# Hot Topic

## Farewell message from Dr. Martin Skrodzki

2020-12-03

When my scholarship program first notified me that I would be able to spend a year, working in the RIKEN iTHEMS program, I was extremely happy for this great interdisciplinary and intercultural opportunity. Now, this year has ended and I will return to Germany to continue my research there and possibly at other locations in Europe.

I arrived at RIKEN iTHEMS on January 20. While I expected the year to be very exciting, I did not at all imagine what 2020 had in store for all of us. After my first talk in the math seminar, we went to an Izakaya and had a Nomikai there. From the current perspective, this appears quite unreal to me. The Covid-19 pandemic caused this year to be run quite different from any other year. We all had to get used to working from home, working online, and even enjoying each other’s company during online coffee as well as online parties. Despite these restrictions, I tremendously enjoyed my stay in Wako and in Japan in general.

Several aspects at iTHEMS contribute to a very friendly and positive atmosphere in the program. For instance, the coffee afternoons were a great way throughout my first couple of weeks to get to know many members of the workgroup. Also the all-hands coffee meetings on Friday are a great installment to stay in touch with each other. Furthermore, the dedicated math seminars were great for me to get new input, learn new mathematics, and chat with the colleagues. The organizers of the math seminar, but also all other chairs and the speakers, made the seminars very enjoyable events. Aside from these events, I was very happy to pursue my own research projects. As I am in the first phase of my postdoctoral career, this was a great opportunity to sharpen my research foci.

Thus, I would like to close by thanking everyone for making my stay at RIKEN iTHEMS as enjoyable as it was. Once the global situation has settled down, I would love to return for a short visit or even another extended research stay. Let’s stay in touch!

# Seminar Report

## Biology Seminar by Dr. Hidenori Tanaka on November 20, 2020

2020-11-30

In iTHEMS Biology Seminar on November 20th, Dr. Hidenori Tanaka (Physics & Informatics Laboratories, NTT Research) gave us an exciting talk about physics principles in neural networks. He first reviewed the basic scheme of deep learning using neural networks. Then, he presented three questions regarding both neural science and machine learning and explained his recent works which address these questions. He stressed how physics principles like symmetry and conservation laws are useful in extracting minimal features of biological circuit models, improving algorithms to simplify neural networks, and predicting learning dynamics of neural networks. As his talk was clear and kind to both specialists and non-specialists, there were various questions from the audience. Hidenori is a very active researcher, and I was happy to invite him as a guest speaker.

Kyosuke Adachi (BDR/iTHEMS)

Symmetry and conservation laws in neural networks

November 20 at 10:00 - 11:00, 2020

# Upcoming Events

## External Event

### Presented by iTHEMS & academist - Do you really know what the black hole is?

December 6 at 14:00 - 16:00, 2020

Dr. Shigehiro Nagataki (Deputy Program Director, iTHEMS / Chief Scientist, RIKEN Astrophysical Big Bang Laboratory)

Dr. Yoshiyuki Inoue (Senior Visiting Scientist, iTHEMS / Associate Professor, Department of Earth and Space Science, Graduate School of Science, Osaka University)

Dr. Yuki Yokokura (Senior Research Scientist, iTHEMS)

–From the latest theories and observations to the explanation of the Nobel Prize in Physics! An introduction to black holes from active physicists–

The 2020 Nobel Prize in Physics was awarded to Sir Roger Penrose, Prof. Dr. Reinhard Genzel and Prof. Andrea Ghez for their contributions to the theory and observation of black holes. Black holes have continued to provide hot topics in recent years, such as gravitational waves from black hole coalescence and black hole imaging, but do you really know what black holes are? Three cutting-edge black hole researchers will explain its identity and mystery.

Venue: via Online

Event Official Language: Japanese

## Seminar

### Scattering theory for half-line Schrödinger operators: analytic and topological results

December 7 at 16:00 - 18:10, 2020

Dr. 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 half-line. This talk is based on my Ph.D thesis.

Venue: via Zoom

Event Official Language: English

## Seminar

### Conserved charges in gravity and entropy

December 10 at 13:00 - 14:30, 2020

Dr. Shinya Aoki (Professor, Yukawa Institute for Theoretical Physics, Kyoto University)

We propose a manifestly covariant definition of a conserved charge in gravity. We first define a charge density from the energy momentum tensor with a Killing vector, if exists in the system, and calculate the energy (and angular momentum) of the black hole by a volume integral. Our definition of energy leads to a correction of the known mass formula of a compact star, which includes the gravitational interaction energy and is shown to be 68\% of the leading term in some case. Secondly we propose a new method to define a conserved charge in the absence of Killing vectors, and argue that the conserved charge can be regarded as entropy, by showing the 1st law of thermodynamic for a special case. We apply this new definition to the expanding universe, gravitational plane waves and the black hole. We discuss future directions of our research.

Venue: via Zoom

Event Official Language: English

## Colloquium

### Geometry (形); Inconspicuous regulator that determines the fate of cells

December 14 at 10:00 - 11:30, 2020

Prof. Sungrim Seirin-Lee (Professor, Hiroshima University)

December 14 at 10:00-11:30, 2020 (JST)

December 13 at 20:00-21:30, 2020 (EST)

In the history of mathematical study in pattern formation, the effect of domain has been considered as an important factor that can regulate spatial patterning. However, it is still unknown in biology how the geometry of the domain such as nuclear or cellular shapes can directly regulate the cell fate. In this talk, I will introduce two studies of spatial reorganization in chromatin and cellular dynamics and show that the domain is likely to play a critical role in determining the cell function.

Venue: via Zoom

Event Official Language: English

## Seminar

### Non-perturbative tests of duality cascades in three dimensional supersymmetric gauge theories

December 14 at 16:00 - 18:10, 2020

Dr. Naotaka Kubo (Postdoctoral Researcher, Yukawa Institute for Theoretical Physics, Kyoto University)

M2-brane is an interesting object in M-theory and string theory. A three-dimensional 𝒩=6 super conformal Chern Simons theory with gauge group U(𝑁1)×𝑈(𝑁2), called ABJ theory, describes the low energy behavior of M2-brane On the one hand, it has been considered that when |𝑁1−𝑁2| is larger than the absolute value of Chern Simons level, the supersymmetry is broken. On the other hand, it was predicted that an interesting phenomenon called duality cascade occurs, and supersymmetry is not broken in some cases.

Motivated by this situation, we performed non-perturbative tests by focusing on the partitionfunction on 𝑆3. The result strongly suggests that the duality cascade indeed occurs. We also proposed that the duality cascade occurs in theories with more general gauge groups and we performed non-perturbative tests in the same way. I will review and explain our physical prediction in the first half of my talk. In the second half of my talk , I will explain the non-perturbative tests . This part is mathematical because the partition function reduces to a matrix model by using the supersymmetric localization technique.

Venue: via Zoom

Event Official Language: English

## Seminar

### Classical liquids and functional renormalization group

December 17 at 13:00 - 14:30, 2020

Dr. Takeru Yokota (Postdoctoral Researcher, Institute for Solid State Physics, The University of Tokyo)

Development of methods for classical statistical mechanics is desired for accurate predictions of the structures and thermodynamic properties of liquids. A powerful framework to describe classical liquids is density functional theory (DFT). In the quantum case, there have been recent attempts to develop accurate methods with combining DFT and the functional renormalization group (FRG), which is another framework to deal with many-body systems utilizing evolution equations, and such approaches are expected to work also in the classical case. In this presentation, I will talk about a new approach for classical liquids aided by FRG. The formalism and some ideas to incorporate higher-order correlation functions to systematically improve the accuracy will be shown. I will also present a numerical demonstration in a one-dimensional exactly solvable system and discuss the results by comparing to other conventional methods such as the hypernetted chain.

Venue: via Zoom

Event Official Language: English

# Paper of the Week

## Week 1 of December

2020-12-03

Title: Consistent description for cluster dynamics and single-particle correlation

Author: Naoyuki Itagaki, Tomoya Naito

arXiv: http://arxiv.org/abs/2011.12642v1

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