# Coffee Meeting Log

2022-11-25

## Counting by Drawing

Michel Van Garrel (Assistant Professor, School of Mathematics, University of Birmingham, UK)

How many other circles can you find that meet each of the 3 circles in exactly 1 point? I will talk about fun questions like this one that are solved by drawing, and have been with mathematics all along.

2022-11-18

## Knot theory and its interactions with other fields

Masaki Taniguchi (Special Postdoctoral Researcher, iTHEMS)

Knot theory is one of subjects in the field of topology. I'll exlain what are purposes of knot theory and its interactions with other fields.

2022-11-11

## New approach to spontaneous symmetry breaking by gradient flow

Kengo Kikuchi (Special Postdoctoral Researcher, iTHEMS)

The spontaneous symmetry breaking (SSB) is one of the most important concept in the elementary particle physics. In this coffee meeting, I explain the fundamental content of the SSB briefly, and after that, I talk about our recently research, new approach to analyze the phase structure of SSB using the gradient flow method.

2022-11-04

## Waveform analysis of biological oscillatory models

Shingo Gibo (Postdoctoral Researcher, iTHEMS)

In biological systems, many oscillatory phenomena emerge. For example, our sleep-wake rhythms are regulated by gene activity oscillation with a period of 24 hours. Time series of these biological oscillations are of various shapes. In this talk, I would like to talk about the effect of the waveform on period stability and synchronization.

2022-10-28

## A new B.O.A.T. in astrophysics

Don Warren (Research Scientist, iTHEMS / Research Scientist, Astrophysical Big Bang Laboratory, RIKEN Cluster for Pioneering Research (CPR))

On October 9 of this year, an extremely bright gamma-ray burst was detected — it has been called the "brightest of all time". I will briefly describe some of the ways this burst was so extraordinary, and what science we can do with such an unusual opportunity.

2022-10-21

## Geometirical chracteristics of a polymer chain: twist and writhe

Hiroshi Yokota (Postdoctoral Researcher, iTHEMS)

Polymer is a string-like molecules composed of molecular units. Many physical phenomena on polymers are described by using consecutive beads connected by springs (bead-spring model). Although this model is widely used, sometimes this model is not sufficient when the twist or writhe structures are considered. In this talk, I would like to introduce the kink structure (twist and writhe) and its mathematical and physical description. And then, I would like to talk about the computational treatment of twist and writhe.

2022-10-14

## Evolution and our daily rhythms

Gen Kurosawa (Senior Research Scientist, iTHEMS)

Imagine that you are in a room without information of time. The room is in a cave so that temperature and light-intensity are constant over time. Can you wake up tomorrow or day after tomorrow? In fact, most humans can wake up tomorrow and day after tomorrow almost regularly. It is because we have daily rhythms in our body. Biological experiments have shown that not only humans but also other many species on the Earth have these daily rhythms. In this talk, unsolved problems about the rhythms, and some approaches from the point of view of dynamical system will be introduced.

2022-10-07

## Various approaches to the sign problem

Akira Matsumoto (Postdoctoral Researcher, iTHEMS)

The Monte Carlo simulation is a powerful tool to study the non-perturbative aspect of quantum field theory. However, the Monte Carlo method is applicable to the system with a real action only. If the action is complex, it is difficult to handle the rapidly oscillating phase in the path integral, which is known as the sign problem. This problem prevents us from simulating various interesting systems, such as finite density QCD, topological theta term and real time evolution. In this talk, I introduce several approaches to overcome the sign problem and compare their features.

2022-09-30

## A primitive derivation of black hole entropy

Yuki Yokokura (Senior Research Scientist, iTHEMS)

Black holes have entropy. While a black hole occupies a three-dimensional spatial domain, the entropy is given by its two-dimensional surface area. In this sense, the entropy is holographic. However, its true origin is still unknown, and many researchers are studying it using various approaches. In this talk, I will provide an intuitive derivation of the entropy according to Bekenstein's first discussion in 1973. In particular, I will emphasize that it is the result of a combination of quantum theory and gravity. I will also give a brief review of the basics of physics so that people in other fields can enjoy how this mysterious formula appears.

2022-09-16

## Area law of entanglement entropy in quantum many body systems and its implication in tensor network calculation

Yantao Wu (Postdoctoral Researcher, iTHEMS)

In this coffee talk, I will explain the idea of entanglement entropy and how it has instructed people to construct a class of ground state wavefunction ansatz for quantum many-body systems. I will be pedagogical and explain the general construction in 1D and 2D. If time permits, I will explain how fermionic versions of them are realized.

2022-09-09

## What’s the value of reproductive value?

Thomas Hitchcock (Special Postdoctoral Researcher, iTHEMS)

Populations are often heterogenous, composed of individuals of different sexes, ages, and condition. The way that genes flow between these different states across time can structure the ancestry of the population, and subsequently generate changes in allele frequency even in the absence of any other evolutionary forces. In this talk I discuss the concept of reproductive value, which provides a description of the expected long-term contribution each state makes to future populations. This tool allows us to aggregate the effects of different evolutionary forces across these different classes of individual, and thus better understand their relative importance. I briefly illustrate the usefulness of these concepts by discussing the evolution of senescence.

2022-09-02

## Coarse Notions of Curvature

Christy Koji Kelly (Special Postdoctoral Researcher, iTHEMS)

Curvature is a fundamental geometric notion with important applications in a variety of physical theories. Typically curvature is defined in smooth (differentiable) contexts but there has been much recent interest in synthetic characterisations of curvature in much rougher spaces than differentiable manifolds---including discrete spaces like networks. In this talk we aim to introduce some of the main coarse curvatures, particular in relation to optimal transport theory.

2022-08-26

## How to carry out a hadron experiment

Natsuki Tomida (Visiting Scientist, iTHEMS / Specially Appointed Assistant Professor, Center for Science Adventure and Collaborative Research Advancement (SACRA), Graduate School of Science, Kyoto University)

I might be the only experimentalist in iTHEMS. I have been working for studying hadrons at SPring-8. Hadron experiments are unique in its large scale of equipment, time, man-power and budget. I would like to introduce how a hadron experimentalist carry out experiments.

2022-08-05

## Operad and consistency of 2d conformal field theories

Yuto Moriwaki (Special Postdoctoral Researcher, iTHEMS)

An operad is a mathematical notion which describes an algebra with infinitely many multiplication structures. I will explain my recent result that "operads can be used to describe the consistency of two-dimensional conformal field theories.

2022-07-29

## Outreach of RIKEN iTHEMS 2022

Takashi Tsuboi (Deputy Program Director, iTHEMS / Musashino University)

2022-07-22

## Overview of artificial selection

Jeffrey Fawcett (Senior Research Scientist, iTHEMS)

Humans have been utilizing many plants, animals, and microorganims for several thousand years. As a result of selective breeding, also called artificial selection, domesticated species that we use differ from their wild progenitor species (e.g. dogs vs wolves) and contain a wide range of morphological diversity within them (e.g. different dog breeds). This process of artificial selection has been an excellent model to study evolution and natural selection ever since Charles Darwin. Moreover, studying artificial selection is important in our current efforts to improve the efficiency of selective breeding, and also provides new insights into human history. Here, I will provide an overview of artificial selection and introduce the projects on buckwheat that I'm involved in.

2022-07-15

## Smale horseshoe: paradigm of chaos

Jizhou Li (Postdoctoral Researcher, iTHEMS)

Ordinary differential equations (ODEs) are mathematical tools ubiquitous in the field of theoretical biology. Quite often, in their implementations, we simply integrate them carefully (or not) with sophisticated numerical packages and accept whatever the outcome as the “correct” answer. However, according to chaos theory, due to sensitive dependence on initial errors, any numerical integrators are doomed to fail if the time scale for the integration is large enough. One of the central goals of chaos theory is to study the solutions of nonlinear ordinary differential equations from an alternative perspective, namely, qualitative and geometric studies of dynamical systems governed by ODEs. In this week’s coffee meeting time, G-Joe will briefly introduce an oversimplified “geometrization” of ODEs systems using Poincare maps and Smale Horseshoes. Such a route aims to reduce the study of general ODE systems into the investigation of generic toy models, i.e., the “Horseshoe”, to gain insights into the fundamental structures otherwise hidden in the numerical solutions. As for almost everything in pure mathematics, such an approach is simple, elegant, and useless (for now), and I hope you will enjoy it with a cup of coffee.

2022-07-08

## Geometry in positive characteristic

Shou Yoshikawa (Special Postdoctoral Researcher, iTHEMS)

Algebraic geometry is a subject to study spaces defined by algebraic equations. It is separated into algebraic geometry in characteristic zero and algebraic geometry in positive characteristic. The first one is studying spaces which are similar to our living world, but the latter geometry is far from our geometric sense. However, we can regard our living world is a limit of spaces in positive characteristic. By the viewpoint, we can reduce problems in characteristic zero to similar problems in positive characteristic, the thechnic is called the reduction to positive characteristic. In this talk, I will introduce the notion of positive chracteristic and what reduction to positive characteristic is.

2022-07-01

## Isospin Symmetry Breaking in Nuclear Physics

Tomoya Naito (Special Postdoctoral Researcher, iTHEMS)

Properties of neutron stars, such as the mass and radius relation, are one of the hot topics in astrophysics. Nuclear interaction determines such properties and available data of neutron stars are rather limited; hence, theoretical and experimental studies on nuclear physics help to understand neutron star properties. Protons and neutrons have almost the same properties apart from their charge, which is called isospin symmetry. Accordingly, the nuclear interaction also has isospin symmetry. However, tiny contribution of isospin symmetry breaking of nuclear interaction gives large systematic uncertainty for discussion of neutron star properties.

2022-06-24

## Categorification of the Jones polynomial

Taketo Sano (Special Postdoctoral Researcher, iTHEMS)

Jones polynomial is a knot invariant discovered by V. F. R. Jones in 1984. Not only that it is a useful mathematical tool, the discovery led to opening up a new research area, quantum topology, which connects quantum mechanics and low-dimensional topology. In 2000, M. Khovanov introduced a “categorification of the Jones polynomial”, which is now called Khovanov homology, and made categorification one of the fundamental concept in knot theory.