iTHEMS Math Seminar
90 events

Seminar
Longtime behavior of moving solids in a fluid and the kinetic theory of gases
April 7 (Wed) at 16:00  18:10, 2021
Kai Koike (JSPS Fellow, Graduate School of Engineering, Kyoto University)
Understanding dynamics of solids in a fluid is a fundamental problem in fluid dynamics. Due to the growing interest in engineering in outofequilibrium situations, moving boundary problems for kinetic equations such as the Boltzmann equation have become an active area of research. In the first part of the talk, I shall explain recent, especially mathematical, developments in this field. Then in the second part, I'd like to explain my results concerning the longtime behavior of a point particle moving in a 1D viscous compressible fluid. These results aim to give some explanation of related numerical simulations for a BGK model of the Boltzmann equation.
Venue: via Zoom
Event Official Language: English

Seminar
The GreenTao theorem for number fields
March 22 (Mon) at 16:00  18:10, 2021
Wataru Kai (Assistant Professor, Mathematical Institute, Tohoku University)
5, 11, 17, 23, 29 are prime numbers which form an arithmetic progression of length 5. A famous theorem of Ben Green and Terence Tao in 2008 says there are arbitrarily long arithmetic progressions of prime numbers. Algebraic number theorists are also interested in more general numbers like square roots of integers. Recently, Mimura, Munemasa, Seki, Yoshino and I have established a generalization of the GreenTao theorem in such a direction. In the first 50 minutes of my talk, I would like to explain some background and technology behind the GreenTao theorem. In the second half after a break, I explain the concept of number fields to formulate our generalization of their result. I will also discuss how one of the new difficulties, which I call the norm vs length conflict, is handled by a technique called Geometry of Numbers. *Please contact Keita Mikami or Hiroyasu Miyazaki's mailing address to get access to the Zoom meeting room.
Venue: via Zoom
Event Official Language: English

Seminar
Scattering theory for halfline Schrödinger operators: analytic and topological results
December 7 (Mon) at 16:00  18:10, 2020
Hideki Inoue (Nagoya University)
Levinson’s theorem is a surprising result in quantum scattering theory, which relates the number of bound states and the scattering part of the underlying quantum system. For the last about ten years, it has been proved for several models that once recast in an operator algebraic framework this relation can be understood as an index theorem for the Møller wave operators. Resulting index theorems are called topological version of Levinson’s theorem or shortly topological Levinson’s theorem. In this talk, we first review the background and the framework of our investigation. New analytical and topological results are provided for Schrödinger operators on the halfline. This talk is based on my Ph.D thesis.
Venue: via Zoom
Event Official Language: English

Flat and spherical surface approximations
November 30 (Mon) at 16:00  17:30, 2020
Martin Skrodzki (Visiting Scientist, iTHEMS / Fellow, German Academic Scholarship Foundation, Germany)
Stateoftheart acquisition devices produce surface representations of increasingly high resolution. While these detailed representations are important for production, they are problematic e.g. when exchanging drafts via the internet or when a quick rendering for comparison is necessary. In the first part of the talk, I will present results and further research questions from a paper I recently coauthored on 'Variational Shape Approximation'. This approach aims at linearizing the input surface and representing it via a set of localized planar segments. In the second part of the talk, I will present some ongoing research on surface representations via balls. This work started with constructions from spherical neodym magnets and provided a set of mathematical questions. These investigations are joint work with FU Berlin and OIST.
Venue: via Zoom
Event Official Language: English

Seminar
Representations of fundamental groups and 3manifold topology
November 16 (Mon) at 16:00  18:10, 2020
Takahiro Kitayama (Associate Professor, Graduate School of Mathematical Sciences, The University of Tokyo)
In 3dimensional topology the great progress during the last two decades revealed that various properties of 3manifolds are well understood from their fundamental groups. I will give an introduction to the study of splittings of 3manifolds along surfaces, with an emphasis on an application of group representations. A fundamental and difficult problem in general is to find surfaces essentially embedded in a given 3manifold. I will explain how such surfaces are detected by deformations of representations of the fundamental group, and what information of detected surfaces is described in terms of topological invariants derived from representations.
Venue: via Zoom
Event Official Language: English

Efficient probabilistic assessment of building performance: sequential Monte Carlo and decomposition methods
November 13 (Fri) at 16:00  18:10, 2020
Tianfeng Hou (Postdoctoral Researcher, iTHEMS / Postdoctoral Researcher, Prediction Science Laboratory, RIKEN Cluster for Pioneering Research (CPR) / Postdoctoral Researcher, Data Assimilation Research Team, RIKEN Center for Computational Science (RCCS))
The use of numerical simulations for complex systems is common. However, significant uncertainties may exist for many of the involved variables, and in order to ensure the reliability of our simulation results and the safety of such complex systems, a stochastic approach providing statistics of the probability distribution of the results is of crucial importance. However, when a highly accurate result is required, the conventional Monte Carlo based probabilistic methodology inherently requires many repetitions of the deterministic analysis and in cases where that deterministic simulation is (relatively) time consuming, such probabilistic assessment can easily become computationally intractable. Hence, to reduce the computational expense of such probabilistic assessments as much as possible, the targets of this seminar are twofold: (1), to exploit an efficient sampling strategy to minimize the number of needed simulations of Monte Carlo based probabilistic analysis; (2), to investigate a surrogate model to reduce the computational expense of single deterministic simulation. This seminar contains two parts and will be accompanied by a set of illustrative building physical case studies (analysis of the heat and moisture transfer through building components). The first part of this seminar focusses on the use of quasiMonte Carlo based probabilistic assessment for building performance, since it has the potential to outperform the standard Monte Carlo method. More specifically, the quasiMonte Carlo sampling strategies and related error estimation techniques will be introduced in detail. In addition, questions on under which conditions the quasiMonte Carlo can outperform the standard Monte Carlo method will be answered by a set of analyses. The second part of this seminar targets the investigation of using model order reduction methods for optimizing the deterministic simulation, given that it generally allows a (large) reduction of the simulation time without losing the dynamic behavior of the conventional models (such as the transient finite element analysis). Particularly, the fundamental concepts of one common model order reduction method – proper orthogonal decomposition (POD) will be provided, and its potential use for simulating (building physical) problems with different levels of nonlinearity and complexity will be illustrated.
Venue: via Zoom
Event Official Language: English

Seminar
Mathematical aspects of quasiMonte Carlo integration
November 5 (Thu) at 16:00  18:10, 2020
Kosuke Suzuki (Assistant Professor, Graduate School of Advanced Science and Engineering, Hiroshima University)
In this talk, I will introduce mathematical aspects of quasiMonte Carlo (QMC) integration. We aim to approximate the integral of a function on the ddimensional hypercube [0,1]^d. A useful approach is MonteCarlo (MC) integration, which uses randomly chosen samples. A drawback of MC is the rate of convergence; the standard deviation of the estimator converges as 1/sqrt(n) asymptotically in n. To have a better rate of convergence as O(log^d N/N) or more, QMC uses deterministic, uniformly distributed points. In the first part, I will give an overview of QMC, such as stardiscrepancy, KoksmaHlawka inequality, and some explicit constructions as lattices and digital nets. In the second part, I will show that QMC using lattices and digital nets can achieve a higher rate of convergence for smooth integrands.
Venue: via Zoom
Event Official Language: English

Math Seminars by Dr. Genki Ouchi and Dr. Kenta Sato
September 24 (Thu) at 16:00  18:10, 2020
Genki Ouchi (Special Postdoctoral Researcher, iTHEMS)
Kenta Sato (Special Postdoctoral Researcher, iTHEMS)[Talk 1] (16:00  17:00) Dr. Genki Ouchi Automorphism groups of cubic fourfolds and K3 categories In this talk, I would like to talk about symmetries of algebraic varieties, especially cubic fourfolds and K3 surfaces. It is known that symmetries of cubic fourfolds and K3 surfaces are related to sporadic finite groups as Mathieu groups and Conway groups in both algebraic geometry and string theory. Relations between cubic fourfolds and K3 surfaces are studied in the context of derived categories, Hodge theory and so on. I would like to explain the direct relation among symmetries of cubic fourfolds and K3 surfaces via their derived categories. [Talk 2] (17:10  18:10) Dr. Kenta Sato An algebraic approach to the four color theorem The four color theorem states that, given any separation of a plane into contiguous regions, no more than four colors are required to color the regions. Although this theorem was already proved about 40 years ago, another proof without using a computer is not found still now. In this talk, I will introduce an algebraic approach to this theorem, which states that a conjecture about singularities of algebraic varieties implies the four color theorem. In particular, I would like to focus on the connection of three different fields in mathematics: graph theory, convex geometry and algebraic geometry. *Detailed information about the seminar refer to the email.
Venue: via Zoom
Event Official Language: English

Maximal Regularity and Partial Differential Equations
September 8 (Tue) at 16:00  18:10, 2020
Ken Furukawa (Postdoctoral Researcher, Prediction Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))
The theory of maximal regularity is a powerful tool to get solutions having the best regularity to linear partial differential equations (PDEs) of parabolic type. The theory is also applicable to show wellposedness of various nonlinear PDEs. In the first part, We introduce the history of the development of the theory of maximal regularity and the way to apply nonlinear PDEs. In the second part, We give some applications to PDEs, e. g. the primitive equations, the NavierStokes equations, and elliptic equations with dynamic boundary conditions. *Please contact Keita Mikami's mail address to get access to the Zoom meeting room.
Venue: via Zoom
Event Official Language: English

Seminar
Stability of ferromagnetism in manyelectron systems
July 31 (Fri) at 16:00  18:10, 2020
Tadahiro Miyao (Associate Professor, Department of Mathematics, Faculty of Science, Hokkaido University)
First part Title: Stability of ferromagnetism in manyelectron systems Abstract: I construct a modelindependent framework describing stabilities of ferromagnetism in strongly correlated electron systems. Within the new framework, I reinterpret the MarshallLiebMattis theorem and Lieb’s theorem; in addition, from the new perspective, I prove that Lieb’s theorem still holds true even if the electronphonon and electronphoton interactions are taken into account. I also examine the NagaokaThouless theorem and its stability. These examples verify the effectiveness of the new viewpoint. Second part Title: Order preserving operator inequalities in manyelectron systems Abstract: In this talk, I will introduce order preserving operator inequalities and explain how these inequalities are applied to the mathematical study of ferromagnetism. As examples of applications, Lieb's theorem of the Hubbard model and its stabilities will be discussed in terms of the inequalities.
Venue: via Zoom
Event Official Language: English

Seminar
Topological data analysis from a practical and mathematical perspective
July 15 (Wed) at 16:00  18:10, 2020
Yuichi Ike (Researcher, Artificial Intelligence, FUJITSU LABORATORIES LTD.)
1. Topological data analysis and its applications In this talk, I will explain some methods in topological data analysis (TDA) and their applications. First I recall persistent homology, which is a central tool to analyze the "shape" of a point cloud set. Then I show several applications to material science and timeseries analysis. I also talk about our collaborative research with Inria on noiserobust persistent homology and an automated vectorization method of persistence diagrams. 2. Persistencelike distance on sheaf category and displacement energy In this talk, I will talk about relation among sheaf theory, persistence modules, and symplectic geometry. We introduce a persistencelike distance on Tamarkin sheaf category and prove a stability result with respect to Hamiltonian deformation of sheaves. Based on this result, we propose a new sheaftheoretic method to give a lower bound of the displacement energy of compact subsets of a cotangent bundle. This is a joint work with Tomohiro Asano.
Venue: via Zoom
Event Official Language: English

Universal Error Bound for Constrained Quantum Dynamics
June 24 (Wed) at 16:00  18:10, 2020
Ryusuke Hamazaki (Senior Research Scientist, iTHEMS / RIKEN Hakubi Team Leader, Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team, RIKEN Cluster for Pioneering Research (CPR))
In quantum mechanics, the existence of large energy gaps allows us to trace out the degrees of freedom of irrelevant energy scale. Consequently, we can treat a system within a constrained subspace obtained by the projection of the total Hilbert space. While this statement has widely been used to approximate quantum dynamics in various contexts, a general and quantitative justification stays lacking. In this talk, we show a universal and rigorous error bound for such a constraineddynamics approximation in generic gapped quantum systems [1,2]. This universal bound is a linear function of time that only involves the energy gap and coupling strength, provided that the latter is much smaller than the former. If time allows, I will briefly talk about generalizations of our result to e.g., quantum manybody systems and open quantum systems.
Venue: via Zoom
Event Official Language: English

Seminar
Information geometry of operator scaling
June 17 (Wed) at 16:00  18:10, 2020
Tasuku Soma (Research Associate, Graduate School of Information Science and Technology, The University of Tokyo)
Matrix scaling is a classical problem with a wide range of applications. It is known that the Sinkhorn algorithm for matrix scaling is interpreted as alternating eprojections from the viewpoint of classical information geometry. Recently, a generalization of matrix scaling to completely positive maps called operator scaling has been found to appear in various fields of mathematics and computer science, and the Sinkhorn algorithm has been extended to operator scaling. In this study, the operator Sinkhorn algorithm is studied from the viewpoint of quantum information geometry through the Choi representation of completely positive maps. The operator Sinkhorn algorithm is shown to coincide with alternating eprojections with respect to the symmetric logarithmic derivative metric, which is a Riemannian metric on the space of quantum states relevant to quantum estimation theory. This talk is based on joint work with Takeru Matsuda.
Venue: via Zoom
Event Official Language: English

Knotted 2spheres in the 4space and YangMills gauge theory
May 27 (Wed) at 16:00  18:10, 2020
Masaki Taniguchi (Special Postdoctoral Researcher, iTHEMS)
The classification problem of knots is one of the central topics in a study of topology. In the first part, we review classical knot theory and theory of 2dimensional knots in the 4dimensional space. In the second part, we focus on a problem considered in differential topology. In the studies of differential topology, people are interested in the difference between continuous and smooth. As the main result of this talk, we introduce a theorem that tells us the difference between continuous and smooth 2dimensional knots. The proof uses YangMills gauge theory for 4manifolds obtained by the surgery of 2knots.
Venue: via Zoom
Event Official Language: English

How many electrons can atoms bind?
May 13 (Wed) at 16:00  18:10, 2020
Yukimi Goto (Special Postdoctoral Researcher, iTHEMS)
In this talk, I will introduce the mathematical studies on the ionization problem. Some experimental & numerical evidences say that any doubly charged atomic ion X^{2} is not stable. This 'fact' is called the ionization conjecture in mathematical physics literatures. My hope is to illustrates the interplay between mathematical and physical ideas. The talk is directed towards researchers on various aspects of quantum mechanics. In the first part, we will discuss the manybody aspects of quantum mechanics and introduce some basic notions. The second part will deal with the mathematical results in some approximation theories.
Venue: via Zoom
Event Official Language: English

From Eigenvalues to Resonances
May 1 (Fri) at 16:00  18:10, 2020
Keita Mikami (Research Scientist, iTHEMS)
Resonance is one of the most studied object in mathematical study of Schrödinger operators. One possible reason is that resonance is appeared in many other fields like arithmetic, physics, and topography. This series of talks target both mathematicians and researchers in other fields. The goal of the talk is to introduce the study of resonances for two body Schrödinger operators. In the first part, we briefly review spectral theory and how we use it in the study of Schrödinger operators. The aim of this part is to introduce the audience some basic notions used in the study of Schrödinger operators. In the second part, we give brief introduction of resonances and its application to both mathematicians and researchers in other fields. We start from mathematical definition of resonances to its applications in the other fields.
Venue: via zoom
Event Official Language: English

Seminar
Index of the WilsonDirac operator revisited: a discrete version of Dirac operator on a finite lattice
February 25 (Tue) at 16:00  18:10, 2020
Mikio Furuta (Professor, The University of Tokyo)
The WilsonDirac operator is a discrete version of Dirac operator defined on regular lattices. When the discrete version is a fine approximation of the Dirac operator on a Z/2graded Clifford module on a torus, it is known that (1) an integervalued index is defined for the WilsonDirac operator, and (2) the index is equal to the AtiyahSinger index of the Dirac operator on the torus. These have been well established up to around 2000. The strategy of all the previous works is to make use of the discrete version of the heat kernel for Neuberger's overlap Dirac operator. Therefore the strategy cannot be generalized to mod 2 index nor family version of index. In this talk I would like to explain a new approach to the index of WilsonDirac operator which can be immediately generalized to these various cases. Joint work with H. Fukaya, S. Matsuo, T. Onogi, S. Yamaguchi and M. Yamashita.
Venue: Seminar Room #160
Event Official Language: English

Solved and open problems regarding the neighborhood grid data structure
February 7 (Fri) at 16:00  18:10, 2020
Martin Skrodzki (Visiting Scientist, iTHEMS / Fellow, German Academic Scholarship Foundation, Germany)
February 7 at 16:0017:00 17:1018:10, 2020 In 2009, Joselli et al. introduced the neighborhood grid data structure for fast computation of neighborhood estimates for point clouds in arbitrary dimensions. Even though the data structure has been used in several applications and was shown to be practically relevant, it is theoretically not yet well understood even in the twodimensional case. The purpose of this talk is to present the data structure, give a timeoptimal building algorithm, and motivate several associated questions from enumerative combinatorics as well as lowdimensional (probabilistic) geometry. In case of questions that have been solved in the past, corresponding proofs will be provided. For the open question, the talk will list them as an outlook to possible future collaboration.
Venue: Seminar Room #160
Event Official Language: English

Seminar
Semiclassical methods in mathematical quantum mechanics
January 23 (Thu) at 16:00  18:10, 2020
Shu Nakamura (Professor, Gakushuin University)
Plan of the seminar: we separate each talk into two. In the first 60 minutes the speaker gives an introductory talk for nonmathematicians. After a short break, the second 60 minutes is spent for a bit more detailed talk for mathematicians (working in other areas). We welcome you joining both parts of the seminar or only the first/second half. Talk 1: Semiclassical analysis, microlocal analysis and scattering theory. I plan to talk about overview on the semiclassical analysis and related topics, especially its intrinsic relationship with microlocal analysis and (microlocal) scattering theory. Roughly speaking, the microlocal analysis is an application of semiclassical idea to the analysis of singularities, and its analogue in momentum space is the microlocal scattering theory. We discuss basic notions of these, and mention several recent results. Talk 2: Microlocal structure of the scattering matrix with longrange perturbations. As an example of topics discussed in Talk 1, we discuss recent results on the scattering matrix with longrange perturbations. In particular, we show that the scattering matrix is expressed as a Fourier integral operator, and in some cases we can decide its spectral properties. Our approach is fairly geometric and abstract, and thus applies not only to usual Schrödinger operators but also to higher order operators and discrete Schrödinger operators.
Venue: #435437, Main Research Building
Event Official Language: English

Multiple Zeta Values: Interrelation of Series and Integrals
December 17 (Tue) at 16:00  18:10, 2019
Syuji Yamamoto (Associate Professor, Keio University)
Plan of the seminar: we separate each talk into two. In the first 60 minutes the speaker gives an introductory talk for nonmathematicians. After a short break, the second 60 minutes is spent for a bit more detailed talk for mathematicians (working in other areas). We welcome you joining both parts of the seminar or only the first/second half. Abstract: This is an introduction to multiple zeta values (MZVs). Although the study of MZVs is related to various areas of mathematics, we will concentrate on the algebraic structures of MZVs themselves. The key point is that MZVs have two kinds of representations: nested series and iterated integrals. We present how these two representations yield rich algebraic relations among MZVs.
Venue: Seminar Room #160
Event Official Language: English
90 events
Events
Categories
series
 iTHEMS Colloquium
 MACS Colloquium
 iTHEMS Seminar
 iTHEMS Math Seminar
 DMWG Seminar
 iTHEMS Biology Seminar
 iTHEMS Theoretical Physics Seminar
 Information Theory SG Seminar
 Quantum Matter Seminar
 ABBLiTHEMS Joint Astro Seminar
 MathPhys Seminar
 Quantum Gravity Gatherings
 RIKEN Quantum Seminar
 Quantum Computation SG Seminar
 DEEPIN Seminar
 NEW WG Seminar
 LabTheory Standing Talks
 QFTcore Seminar
 STAMP Seminar
 QuCoIn Seminar
 Number Theory Seminar
 BerkeleyiTHEMS Seminar
 iTHEMSRNC Meson Science Lab. Joint Seminar
 AcademicIndustrial Innovation Lecture
 RIKEN Quantum Lecture
 Theory of Operator Algebras
 iTHEMS Intensive CourseEvolution of Cooperation
 Introduction to PublicKey Cryptography
 Knot Theory
 iTHES Theoretical Science Colloquium
 SUURICOOL Seminar
 iTHES Seminar