Search Event
655 results
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Seminar
Efficient probabilistic assessment of building performance: sequential Monte Carlo and decomposition methods
November 13 (Fri) 16:00 - 18:10, 2020
Tianfeng Hou (Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Postdoctoral Researcher, Prediction Science Laboratory, RIKEN Cluster for Pioneering Research (CPR) / Postdoctoral Researcher, Data Assimilation Research Team, RIKEN Center for Computational Science (R-CCS))
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 quasi-Monte Carlo based probabilistic assessment for building performance, since it has the potential to outperform the standard Monte Carlo method. More specifically, the quasi-Monte Carlo sampling strategies and related error estimation techniques will be introduced in detail. In addition, questions on under which conditions the quasi-Monte 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 non-linearity and complexity will be illustrated.
Venue: via Zoom
Event Official Language: English
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Seminar
Some idea on quantum tunneling via Lefschetz thimbles
November 12 (Thu) 10:30 - 12:00, 2020
Yuya Tanizaki (Special Postdoctoral Researcher, Theory Group, RIKEN Nishina Center for Accelerator-Based Science (RNC) / Assistant Professor, Yukawa Institute for Theoretical Physics, Kyoto University)
In this talk, I will explain my previous study with Takayuki Koike on a possible approach to quantum tunneling via Lefschetz thimbles. We classified all the complex saddle points for the real-time path integral for the symmetric double-well quantum mechanics. We looked at various properties of those complex solutions, which motivated us to conclude that the computation of tunneling amplitudes for the symmetric double well requires the interference of infinitely many Lefschetz thimbles. I would also like to talk about some speculations, admittingly being very optimistic.
Venue: via Zoom
Event Official Language: English
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Seminar
Mathematical aspects of quasi-Monte Carlo integration
November 5 (Thu) 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 quasi-Monte Carlo (QMC) integration. We aim to approximate the integral of a function on the d-dimensional hypercube [0,1]^d. A useful approach is Monte-Carlo (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 star-discrepancy, Koksma-Hlawka 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
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Seminar
Evolution of a peak of genetic divergence driven by local adaptation
November 5 (Thu) 10:00 - 11:00, 2020
Takahiro Sakamoto (Postdoctoral Researcher, School of Advanced Sciences, The Graduate University for Advanced Studies (SOKENDAI))
In species that are distributed in various environments, each subpopulation adapts to the local environment. In general, when there is migration between subpopulations, genetic divergence does not proceed because the genomes are exchanged between subpopulations. However, around the loci involved in local adaptation, genetic divergence proceeds. This is because different genotypes are favored between subpopulations, so that the alleles of migrants are purged by natural selection and the exchange of genomes is suppressed. It has not been theoretically known how the degree of genetic differentiation evolves over time, making the interpretation of population genomic data difficult. In this study, we constructed and analyzed a model of population genetics to clarify the dynamics of genetic divergence.
Venue: via Zoom
Event Official Language: English
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Seminar
Basics of population genomic data analysis
October 29 (Thu) 10:00 - 11:00, 2020
Jeffrey Fawcett (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
In recent years, it has become possible to obtain the DNA sequence data of a large number of individuals of the same species. This data set is basically a M (number of samples) x N (number of genomic positions) matrix where each data point is 0 or 1. Using this data set, we try to understand, for example, the relationship between each sample or group of samples, and the population process that has generated the data set. In this talk, I will introduce the basic concepts behind the approaches we use to analyze such data sets.
Venue: via Zoom
Event Official Language: English
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Seminar
Toward simulating Superstring/M-theory on a Quantum Computer
October 23 (Fri) 17:00 - 18:00, 2020
Masanori Hanada (Rutherford Fellow, Department of Mathematics, University of Surrey, UK)
We present a framework for simulating superstring/M-theory on a quantum computer, based on holographic duality. Because holographicduality maps superstring/M-theory to quantum field theories (QFTs), we can study superstring/M-theory if we can put such QFTs on a quantum computer --- but it still looks like a complicated problem, if we use a usual lattice regularization. Here we propose an alternative approach, which turns out to be rather simple: we map the QFT problems to matrix models, especially the supersymmetric matrix models such as the Berenstein-Maldacena-Nastase (BMN) matrix model. Supersymmetric matrix models have natural applications to superstring/M-theory and gravitational physics, in an appropriate limit of parameters. Furthermore, for certain states in the Berenstein-Maldacena-Nastase (BMN) matrix model, several supersymmetric quantum field theories dual to superstring/M-theory can be realized on a quantum device. It is straightforward to put the matrix models on a quantum computer, because they are just quantum mechanics of matrices, and the construction of QFTs is mapped to the preparation of certain states. We show the procedures are conceptually rather simple and efficient quantum algorithms can be applied. In addition, as a (kind of) byproduct, we provide a new formulation of pure Yang-Mills on quantum computer. If you would like to participate, please register using the form below.
Venue: via Zoom
Event Official Language: English
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Seminar
Realistic shell model and chiral three-body force
October 22 (Thu) 13:30 - 15:00, 2020
Tokuro Fukui (Researcher, Yukawa Institute for Theoretical Physics, Kyoto University)
We show an evolution to derive the effective Hamiltonian in the shell-model framework starting from two- and three-body interactions based on the chiral effective field theory. A new way to calculate three-body matrix elements of the chiral interaction with the nonlocal regulator is proposed. We apply our framework to the p-shell nuclei and perform benchmark calculations to compare our results with those by an ab initio no-core shell-model. We report that our results are satisfactory and the contribution of the three-body force is essential to explain experimental low-lying spectra of the p-shell nuclei. We discuss the contribution of the three-body force on the effective single-particle energy extracted from the monopole interaction. Next, we investigate the shell evolution on the pf-shell nuclei. We show that the monopole component of the shell-model effective Hamiltonian induced by the three-body force plays an essential role to account for the experimental shell evolution. Moreover, we present our latest results on the investigation of the possible neutron dripline of the Ca isotopes. Finally, we discuss very neutron-rich systems, namely, the oxygen isotopes at the dripline and beyond, where the interplay between the three-body force and continuum states plays an important role. If you would like to participate, please register using the form below.
Venue: via Zoom
Event Official Language: English
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External Event
Daikanyama College of Music Lecture Series Vol.1 - Music x Mathematics : May not music be described as the mathematics of the sense, mathematics as music of the reason?
October 21 (Wed) 20:00 - 21:00, 2020
Takashi Tsuboi (Deputy Program Director, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Professor, Musashino University)
Prof. Takashi Tsuboi, Deputy Program Director of iTHEMS will explain the beauty and harmony of Mathematics together with the piano performance by Rutsuko Yamagishi.
Venue: via Online
Event Official Language: Japanese
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Seminar
Bayesian nonparametric estimation of Random Dynamical Systems
October 21 (Wed) 14:00 - 15:00, 2020
Christos Merkatas (Postdoctoral Researcher, Aalto University, Finland)
In this talk, a Bayesian nonparametric framework for the estimation and prediction, from observed time series data, of discretized random dynamical systems is presented [1]. The size of the observed time series can be small and the additive noise may not be Gaussian distributed. We show that as the dynamical noise departs from normality, simple Markov Chain Monte Carlo method (MCMC) models are inefficient. The proposed models assume an unknown error process in the form of a countable mixture of zero mean normals, where a–priori the number of the countable normal components and their variances is unknown. Our method infers the number of unknown components and their variances, i.e., infers the density of the error process directly from the observed data. An extension for the joint estimation and prediction of multiple discrete time random dynamical systems based on multiple time-series observations contaminated by additive dynamical noise is presented [2]. In this case the model assumes an unknown joint error process with a pairwise dependence in the sense that to each pair of unknown dynamical error processes, we assign a– priori an independent Geometric Stick-Breaking process mixture of normals with zero mean. These mixtures a–posteriori will capture common characteristics, if there are any, among the pairs of noise processes. We show numerically that when the unknown error processes share common characteristics, it is possible under suitable prior specification to induce a borrowing of strength relationship among the dynamical error pairs. Then time-series with an inadequate sample size for an independent Bayesian reconstruction can benefit in terms of model estimation accuracy. Finally, possible directions for future research will be discussed.
Venue: via Zoom
Event Official Language: English
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Seminar
A PDE model for the localization and spread of flu in the human respiratory tract
October 14 (Wed) 10:00 - 11:00, 2020
Christian Quirouette (Ph.D. Student, Department of Medical Physics, Ryerson University, Canada)
Within the human respiratory tract (HRT), virus diffuses through the periciliary fluid (PCF) bathing the epithelium. But it also undergoes advection: as the mucus layer sitting atop the PCF is pushed along by the ciliated cell's beating cilia, the PCF and its virus contents are also pushed along, upwards towards the nose and mouth. Our PDE model represents the HRT as a one-dimensional track extending from the nose down to the lower HRT, wherein stationary cells interact with virus which moves within (diffusion) and along with (advection) the PCF. In the PDE model, diffusion is negligible in the presence of advection which effectively sweeps away virus, preventing infection from spreading below the depth of deposition. Higher virus production rates (10-fold) are required at higher advection speeds (40 micron/s) to maintain equivalent infection severity and timing. Because virus is entrained upwards, upper parts of the HRT located downstream of the advection flow see more virus than lower parts, and so infection grows, peaks, and resolves later in the lower HRT. Clinically, the infection would appear to progress from the upper towards the lower HRT, as reported in mice. When the PDE model is expanded to include cellular regeneration and an immune response, it reproduces tissue damage levels reported in patients. This new PDE model offers a convenient and unique platform from which to study the localization and spread of respiratory viruses (flu, RSV, COVID-19) within the HRT during an infection.
Venue: via Zoom
Event Official Language: English
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External Event
Tetsuo Hatsuda x Koji Hashimoto x Aiko Teranishi "Science for the Future"-"The Usefulness of Useless Knowledge" (University of Tokyo Press) Commemorative Publication
October 10 (Sat) 15:00 - 17:00, 2020
Tetsuo Hatsuda (Program Director, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Koji Hashimoto (Professor, Department of Physics, Osaka University)
Aiko Teranishi (President and CEO, indigo.inc)Dr. Tetsuo Hatsuda, iTHEMS Program Director, published "The Usefulness of Useless Knowledge" in this July by the University of Tokyo Press. This book is a collection of essays by Abraham Flexner, the first director of the Institute for Advanced Study in Princeton, USA, and Robbert Dijkgraaf, the current director. To celebrate the publication of this book, a talk event will be held at Bookstore B&B. For more information and to book the event, please visit the related links (in Japanese).
Venue: via Zoom
Event Official Language: Japanese
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External Event
Fast Radio Bursts: a cosmic mystery
September 28 (Mon) 10:30 - 11:00, 2020
Don Warren (Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
On Monday morning, iTHEMS researcher Don Warren will give a public talk on fast radio bursts. He will describe this cosmic mystery, and reveal how impatient Australians play an important role in the story. This talk is part of the global Stream You series, hosted by National Geographic and Nerd Nite. Don’s talk begins at 10:30am, but there will be three talks before his starting at 9:00am. To watch Don’s talk, or any of the others, visit facebook.com/nerdnite on Monday and look for the YouTube link.
Venue: via Online
Event Official Language: English
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Seminar
Math Seminars by Dr. Genki Ouchi and Dr. Kenta Sato
September 24 (Thu) 16:00 - 18:10, 2020
Genki Ouchi (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Kenta Sato (Special Postdoctoral Researcher, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (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
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Seminar
Phase Transitions in Biological Systems
September 23 (Wed) 10:00 - 11:00, 2020
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))
Biological systems are built hierarchically by DNA, proteins, cells, tissues, organs, individuals, etc. Recent experiments have clarified the existence of interesting mesoscale phenomena inside cells, where the concept of condensed matter physics such as phase transition can be useful in its understanding. For example, interacting nucleosomes in a chromatin chain can cause the mega-base scale structural change, and sub-micron scale dense droplets of proteins/mRNAs can appear through phase separation. In this talk, I will discuss our recent topics: (i) structural transition of a chromatin with epigenetic marks, (ii) intracellular wetting of phase-separated droplets, and (iii) spontaneous aggregation of self-propelled individuals.
Venue: via Zoom
Event Official Language: English
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Seminar
Singular point implies coexistence in adaptive dynamics
September 9 (Wed) 10:00 - 11:00, 2020
Masashi Tachikawa (Visiting Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS) / Associate Professor, Institute for Frontier Life and Medical Sciences, Kyoto University)
Adaptive dynamics is a relatively new mathematical framework for studying evolution(~1990s). Under the influence of the mathematical ecology and the game theory, adaptive dynamics considers the effect of resident populations on the fitness landscape. As a result, it explains a possible mechanism of evolutionary branching. In this talk, I introduce adaptive dynamics and Pairwise Invasibility Plot (PIP) analysis, a standard method for understanding the adaptive dynamics. Then, I propose a new approach to analyze the adaptive dynamics which enable us to understand higher dimensional systems than PIP does.
Venue: via Zoom
Event Official Language: English
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Seminar
Maximal Regularity and Partial Differential Equations
September 8 (Tue) 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 well-posedness of various non-linear PDEs. In the first part, We introduce the history of the development of the theory of maximal regularity and the way to apply non-linear PDEs. In the second part, We give some applications to PDEs, e. g. the primitive equations, the Navier-Stokes 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
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Seminar
Modeling biological timing
August 26 (Wed) 10:00 - 11:00, 2020
Gen Kurosawa (Senior Research Scientist, RIKEN Interdisciplinary Theoretical and Mathematical Sciences Program (iTHEMS))
Under stay-at-home situation, some of you may suffer from sleep disorder. Efficacy of a drug often depends on the timing of its prescription. We know this fact about our "timing", but we don't know why. This time, I wish to introduce two big mysteries in regard to biological timing. First is our internal daily clock. In general, biochemical process is believed to accelerate with temperature. In contrast, the period of our daily clock, made up of biochemical reactions is somehow stable to temperature. The prediction from simpler biochemical mathematical model, and its experimental verification will be presented. Second is hibernation. During winter, some birds and mammals decrease drastically their body temperature possibly to decrease their energy expenditure. Many studies about hibernation have been conducted for many years. However, basic mechanisms of hibernation (e.g. how the duration of hibernation is determined?) are largely unknown. Recently, we started to investigate body temperature time-series of hibernating hamsters over 100 days in the collaboration with experimental biologists. Preliminary results will be presented.
Venue: via Zoom
Event Official Language: English
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Seminar
Lefschetz-thimble inspired analysis of the Dykhne–Davis–Pechukas method and an application for the Schwinger Mechanism
August 21 (Fri) 13:00 - 14:30, 2020
Takuya Shimazaki (Researcher, Hadron Theory Group, The University of Tokyo)
Dykhne–Davis–Pechukas (DDP) method is a common approximation scheme for the transition probability in two-level quantum systems, as realized in the Landau–Zener effect, leading to an exponentially damping form comparable to the Schwinger pair production rate. We analyze the foundation of the DDP method using a modern complex technique inspired by the Lefschetz-thimble method. We derive an alternative and more adaptive formula that is useful even when the DDP method is inapplicable. As a benchmark, we study the modified Landau–Zener model and compare results from the DDP and our methods. We then revisit a derivation of the Schwinger Mechanism of particle production under electric fields using the DDP and our methods. We find that the DDP method gets worse for the Sauter type of short-lived electric pulse, while our method is still a reasonable approximation. We also study the Dynamically Assisted Schwinger Mechanism in two methods.
Venue: via Zoom
Event Official Language: English
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Seminar
Nambu-Goldstone fermion in a Bose-Fermi mixture with an explicitly broken supersymmetry
August 7 (Fri) 13:00 - 14:30, 2020
Hiroyuki Tajima (Assistant Professor, Department of Natural Science, Kochi University)
Supersymmetry, which is a symmetry associated with interchange between bosons and fermions, is one of the most important symmetries in high-energy physics but its evidence has never been observed yet. Apart from whether supersymmetric partners such as squark exist or not in our world, it is an interesting problem to explore the consequences of the supersymmetry in an ultracold atomic gas. In this study, we address the Nambu-Goldstone mode called Goldstino associated with the spontaneous supersymmetry breaking in a Bose-Fermi mixture. While the explicit supersymmetry breaking is unavoidable even in cold atomic systems, the energy gap in Goldstino spectra can be measured in such atomic systems. By comparing the energy gaps obtained from the Gell-Mann-Oakes-Renner relation and the random phase approximation, we elucidate how the Goldstino acquires the energy gap due to the explicit breakings. We also show effects of Goldstino pole on the fermionic single-particle spectral functions, which can be measured in the recent experiments.
Venue: via Zoom
Event Official Language: English
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Seminar
Heavy tails in the brain
August 5 (Wed) 10:00 - 11:00, 2020
Lukasz Kusmierz (Research Scientist, RIKEN Center for Brain Science (CBS))
In my talk I will discuss the relation between two seemingly unrelated measures in the brain that exhibit heavy tails: neuronal avalanches, i.e. bursts of activity with power-law distributions of sizes and lifetimes, and synaptic weights that are believed to be distributed according to the log-normal distribution. Many current models of neuronal avalanches do not rely on heavy-tailed synaptic weight distributions, suggesting that heavy tails of these two quantities may not be related. However, our recent theoretical considerations indicate that this independence no longer holds if two biologically relevant constraints are introduced, i.e., that neurons (1) receive many incoming connections and (2) do not spike if the membrane potential is below some positive threshold, e.g., in the absence of inputs. Under these assumptions we have shown that heavy tails of synaptic weights are necessary to generate biologically plausible low activity levels and associated neuronal avalanches. Our results suggest that the observed distributions of synaptic weights may play important functional roles in the brain.
Venue: via Zoom
Event Official Language: English