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Black Hole Information Paradox and Wormholes

June 21 (Mon) 13:00 - 14:30, 2021

Kanato Goto (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

In this talk, I will explain about the recent progress in the black hole information paradox that I am involved with. The information paradox arises when a black hole evaporates by emitting Hawking radiation due to the quantum effects. Time dependence of the entropy of Hawking radiation is diagnosis of information loss caused by the black hole evaporation. If information is not lost, the entropy of Hawking radiation should obey the so-called Page curve. In recent research developments, it was found that “the quantum extremal islands” reproduce the unitary Page curve in an evaporating black hole. I will argue about how the quantum extremal islands are derived from the computation of the entropy of Hawking radiation using the gravitational path-integral.

Venue: via Zoom

Event Official Language: English

Stable eigenvalues of compact anti-de Sitter 3-manifolds

June 18 (Fri) 16:00 - 18:10, 2021

Kazuki Kannaka (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

Geometric objects that have been investigated in detail so far, such as closed Riemann surfaces, are sometimes locally homogeneous. Loosely speaking, their infinitesimal behavior is the same at each point. In this talk, I would like to explain the idea of investigating such objects using the Lie group theory.In the first part of the talk, I will recall the notions of Lie group actions and their quotient spaces with examples, and then explain the definitions of locally homogeneous spaces and their deformations (Teichmüller spaces). In the second part of the talk, I will consider anti-de Sitter manifolds as a special case, i.e., Lorentzian manifolds of negative constant curvature. As in the Riemannian case, a differential operator called the Laplacian (or the Klein-Gordon operator) is defined on Lorentzian manifolds. Unlike the Riemannian case, it is no longer an elliptic differential operator but a hyperbolic differential operator. In its spectral analysis, new phenomena different from those in the Riemannian case have been discovered in recent years, following pioneering works by Toshiyuki Kobayashi and Fanny Kassel. I would like to explain stable eigenvalues of the hyperbolic Laplacian of anti-de Sitter 3-manifolds with recent progress.

Venue: via Zoom

Event Official Language: English

The origin and dispersal of buckwheat

June 10 (Thu) 10:00 - 11:00, 2021

Jeffrey Fawcett (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

Buckwheat, which soba noodles is made from, originated from a wild species that is distributed in southwest China, around Yunnan, Sichuan, and Tibet. We are trying to understand when, where, and how it originated and then spread across the world and came to Japan. To do so, we are using genomic data of wild samples from China and cultivated samples from various parts of the world. I will give a brief introduction about buckwheat, explain what we already know about its origin and dispersal, and show a bit of our results. I will also explain the significance of studying "domestication", that is, the process that plants/animals that humans currently use originated from their ancestral wild species. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

An overview of genome-wide epistasis and co-selection analysis

June 3 (Thu) 10:00 - 11:00, 2021

Yingying Xu (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

This talk is a summary of research that have done by me and my team during 2016~2019. I was a postdoc researcher in Aalto university/Helsinki university in Finland. In the team, a worldwide active collaboration has happened between many fields including statistical physics, biology, computer science and statistics. The target is to analyze ultra-high dimensional large population genomic datasets of two major human pathogens, Streptococcus pneumoniae and Neisseria meningitidis, without phenotypic data. Interacting networks of resistance, virulence and core machinery genes are identified. Many different approaches have been invented and they can be generally applied to other datasets with similar mathematical setting. I will explain methods based on statistical model [1,2], mutual information [3], and theoretical performance analysis for statistical model [4]. In the end, I will briefly introduce a new phenomenon of random matrix which is discovered during the research process for statistical significance filtering [5]. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Magnetorotational Instability: Current Understanding and Perspective

May 28 (Fri) 16:00 - 17:00, 2021

Takashi Minoshima (Researcher, Japan Agency for Marine-Earth Science and Technology (JAMSTEC))

The differentially rotating flow can be destabilized in the presence of a weak magnetic field through the magnetorotational instability (MRI). The MRI is considered as a possible mechanism for outward angular momentum transport and subsequent mass accretion in accretion disks. Numerous studies have been devoted to understand its nature and judge whether it can supply the power sufficient for observed transport efficiency. For example, the MHD simulation studies have attempted to reveal the scaling of the MRI on numerical (e.g., resolution and domain size) as well as physical parameters (e.g., magnetic field intensity and configuration). In this talk, I would like to discuss current understanding and perspective of the MRI through theoretical and numerical studies. I will especially focus on the impact of transport coefficients (viscosity, resistivity, and their ratio) on the evolution of the MRI and disk.

Venue: via Zoom

Event Official Language: English

A Mathematical Model for Stem Cell Competition to Maintain a Cell Pool Injured by Radiation Exposure

May 27 (Thu) 10:00 - 11:00, 2021

Kouki Uchinomiya (Central Research Institute of Electric Power Industry)

Cancer risk of low-dose-rate ionizing radiation exposure is one of the most important issues in radiation protection. Tissue stem cells have been considered one of the targets of radiation-induced carcinogenesis. There has been a hypothesis that the carcinogenic effects of radiation can be reduced if damaged stem cells are eliminated via stem cell competition between damaged and intact stem cells. This would be particularly effective under very low-dose-rate conditions, where only a few stem cells in a stem cell pool are affected by radiation. In this presentation, I will introduce a simple mathematical model to discuss the influence of stem cell competition on the accumulation of radiation damage and show that the character of damaged cells and the size of the stem cell pool may affect the accumulation of radiation damage. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Loewner's theorem for maps on operator domains / The structure of maps on the space of all quantum pure states that preserve a fixed quantum angle

May 24 (Mon) 16:00 - 18:10, 2021

Michiya Mori (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

This talk is divided into two independent topics. In the first part of my talk we consider the order structure of hermitian matrices. Given two matrix domains (open connected sets of n-by-n hermitian matrices), what is the general form of order isomorphisms between them? I will explain that there is a complete correspondence between the class of order isomorphisms and that of biholomorphic mappings. In the second part we consider the metric structure of the space P(H) of all quantum pure states (= the projective space of a complex Hilbert space H). Wigner's theorem asserts that every surjective isometry of P(H) onto itself is implemented by a unitary or an antiunitary operator. Uhlhorn generalized Wigner's theorem by showing that every bijective transformation of P(H) that preserves orthogonality is implemented by a unitary or an antiunitary operator. We consider some variants of Uhlhorn's result. The first part is joint work with P. Semrl (Univ. of Ljubljana), and the second part with G.P. Geher (Univ. of Reading). Only basic linear algebra is assumed in both parts.

Venue: via Zoom

Event Official Language: English

Organ-to-organ two-way coupling contributes to the stabilization of circadian clock in Arabidopsis.

May 20 (Thu) 10:00 - 11:00, 2021

Kyohei Uemoto (Kyoto University / NARA Institute of Science and Technology)

Circadian clocks increase plant fitness by anticipating periodic environmental changes using unstable temporal information perceived in each tissue/organ. However, little is known about how such local and noisy temporal information regulates global and coordinated plant growth. Here, we show that nutrient-mediated two-way communication between shoots and roots stabilizes circadian rhythms and aids plant growth. The photosynthetic product, sucrose, is transported from the shoot to the root, where it regulates the expression of a clock gene and nutrients uptake rhythms. A lack of nutrient rhythms destabilizes the shoot circadian rhythms and reduces the growth rate. Our mathematical model supports that two-way communication between organs reduces the heterogeneity of the circadian rhythm, thereby maintaining the robustness of the circadian clock in a noisy environment. As with feedback loops in other hierarchies, nutrient-mediated shoot-root interorgan communication of the circadian clock is advantageous for proper growth under fluctuating environmental conditions. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Geometry of canonical metrics on Kähler manifolds

May 14 (Fri) 16:00 - 18:10, 2021

Eiji Inoue (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

The aim of this talk is to report recent trends in Kähler geometry. Kähler geometry consists of two aspects: the one is algebraic geometry and the other is metric geometry.The first one hour is an introduction for non-mathematicians. I begin with a simple example of algebraic variety from ancient Greek, which I believe is the simplest example illustrating motivation for compact complex manifolds. On the other hand, I explain the first motivation for canonical metrics in Kähler geometry via Riemann’s uniformization theorem.The last one hour is an introduction to recent trends in Kähler geometry, especially Kähler-Einstein metrics. The existence of Kähler-Einstein metrics turns out to be related to geometry of degenerations of space, which is so called Yau-Tian-Donaldson conjecture. I explain various aspects of this topic. We encounter deep studies in metric geometry, birational geometry and non-archimedean geometry. I finally explain recent breakthrough on Kähler-Ricci flow.The goal of this talk is the starting point of my study. I briefly explain my study if time permits.

Venue: via Zoom

Event Official Language: English

Alternative tsunami observing and forecasting systems

April 22 (Thu) 16:00 - 18:10, 2021

Iyan Mulia (Research Scientist, Prediction Science Laboratory, RIKEN Cluster for Pioneering Research (CPR))

Dedicated tsunami observing systems are mostly expensive and are often not sustainable. Therefore, alternative approaches should be implemented to overcome the issues. We introduced innovative ways to observe tsunamis using existing instrumentation available on unconventional platforms such as commercial vessels and airplanes. Our study demonstrated that the accuracy of the proposed observing systems is adequate for detecting large tsunamis offshore. The use of such systems is expected to provide more cost-effective and sustainable observations for the future. Additionally, we also developed a tsunami forecasting system based on machine learning to improve or complement the conventional methods that typically require considerable computational resources. On the contrary, the main appealing feature of the machine learning is the computational speed that would be suitable for a real-time prediction of tsunami inundation or flooding. We found that the application of machine learning can significantly improve the computing time without sacrificing the accuracy compared to the conventional methods.

Venue: via Zoom

Event Official Language: English

The rheotaxis mechanism of swimming ciliates

April 22 (Thu) 10:00 - 11:00, 2021

Yukinori Nishigami (Research Institute for Electronic Science, Hokkaido University)

The swimming unicellular organisms, which live in freshwater, need to resist currents in the environment. Without this ability, their habitat changes with the flow, and they cannot survive in nature for a long time. It is reported that a kind of swimming microorganism, Paramecium, exhibits upstream swimming in 1904. However, the mechanism of the behavior has been still unclear. To elucidate the mechanism, we observed the behavior of a ciliate in a flow field and performed numerical fluid calculations. My results suggest that the rheotaxis is realized by cell shape and inhibition of ciliary beating near the wall. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

iTHEMS x academist online open to the public "World of Mathematical Sciences 2021"

April 18 (Sun) 10:00 - 16:30, 2021

Kanato Goto (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Jeffrey Fawcett (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Takuya Sugiura (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Naomi Tsuji (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Keita Mikami (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Tetsuo Hatsuda (Program Director, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))

Venue: via Zoom

Event Official Language: Japanese

Mirror symmetry and KAM theory

April 16 (Fri) 13:30 - 15:00, 2021

Kenji Fukaya (Permanent Member, Simons Center for Geometry and Physics, Stony Brook University, New York, USA)

13:30pm-15:00pm (JST) Mirror symmetry is a phenomenon discovered in String theory and is much discussed recently in mathematics especially in the field of complex (algebraic) geometry and symplectic geometry. Strominger-Yau-Zaslow found that this phenomenon is closed related to a Lagrangian torus fibration. In an integrable system in Hamiltonian dynamics, the phase space is foliated by Lagrangian tori. I would like to explain a program that the Lagrangian torus fibration found by Strominger-Yau-Zaslow could be regarded as one appearing certain integrable system and KAM theory (which describes a amiltonian dynamics that is a perturbation of an integrable system) could appear in the situation of Mirror symmetry.

Venue: via Zoom

Event Official Language: English

Liquid condensates in cell nucleus

April 15 (Thu) 10:00 - 11:00, 2021

Kyosuke Adachi (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Special Postdoctoral Researcher, Nonequilibrium Physics of Living Matter RIKEN Hakubi Research Team, RIKEN Center for Biosystems Dynamics Research (BDR))

I will give a talk in a journal club style. I will introduce recent papers regarding liquid-liquid phase separation in cells. In cell biology, several types of liquid condensates of proteins/RNAs have been found recently. The functions of such condensates and the physical mechanism of controlling the liquid state are intensely discussed. In this talk, I will focus on liquid condensates in the cell nucleus and review recent experimental results. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Simple models of cancer growth, MCMC parameter estimation and identifiability

April 8 (Thu) 10:00 - 11:00, 2021

Catherine Beauchemin (Deputy Program Director, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Professor, Department of Physics, Ryerson University, Canada)

I would like to introduce some basic concepts about (very simple) mathematical model of cancer growth, the basic math behind parameter estimation via Markov chain Monte Carlo (MCMC) based on Bayes' theorem, and the different diagnostics you can use to know if the parameters are correctly estimated. I will use a recent example with cancer data in mice. I think this seminar can be interesting to mathematicians (because of the models and the math behind the parameter estimation, but the math is very basic!), to physicists (especially those that have to do some parameter estimation), and to biologists (the cancer model/data and the parameter estimation). I think it will also be interesting to the information theory and prediction science people. MCMC parameter estimation based on physical models is more valuable in my field than machine learning, so I think those interested in machine learning but maybe are not so familiar with MCMC should join to consider them as an alternative approach in certain contexts. *Please refer to the email to get access to the Zoom meeting room.

Venue: via Zoom

Event Official Language: English

Long-time behavior of moving solids in a fluid and the kinetic theory of gases

April 7 (Wed) 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 out-of-equilibrium 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 long-time 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

Structural reduction of chemical reaction networks based on topology

April 1 (Thu) 10:00 - 11:00, 2021

Yuji Hirono (Junior Research Group Leader/Assistant Professor, Research Division, Asia Pacific Center for Theoretical Physics, Republic of Korea)

Chemical reactions form a complex network in living cells and they play vital roles for physiological functions. An amusing question is how the structure of a reaction network is linked to its chemical functionalities. I’ll talk about a method of the reduction of chemical reaction networks, which is convenient for extracting important substructures. Mathematical concepts such as homology and cohomology groups are found to be useful for characterizing the shapes of reaction networks and for tracking the changes of them under reductions. For a given chemical reaction network, we identify topological conditions on its subnetwork, reduction of which preserves the steady state of the remaining part of the network.

Venue: via Zoom

Event Official Language: English

Evolutionary conservativeness and diversification of cycads: Understanding the evolution of living fossils

March 25 (Thu) 10:00 - 11:00, 2021

José Said Gutiérrez-Ortega (Assistant Professor, Institute for Excellence in Educational Innovation, Chiba University)

The cycads are a lineage of gymnosperms that represent an example of biological stasis success. Despite their early origin in the seed plant evolution, they survived multiple events of mass extinction and could diversify in modern tropical ecosystems during the Cenozoic, especially in countries known for their great biodiversity such as Mexico. What factors have allowed their persistence and diversification despite their conservative nature? I have studied the cycad genus Dioon, a group of 17 species occurring in habitats ranging from tropical forests to arid zones in Mexico and Honduras. Phylogenetic and phylogeographic analyses revealed that the diversification of Dioon has been driven by the long-term process of aridification of Mexico since the Miocene. The lineages that shifted from mesic forests to arid zones show leaf trait variations beneficial against water stress; this feature can be also observed at the inter-population level when comparing mesic versus arid sister pairs. What mechanism allows this aridification-driven diversification? Using population genetics and ecological niche modeling on sister lineage pairs, I have revealed that lineages at arid zones might tolerate arid environments, but within the arid habitat, they retain the same ancestral niche also observed on their mesic sisters. The surrounding areas that are suboptimal for their niches serve as barriers against gene flow: this promotes allopatric speciation. This research has revealed that the mechanism that allows the diversification process in Dioon involves three factors: 1) a habitat shift due to aridification, 2) niche conservatism that facilitates geographic isolation, 3) gaining unique morphological and anatomical features that help to counteract water stress, probably through long-term stabilizing selection. This research highlights the importance of biological conservatism in evolution, and how “living fossils” can still diversify into modern ecosystems.

Venue: via Zoom

Event Official Language: English

The Green-Tao theorem for number fields

March 22 (Mon) 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 Green-Tao theorem in such a direction. In the first 50 minutes of my talk, I would like to explain some background and technology behind the Green-Tao 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