Seminar
490 events

Seminar
On A_2liftings of sum formulas and BowmanBradley type formulas for finite multiple zeta values
November 22 (Thu) at 11:40  12:40, 2018
Dr. Shinichiro Seki (Tohoku University)
Both the sum formula and BowmanBradley's theorem for multiple zeta values are well known. Recently, Saito and Wakabayashi proved counterparts of these two formulas for Afinite multiple zeta values. In this talk, I will explain that A_2liftings of some parts of SaitoWakabayashi's results have simple forms using SekiBernoulli numbers. The first part of this talk is a joint work with Shuji Yamamoto. The second part is a joint work with Hideki Murahara and Tomokazu Onozuka.
Venue: Seminar Room #160
Event Official Language: English

Seminar
Generating functions of CM & RM values
November 22 (Thu) at 10:30  11:30, 2018
Dr. Toshiki Matsusaka (Kyushu University)
The special values of the elliptic modular j function j(z) at imaginary quadratic points are known as singular moduli (CM values), and play important roles in algebraic number theory. As a real quadratic analogue, Kaneko (2009) defined the `values’ of j(z) at real quadratic points (RM values). In 2011, DukeImamogluToth showed that the generating function of the traces of these CM & RM values becomes a harmonic Maass form of weight 1/2. In this talk, I shall introduce a new class called polyharmonic weak Maass forms, inspired by works of LagariasRhoades on the Kronecker limit formula, and give a generalization of DukeImamogluToth’s work for any polyharmonic weak Maass form.
Venue: Seminar Room #160
Event Official Language: English

Seminar
Study of density dependent nuclear symmetry energy by using heavy RI collisions at RIKENRIBF
November 16 (Fri) at 14:00  16:00, 2018
Dr. Tadaaki Isobe (Senior Research Scientist, Radioactive Isotope Physics Laboratory, RIKEN Nishina Center for AcceleratorBased Science (RNC))
The nuclear Equation of State (EoS) is a fundamental property of nuclear matter. An international collaboration, named SPiRIT, to study the density dependence of asymmetry term in nuclear EoS has been formed since 2009. The main aim of this collaboration is to make the constraint on the asymmetry term of nuclear EoS for higher dense region($\rho>\rho_0$). In this talk, the conceptual idea of the project will be given in addition to some result of data analysis for the physics run performed at 2016 spring.
Venue: Seminar Room #160
Event Official Language: English

Seminar
Spintronics in NonInertial Frames
November 15 (Thu) at 13:00  18:00, 2018
Prof. Mamoru Matsuo (University of Chinese Academy of Sciences, China)
Venue: Seminar Room #160
Event Official Language: Japanese

Seminar
New Venues in Formation and Detection of Primordial Black Hole Dark Matter
November 12 (Mon) at 14:00  15:00, 2018
Dr. Volodymyr Takhistov
Abstract: Primordial black holes (PBH) provide an attractive nonparticle dark matter (DM) candidate. I will discuss a novel PBH production mechanism that can appear generically in models with scalar fields. Recent reevaluations of PBH constraints suggest that the open parameter space for PBHs to constitute all of dark matter is appreciably larger than previously thought. I will show how compact stars can serve as laboratories for probing it. The variety of resulting novel astrophysical signals are of particular interest to the vibrant field of multimessenger astronomy. Moreso, PBHstar interactions suggest an elegant resolution to some of the most puzzling questions in astrophysics, such as the origin of gold and other heavy elements.
Venue: Seminar Room #160
Event Official Language: English

Seminar
The 28th QCD Club
November 9 (Fri) at 15:00  17:00, 2018
Title: Application of a gradient flow method to thermodynamics of QCD with dynamical quarks Language: Japanese or English
Event Official Language: English

Seminar
Population genetics of duplicated genes
November 1 (Thu) at 15:30  18:00, 2018
Prof. Hideki Innan (The Graduate University for Advanced Studies (SOKENDAI))
Venue: Seminar Room #160
Event Official Language: English

Seminar
Assembly rules and a theory for invasion and extinction in minimal food webs
October 29 (Mon) at 15:00  18:00, 2018
Dr. Namiko Mitarai (Associate Professor, Niels Bohr Institute, University of Copenhagen, Denmark)
We propose a theory of the evolution of a minimal food web by sequential invasion of new species [1]. Our theory is based on the standard generalized LotkaVolterra equations, where basal species compete through resource depletion [2]. The considered food webs are “minimal”, as each species only feeds on a single resource, leading to a hierarchical, treelike food web [1,3]. We prove that at each invasion step there is one uniquely determined outcome: either the invader peacefully coexists with the residents and resources are redistributed; the invader is eliminated; or one or several of the resident species are removed in a uniquely defined extinction cascade. At the end of either of these processes the resulting food web relaxes to a globally stable (and feasible) steady state. We break down the essence of our theory in the conceptual “invasion extinction model” (IEM), which allows us to analytically compute the persistence time and the extinction size distribution.
Venue: Seminar Room #160
Event Official Language: English

Seminar
Relations between fractal dimensions and arithmetic progressions
October 23 (Tue) at 11:35  12:35, 2018
Mr. Kota Saito (Nagoya University)
In this talk we give estimates for the dimensions of sets in real numbers which uniformly avoid finite arithmetic progressions. More precisely, we say that $F$ uniformly avoids arithmetic progressions of length $k\geq 3$ if there is an $\epsilon>0$ such that one cannot find an arithmetic progression of length $k$ and gap length $\Delta>0$ inside the $\epsilon\Delta$ neighbourhood of $F$. Our main result is an explicit upper bound for the Assouad (and thus Hausdorff) dimension of such sets in terms of $k$ and $\epsilon$. In the other direction, we give examples of sets which uniformly avoid arithmetic progressions of a given length. We also consider higher dimensional analogues of these problems, where arithmetic progressions are replaced with arithmetic patches lying in a hyperplane. As a consequence, we obtain a discretised version of a `reverse Kakeya problem': we show that if the dimension of a set in $\mathbb{R}^d$ is sufficiently large, then it closely approximates arithmetic progressions in every direction. The above is a joint work with Fraser and Yu. Finally we show that the converse of `reverse Kakeya problem' does not hold. This is a singleauthor work.
Venue: Large Meeting Room, 2F Welfare and Conference Building (Cafeteria)
Event Official Language: English

Seminar
Generalized Erdös and Obláth theorem for polynomialfactorial Diophantine equations
October 23 (Tue) at 10:30  11:30, 2018
Mr. Wataru Takeda (Nagoya University)
Diophantine equations are equations where only integer solutions are accepted. There are many types of Diophantine equations and many results are known. Our Diophantine equation is of the form x^n+y^n=m!. Erdös and Obláth showed that the Diophantine equation x^2+y^2=m! has only two positive integer solutions (x,y,m)=(1,1,2),(12,24,6). In this talk, the factorial function m! is replaced with a generalized factorial function Π(m) over number fields. Then whether there are infinitely many solutions or not depends on the number field. We give necessary and sufficient condition for existence of infinitely many solutions of x^2+y^2=Π(m). More generally, we introduce an observation for higher degree equation x^n+y^n=Π(m).
Venue: Large Meeting Room, 2F Welfare and Conference Building (Cafeteria)
Event Official Language: English

Seminar
NonHermitian phase transition from a polariton BoseEinstein condensate to a photon laser
September 28 (Fri) at 13:30  17:00, 2018
Dr. Ryo Hanai (Osaka University)
Venue: Seminar Room #160
Event Official Language: Japanese

Seminar
Number Theory Seminar: 3 Talks
July 24 (Tue) at 10:00  12:35, 2018
Dr. Masataka Ono (Keio University)
Dr. Shingo Sugiyama (Nihon University)
Mr. Yoshinosuke Hirakawa (Keio University)This seminar is aimed at scientists in general, not only to mathematicians. 10:0010:45 Title: Multiple zeta functions associated with 2colored rooted trees Speaker: Dr. Masataka Ono (Keio University) Abstract: In our recent work, we introduced a combinatorial object and finite sum associated with them which we call finite multiple zeta values associated with 2colored rooted trees and gave a unified interpretation to some types of finite multiple zeta values. In this talk, we introduce multiple zeta function associated with 2colored rooted tree and discuss its analytic properties, for example, the possible singularities and functional equations. 10:5511:40 Title: Modular forms and trace formulas with applications to equidistributions of their Fourier coefficients Speaker: Dr. Shingo Sugiyama (Nihon University) Abstract: Modular forms are interesting objects in number theory as they are related to arithmetic problems. Trace formulas of Hecke operators acting on modular forms are very useful tools to study arithmetic invariants: Fourier coefficients, special values of Lfunctions, Hurwitz class numbers. We will start fundamental notions on modular forms and trace formulas of Hecke operators for nonexperts, and introduce our results on a generalization of Serre’s vertical SatoTate law. Some results in this talk are based on a joint work with Masao Tsuzuki (Sophia University). 11:5012:35 Title: On a generalization of Dobinski's formula Speaker: Yoshinosuke Hirakawa (Keio University) Abstract: Dobinski's formula is a very classical formula, which expresses the Bell number as an infinite series. Here, the Bell number is the number of partitions of a finite set. Such a "combinatorialanalytic" formula should lead us to more beautiful number theory. In this talk, we introduce a generalization of Dobinski's formula by means of a certain multiple generalization of the exponential function.
Venue: Seminar Room #160
Event Official Language: English

Introduction to topological band structure
July 6 (Fri) at 10:30  17:30, 2018
Dr. Tomoki Ozawa (Senior Research Scientist, iTHEMS)
In this seminar, I give an introduction to topological band structures. Topological band structure is the fundamental idea to understand phenomena such as the quantum Hall effect, topological insulators, and topological superconductors. Although originally found in fermionic electron systems, topological band structure is essentially a singleparticle property, and thus the same phenomenon can arise also in bosonic systems. In fact, topological band structure does not even need quantum mechanics; topological band structure can arise in classical waves inside a periodic medium, such as electromagnetic waves or classical mechanical waves. In this seminar, I first give a brief introduction to key concepts in topological band structures, such as Berry curvature, Chern number, bulkedge correspondence, and tenfold way classification of topological insulators. I then explain how topological band structure can arise in classical systems, giving a brief introduction to the field of topological photonics and topological mechanics.
Venue: Seminar Room #160
Event Official Language: Japanese

Topological Photonics and the fourdimensional quantum Hall effect
July 3 (Tue) at 15:00  17:00, 2018
Dr. Tomoki Ozawa (Senior Research Scientist, iTHEMS)
In this seminar, I will give an introduction to topological photonics, that is, the study of topological band structures and resulting topological phenomena in photonic systems. I will first review basic concepts of topological band structures, and then explain what it means to realize topological band structures in photonic systems. I will particularly emphasize some important differences with respect to solidstate electron systems. I then present some of my own works in topological photonics, such as the synthetic dimensions in photonic systems, which allows one to explore models and phenomena in high dimensions including the fourdimensional quantum Hall effect.
Venue: 1F Meeting Room, AIMR Main Building
Event Official Language: English

Seminar
Denso IT Lab. and RIKEN Joint Innovation Seminar
June 29 (Fri) at 16:00  17:40, 2018
16:0017:00 Denso IT Laboratory "About Denso IT lab." H. Iwasaki (Denso) "HumanCar interface" H. Tsukahara (Denso) "Math in machine leaning and computer vision" K. Ishikawa (Denso) 17:0017:40 RIKEN "Math bridging between classical and quantum physics: quantum hall effect in classical system" T. Ozawa (RIKEN) "DiOmega: a new particle predicted by Kcomputer with mathematical science" S. Gongyo (RIKEN)
Venue: Common Room #246248
Event Official Language: Japanese

Seminar
Introduction to spinboson model
June 22 (Fri) at 10:30  17:30, 2018
Dr. Takeo Kato (The Institute for Solid State Physics (ISSP), The University of Tokyo)
10:3012:30 Introduction to spinboson model @DR3 (14th Building 213) 14:0015:30 Kondo effect @DR7 (14th Building 217) 16:0017:30 Detail of NIBA formalism @DR7 (14th Building 217)
Venue: Yagami Campus, Keio University
Event Official Language: Japanese

Relative and equivariant Lagrangian Floer homology and AtiyahFloer conjecture
June 19 (Tue) at 14:30  16:00, 2018
Prof. Kenji Fukaya (Senior Visiting Scientist, iTHEMS / Permanent Member, Simons Center for Geometry and Physics, Stony Brook University, New York, USA)
AtiyahFloer conjecture concerns a relationship between Floer homology in Gauge theory and Lagrangian Floer homology. One of its difficulty is that the symplectic manifold on which we consider Lagrangian Floer homology is in general singular. In this talk I will explain that, by using relative and equivariant version of Lagrangian Floer homology, we can resolve this problem and can at least state the conjecture as rigorous mathematical conjecture. Supported by RIKEN iTHEMS and Tuesday Seminar on Topology (Graduate School of Mathematical Sciences, The University of Tokyo).
Venue: Room 056, Graduate School of Mathematical Sciences, The University of Tokyo
Event Official Language: Japanese

Seminar
1form LiebSchultzMattis theorem and anomaly matching in quantum dimer model
June 8 (Fri) at 13:30  17:00, 2018
Dr. Yuta Kikuchi (RBRC Researcher, Theory Group, RIKEN BNL Research Center, RIKEN Nishina Center for AcceleratorBased Science (RNC))
The LiebSchultzMattis theorem dictates that a trivial symmetric insulator in lattice models is prohibited if lattice translation symmetry and U(1) charge conservation are both preserved. In this talk, we discuss the generalization of the LiebSchultzMattis theorem to systems with higherform symmetries, which act on extended objects of dimension n > 0. The prototypical lattice system with higherform symmetry is the pure abelian lattice gauge theory whose action consists only of the field strength. We first construct the higherform generalization of the LiebSchultzMattis theorem with a proof. We then apply it to the U(1) lattice gauge theory description of the quantum dimer model on bipartite lattices. Finally, using the continuum field theory description in the vicinity of the RokhsarKivelson point of the quantum dimer model, we diagnose and compute the mixed ’t Hooft anomaly corresponding to the higherform LiebSchultzMattis theorem.
Venue: SUURICOOL (Kyoto)
Event Official Language: Japanese

Seminar
AbelianHiggs dualities in quantum defectmediated melting phase transitions
April 27 (Fri) at 14:30  17:30, 2018
Dr. Aron Beekman (Keio University)
In the study of zerotemperature quantum phase transitions, instead of looking how symmetry is broken, it is often useful to see how symmetry can be restored by the condensation of topological defects. Through a duality mapping, NambuGoldstone modes are represented as gauge bosons, mediating longrange interactions between topological defects. When the latter condense, those bosons get as mass via the AndersonHiggs mechanism, which signals the loss of rigidity and the restoration of symmetry. I will first review the beststudied case: the 2+1D superfluidinsulator transitions where the defects are U(1) vortices. Consecutively several extensions are discussed: going to 3+1D where the defects are not point particles but strings, and quantum elasticity, which studies breaking of spatial translations and rotations.
Venue: Seminar Room #160
Event Official Language: English

Seminar
Thermodynamic of a onedimensional Bose gas at low temperature & superfluidity in neutronstar matter
April 24 (Tue) at 15:30  16:30, 2018
Dr. Giulia De Rosi (The Institute of Photonic Sciences, Barcelona, Spain)
Ultracold atomic gases and neutronstar matter play a paramount importance in manybody physics, by exhibiting important analogies and differences. One of the most interesting phenomena shared by both systems is provided by superfluidity. In the first part of the talk, I will show that the chemical potential of a onedimensional (1D) interacting Bose gas exhibits a nonmonotonic temperature dependence which is peculiar of superfluids. The effect is a direct consequence of the phononic nature of the excitation spectrum at large wavelengths exhibited by 1D Bose gases. For low temperatures T, I demonstrate that the coefficient in the $T^2$ expansion of the chemical potential is defined by the zerotemperature density dependence of the sound velocity and it has been calculated along the crossover between the Bogoliubov weaklyinteracting gas and the TonksGirardeau gas of impenetrable bosons. The theoretical predictions along the crossover are confirmed by comparison with the exactly solvable YangYang model in which the finitetemperature equation of state is obtained numerically by solving Betheansatz equations. In the second part of the talk, I will show the results of a study of the superfluid gap in pure neutron matter, associated with the formation of Cooper pairs in the $^1S_0$ channel. The interaction responsible of the onset of superfluidity is an effective interaction coming from a nuclear Hamiltonian strongly constrained by phenomenology and obtained from the correlated basis function (CBF) perturbation theory and the formalism of cluster expansions. The calculations have been carried out using an improved version of the CBF effective interaction, in which threenucleon forces are taken into account using a microscopic model. Our results show that the superfluid transition occurs at values of densities corresponding to the neutronstar inner crust.
Venue: Seminar Room #160
Event Official Language: English
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