParisTech)
(3)Sparsity and low rank matrix learning. (ENGLISH)
[ 講演概要 ]
In this third lecture, we will present extensions of the previously introduced sparse recovery techniques to the problems of machine learning and statistics in which a large matrix should be learned from data. The analogue of the sparsity, in this context, is the low-rankness of the matrix. We will show that such matrices can be effectively learned by minimizing the empirical risk penalized by the nuclear norm. The resulting problem is a problem of semi-definite programming and can be solved efficiently even when the dimension is large. Theoretical guarantees for this method will be established in the case of matrix completion with known sampling distribution.
[ 講演参考URL ]
http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Dalalyan.pdf

2015年12月02日(水)

作用素環セミナー

16:45-18:15   数理科学研究科棟(駒場) 118号室
山下真 氏 (お茶の水女子大)
Drinfeld center and representation theory for monoidal categories

統計数学セミナー

14:55-18:00   数理科学研究科棟(駒場) 056号室
本講演は,数物フロンティア・リーディング大学院のFMSPレクチャーズとして行います.
Arnak Dalalyan 氏 (ENSAE ParisTech)
Learning theory and sparsity ~ Lasso, Dantzig selector and their statistical properties ~
[ 講演概要 ]
In this second lecture, we will focus on the problem of high dimensional linear regression under the sparsity assumption and discuss the three main statistical problems: denoising, prediction and model selection. We will prove that convex programming based predictors such as the lasso and the Dantzig selector are provably consistent as soon as the dictionary elements are normalized and an appropriate upper bound on the noise-level is available. We will also show that under additional assumptions on the dictionary elements, the aforementioned methods are rate-optimal and model-selection consistent.

FMSPレクチャーズ

14:55-18:00   数理科学研究科棟(駒場) 056号室
"Learning theory and sparsity" 全3回講演の(2)
Arnak Dalalyan 氏 (ENSAE ParisTech)
(2)Lasso, Dantzig selector and their statistical properties. (ENGLISH)
[ 講演概要 ]
In this second lecture, we will focus on the problem of high dimensional linear regression under the sparsity assumption and discuss the three main statistical problems: denoising, prediction and model selection. We will prove that convex programming based predictors such as the lasso and the Dantzig selector are provably consistent as soon as the dictionary elements are normalized and an appropriate upper bound on the noise-level is available. We will also show that under additional assumptions on the dictionary elements, the aforementioned methods are rate-optimal and model-selection consistent.
[ 講演参考URL ]
http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Dalalyan.pdf

数理人口学・数理生物学セミナー

14:55-16:40   数理科学研究科棟(駒場) 128号室
八島健太 氏 (総合研究大学院大学)
基本増殖率の感受性解析を用いたネットワーク中心性指標
[ 講演概要 ]
ネットワーク上における感染症流行を阻止するために,中心性指標を用いた
危険箇所の同定が行われている.感染症侵入の有無は基本増殖率R0により規定さ
れることから,我々は基本増殖率の感受性解析を用いた新たなネットワーク中心
性指標の提案を行った.これを用いて東京都市圏における感染症流行の解析を行
ったところ,既存の中心性指標では見落とされてきた流行動態を明らかにできた.
利用者数最大の新宿駅は2位の東京駅の1.5倍程度の利用者数規模であるが,隔離
政策を行った際の基本増殖率低減への寄与率が約1,000倍も大きいことが分かった.
また,侵入阻止(基本増殖率低下)のために注力すべき箇所と,侵入が起こった
さいに被害低減(最終規模低下)のために注力すべき箇所が必ずしも一致しない
ことが分かった.本講演では,提案したネットワーク中心性指標の紹介および上
記の結果を紹介したい.

[ 講演参考URL ]
http://www.soken.ac.jp/

2015年12月01日(火)

解析学火曜セミナー

16:50-18:20   数理科学研究科棟(駒場) 126号室
Stéphane Malek 氏 (Université de Lille, France)
On complex singularity analysis for some linear partial differential equations
[ 講演概要 ]
We investigate the existence of local holomorphic solutions Y of linear partial differential equations in three complex variables whose coefficients are holomorphic on some polydisc outside some singular set S. The coefficients are written as linear combinations of powers of a solution X of some first order nonlinear partial differential equation following an idea :we have initiated in a previous joint work with C. Stenger. The solutions Y are shown to develop singularities along the singular set S with estimates of exponential type depending on the growth's rate of X near the singular set. We construct these solutions with the help of series of functions with infinitely many variables which involve derivatives of all orders of X in one variable. Convergence and bounds estimates of these series are studied using a majorant series method which leads to an auxiliary functional equation that contains differential operators in infinitely many variables. Using a fixed point argument, we show that these functional equations actually have solutions in some Banach spaces of formal power series. (Joint work with A. Lastra and C. Stenger).

トポロジー火曜セミナー

17:00-18:30   数理科学研究科棟(駒場) 056号室
Tea : Common Room 16:30 -- 17:00
奥田 喬之 氏 (東京大学大学院数理科学研究科)
Monodromies of splitting families for singular fibers (JAPANESE)
[ 講演概要 ]
A degeneration of Riemann surfaces is a family of complex curves
over a disk allowed to have a singular fiber.
A singular fiber may split into several simpler singular fibers
under a deformation family of such families,
which is called a splitting family for the singular fiber.
We are interested in the topology of splitting families.
For the topological types of degenerations of Riemann surfaces,
it is known that there is a good relationship with
the surface mapping classes, via topological monodromy.
In this talk,
we introduce the "topological monodromies of splitting families",
and give a description of those of certain splitting families.

2015年11月30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) 128号室
Jean-Pierre Demailly 氏 (Univ. de Grenoble I)
Extension of holomorphic functions defined on non reduced analytic subvarieties (English)
[ 講演概要 ]
The goal of this talk will be to discuss $L^2$ extension properties of holomorphic sections of vector bundles satisfying weak semi-positivity properties. Using techniques borrowed from recent proofs of the Ohsawa-Takegoshi extension theorem, we obtain several new surjectivity results for the restriction morphism to a non necessarily reduced subvariety, provided the latter is defined as the zero variety of a multiplier ideal sheaf. These extension results are derived from $L^2$ approximation techniques, and they hold under (probably) optimal curvature conditions.

代数幾何学セミナー

15:30-17:00   数理科学研究科棟(駒場) 122号室
Fabrizio Catanese 氏 (Universität Bayreuth)
Interesting surfaces which are coverings of a rational surface branched over few lines (English)
[ 講演概要 ]
Surfaces which are covers of the plane branched over 5 or 6 lines have provided answers to long standing questions, for instance the BCD surfaces for Fujita's question on semiampleness of VHS (Dettweiler-Cat); and examples of ball quotients (Hirzebruch), automorphisms acting trivially on integral cohomology (Cat-Gromadtzki), canonical maps with high degree or image-degree (Pardini, Bauer-Cat). I shall speak especially about the above Abelian coverings of the plane, the geometry of the del Pezzo surface of degree 5, the rigidity of BCD surfaces, and a criterion for a fibred surface to be a projective classifying space.

東京確率論セミナー

16:50-18:20   数理科学研究科棟(駒場) 128号室
Raoul Normand 氏 (中央研究院 數學研究所)
Self-organized criticality in a discrete model of limited aggregation
[ 講演概要 ]
We consider a discrete model of coagulation, where a large number of particles are initially given a prescribed number of arms. We successively choose arms uniformly at random and bind them two by two, unless they belong to "large" clusters. In that sense, the large clusters are frozen and become inactive. We study the graph structure obtained, and describe what a typical cluster looks like. We show that there is a fixed time T such that, before time T, a typical cluster is a subcritical Galton-Watson tree, whereas after time T, a typical cluster is a critical Galton-Watson tree. In that sense, we observe a phenomenon called self-organized criticality.

2015年11月27日(金)

談話会・数理科学講演会

16:50-17:50   数理科学研究科棟(駒場) 056号室
木田良才 氏 (東京大学大学院数理科学研究科)
従順群に関する最近の進展について (JAPANESE)
[ 講演概要 ]
群の従順性は、Banach-Tarski のパラドックスを理解する上で von Neumann により導入された概念である。その過程で未解決となった問題の一つが、非可換な自由群を含まない非従順群の存在を問うものであり、これは後に von Neumann-Day の問題と呼ばれるようになる。1980年頃にそのような群が構成されこの問題は解決されたが、最近 Nicolas Monod によりそのような群の例で全く異なるタイプのものが発見された。この新しい例は、区分的に PSL_2(R) の元であるような円周上の同相写像から成る群であり、従来のものに比べると格段に扱いやすいという利点をもっている。また、その非従順性の証明は群作用の従順性を応用するという新たな手法に基づいている。講演では、従順群の紹介からはじめ、非従順性の証明やその背景を中心として Monod の例を紹介したい。
[ 講演参考URL ]
http://www.ms.u-tokyo.ac.jp/~kida/

幾何コロキウム

10:00-11:30   数理科学研究科棟(駒場) 126号室
久本智之 氏 (名古屋大学)
Kエネルギーの coercivity に由来する安定性 (Japanese)
[ 講演概要 ]
S. Boucksom氏とM. Jonsson氏との共同研究について話します。Kエネルギーのcoercivityという有名な増大度条件があります。我々はこのcoercivity に対応するJ一様K安定性の概念を導入し、J一様K安定性が従来のK安定性よりも真に強い定義でありながら、Kähler-Einstein多様体などについて実際に成り立つことを示しました。さらに最近Berman-Boucksom-Jonssonによって、J一様K安定性から出発すればCheeger-Coldingの非崩壊理論を用いることなくKähler-Einstein計量を得られることが明らかにされました。時間が許せばこのあたりについても解説したいと思います。

2015年11月26日(木)

Lie群論・表現論セミナー

17:00-18:45   数理科学研究科棟(駒場) 号室
Birgit Speh 氏 (Cornell University)
Introduction to the cohomology of discrete groups and modular symbols 2 (English)
[ 講演概要 ]
The course is an introduction to the cohomology of torsion free discrete subgroups $\Gamma \subset G $ of a semi simple group $G$. The discrete group $\Gamma$ acts freely on the symmetric space $X= G/K$ and we will always assume that $\Gamma \backslash G/K$ is compact or has finite volume. An example is a torsion free subgroup $\Gamma_n $ of finite index n in Sl(2,Z) acting on $Sl(2.R)/SO(2) \simeq {\mathcal H}=\{z=x+iy \in C| y >0 \}$ by fractional linear transformations. $\Gamma_n \backslash {\mathcal H}$ can be determined explicitly and it can be visualized as an area in the upper half plane glued at the boundary. It is easy to see that it has some nice compactifications.

The cohomology $H^*(\Gamma, C)$ of the group $\Gamma$ is equal to the deRham cohomology $H^*_{deRham}(\Gamma \backslash X, C)$ of the manifold $\Gamma\backslash X$. This cohomology is studied by proving that it is isomorphic to the $H^*(g,K,{\mathcal A}(\Gamma \backslash G))$. Here ${\mathcal A}(\Gamma \backslash G)$ of automorphic functions on $\Gamma \backslash G$. In the case $\Gamma_n \subset Sl(2,Z)$ the space ${\mathcal A}(\Gamma \backslash G)$ is the space of classical automorphic functions on the upper half plane containing holomorphic cusp form, Eisenstein series, Maass forms and it is often introduced in an introductory course in analytic number theory.


On the geometric side we will construct some of the cycles (modular symbols) in the homology $H_*(\Gamma\backslash X)$ which are dual to the cohomology classes we constructed. In our example $\Gamma_n\backslash Sl(2,R)/SO(2)$ these cycles correspond to geodesics and can easily be visualized.


In this course I will explain these results and show how to use them to prove vanishing and non vanishing theorem for $H^*_{deRham}(\Gamma \backslash X)$. I will state the results in full generality, but I will prove them only in the classical case: G=SL$(2,R)$ and the subgroup $\Gamma= \Gamma_n$ a congruence subgroup. Some familiarity with Lie groups and Lie algebras is only prerequisite for the course.

2015年11月25日(水)

作用素環セミナー

16:45-18:15   数理科学研究科棟(駒場) 118号室
Max Lein 氏 (東北大AIMR)
Combining Pseudodifferential and Vector Bundle Techniques, and Their Applications to Topological Insulators

統計数学セミナー

14:55-18:00   数理科学研究科棟(駒場) 056号室
本講演は,数物フロンティア・リーディング大学院のFMSPレクチャーズとして行います.
Arnak Dalalyan 氏 (ENSAE ParisTech)
Learning theory and sparsity ~ Introduction into sparse recovery and compressed sensing ~
[ 講演概要 ]
In this introductory lecture, we will present the general framework of high-dimensional statistical modeling and its applications in machine learning and signal processing. Basic methods of sparse recovery, such as the hard and the soft thresholding, will be introduced in the context of orthonormal dictionaries and their statistical accuracy will be discussed in detail. We will also show the relation of these methods with compressed sensing and convex programming based procedures.

FMSPレクチャーズ

14:55-18:00   数理科学研究科棟(駒場) 056号室
"Learning theory and sparsity" 全3回講演の(1)
Arnak Dalalyan 氏 (ENSAE ParisTech)
(1)Introduction into sparse recovery and compressed sensing. (ENGLISH)
[ 講演概要 ]
In this introductory lecture, we will present the general framework of high-dimensional statistical modeling and its applications in machine learning and signal processing. Basic methods of sparse recovery, such as the hard and the soft thresholding, will be introduced in the context of orthonormal dictionaries and their statistical accuracy will be discussed in detail. We will also show the relation of these methods with compressed sensing and convex programming based procedures.
[ 講演参考URL ]
http://fmsp.ms.u-tokyo.ac.jp/FMSPLectures_Dalalyan.pdf

2015年11月24日(火)

解析学火曜セミナー

16:50-18:20   数理科学研究科棟(駒場) 126号室
許 本源 氏 (東大数理)
A local analysis of the swirling flow to the axi-symmetric Navier-Stokes equations near a saddle point and no-slip flat boundary (English)
[ 講演概要 ]
As one of the violent flow, tornadoes occur in many place of the world. In order to reduce human losses and material damage caused by tornadoes, there are many research methods. One of the effective methods is numerical simulations.  The swirling structure is significant both in mathematical analysis and the numerical simulations of tornado. In this joint work with H. Notsu and T. Yoneda we try to clarify the swirling structure. More precisely, we do numerical computations on axi-symmetric Navier-Stokes flows with no-slip flat boundary. We compare a hyperbolic flow with swirl and one without swirl and observe that the following phenomenons occur only in the swirl case: The distance between the point providing the maximum velocity magnitude $|v|$ and the $z$-axis is drastically changing around some time (which we call it turning point). An ``increasing velocity phenomenon'' occurs near the boundary and the maximum value of $|v|$ is obtained near the axis of symmetry and the boundary when time is close to the turning point.

トポロジー火曜セミナー

17:00-18:30   数理科学研究科棟(駒場) 056号室
Tea : Common Room 16:30 -- 17:00
佐藤 正寿 氏 (東京電機大学)
On the cohomology ring of the handlebody mapping class group of genus two (JAPANESE)
[ 講演概要 ]
The genus two handlebody mapping class group acts on a tree
constructed by Kramer from the disk complex,
and decomposes into an amalgamated product of two subgroups.
We determine the integral cohomology ring of the genus two handlebody
mapping class group by examining these two subgroups
and the Mayer-Vietoris exact sequence.
Using this result, we estimate the ranks of low dimensional homology
groups of the genus three handlebody mapping class group.

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