Rational function semifields of dimension one

November 7 (Fri) 13:30 - 15:30, 2025

JuAe Song (Assistant Professor, Faculty of Mathematics, Kyushu University)

Recently some researchers gave many studies toward algebro-geometric foundation for tropical geometry. I focused on rational function semifields of tropical curves and characterized them. With this characterization, in this talk, I suggest a definition of ``rational function semifield of dimension one". This definition can write out weight in the term of $\boldsymbol{T}$-algebra homomorphism, and can write balancing condition together with harmonic functions, where both weight and balancing condition are fundamental concepts for tropical varieties and $\boldsymbol{T}$ is the tropical semifield $(\boldsymbol{R} \cup \{-\infty\}, \operatorname{max}, +)$.

Venue: Seminar Room #359 / via Zoom

Event Official Language: Japanese

Pairing in Bose-Fermi and Fermi-Fermi systems

November 6 (Thu) 15:00 - 16:30, 2025

Pierbiagio Pieri (Associate Professor, Dipartimento di Fisica e Astronomia “Augusto Righi”, Università di Bologna, Italy)

This seminar is co-hosted by GWX-EOS Working Group and iTHEMS-ABBL Joint Astro Study Group. Abstract: In the first part of my talk, I will review recent work on Bose-Fermi mixtures with an attractive interaction inducing pairing between bosons and fermions. After discussing a recent experiment on this system [1], which has confirmed predictions obtained by us some time ago within a many-body diagrammatic approach [2], I will present novel results for the compressibility [3] that suggest a metastable nature for the many-body phase observed in [1]. Then, I will discuss the extension of our calculations to two-dimensional Bose-Fermi mixtures [4,5]. The results obtained in 2D challenge previous beliefs formulated for 3D systems. In the second part, I will discuss attractive polarized Fermi systems, for which the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase was proposed many years ago as a possible superfluid phase. I will discuss how significant precursor FFLO fluctuation effects appear already in the normal phase of polarized Fermi gases at finite temperature [6], and how they could be experimentally detected with ultracold gases. At zero temperature [7], I will discuss how the quasi-particle parameters of the normal Fermi gas change when approaching an FFLO quantum critical point, with a complete breakdown of the quasi-particle picture analogous to what found in heavy-fermion materials at an antiferromagnetic quantum critical point. Finally, I will discuss a recent joint experimental-theoretical work on the motion of a vortex orbiting a pinned anti-vortex in a strongly interacting Fermi gas [8], highlighting the interplay between Andreev bound states in the vortex core and delocalized thermal excitations in shaping the vortex dynamics.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Semiotic Rupture and the Emergence of Writing: Toward a Multimodal Model of Representational Innovation

November 6 (Thu) 13:00 - 14:00, 2025

Joshua Englehardt (Professor, Center of Archeologist Studies, El Colegio de Michoacán, Mexico)
Michael D. Carrasco (Associate Dean for Research / Associate Professor, College of Fine Arts, Florida State University, USA)

Writing is a unique—and distinctively human—creation, one which arose independently in only six locations worldwide. From these primary sites of innovation, this relatively recent technology spread across the world. Its development is routinely lauded as one of humanity’s most important inventions, among its “greatest intellectual and cultural achievements,” and a key to human evolution. The scholar Florian Coulmas labels it “the single most important sign system ever invented on our planet. This presentation presents a theoretical framework for modeling the emergence, development, and structure of writing and other visual representational systems through a formal, processual lens. Building on Noam Chomsky’s distinction between internal language (I-language) and its externalization as E-language, we model writing as the mediated product of E-language and propose a set of visual analogues: I-image and E-image, understood as structurally similar generative systems. We offer a formal, cross- and multimodal model of writing and its development that treats it not as a codified extension of speech, but as a recursive reorganization of visual and linguistic generative systems. Rather than asking what writing is, we ask how it and other semiotic systems emerge. What tensions, pressures, and interactions catalyze their formation, transformation, and typological diversity? We contend that the semiotic dynamics that give rise to writing are not isolated or unique events, but are grounded in deeper processes, such as those underlying the emergence of image-making, that are already established in the cognitive evolution of Homo sapiens and plausibly present in ancestral hominins. That is, we see writing not as a spontaneous invention but as an emergent semiotic modality grounded in cognitive evolution and cultural externalization.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Introduction to Game Theory #2

November 6 (Thu) 11:00 - 12:00, 2025

Yohsuke Murase (Team Director, Mathematical Social Science Team, Division of Applied Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

An introductory lecture on game theory to promote potential interdisciplinary collaborations. No prior knowledge is required — the lecture is intended for non-experts. We will cover the fundamental concepts to help you build an intuitive understanding of how game theory analyzes strategic interactions. After briefly reviewing the previous lecture, we will discuss mixed-strategy Nash equilibria and their computational complexity.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Quantum multi-body problems using unsupervised machine learning

November 5 (Wed) 15:00 - 16:00, 2025

Tomoya Naito (Project Assistant Professor, Department of Nuclear Engineering and Management, Graduate School of Engineering, The University of Tokyo)

I will introduce the recent development of a method to calculate the (anti)symmetrized wave functions and energies of the ground and low-lying excited states using the unsupervised machine learning technique. I will also introduce the recent attempts to consider the spin-isospin degrees of freedom and extend them to the Dirac equation.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Introduction to Lean theorem prover

October 31 (Fri) 14:00 - 17:00, 2025

Yuma Mizuno (Postdoctoral Researcher, University College Cork, Ireland)

A theorem prover is a tool for the formalization of mathematics, that is, for rigorously expressing and verifying theorems and proofs on a computer. In recent years, the Lean theorem prover has seen progress in the formalization of a wide range of areas of mathematics. In this talk, I will explain formalization of mathematics in Lean from the basics and survey the formalized results achieved to date.

Venue: via Zoom / #359, Seminar Room #359

Event Official Language: English

5th ComSHeL Seminar

October 31 (Fri) 11:00 - 12:00, 2025

Motoko Kotani (Executive Director of Science, RIKEN)

Title: Discrete Geometric Analysis and its application to materials science Abstract: Discrete Geometric Analysis is a discrete version of Geometric Analysis. It is however not just its discretization but a development of methods to bridge discrete and continuum. I will explain those and share some applications to materials science with you.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Operational Quantum Frames: from quantum mechanics to quantum field theory and beyond

October 30 (Thu) 16:00 - 17:00, 2025

Jan Głowacki (Postdoc, Institute for Quantum Optics and Quantum Information, Austrian Academy of Sciences, Austria)

In this talk I will begin with a concise overview of the development of the operational approach to quantum reference frames (QRFs), tracing the line from its foundational contributions to its most recent applications. I will then introduce the central ideas of the research direction that I am pursuing which aims at developing relational foundations for relativistic quantum physics. The starting point is the application of the operational QRF formalism to the context of the Poincaré group, and establishing connections between this emerging framework and existing formalisms in quantum field theory. This part of the talk will summarize results from a recent preprint written with Samuel Fedida [1]. I will conclude by outlining a number of open research directions, highlighting selected topics in more detail depending on the available time and the interests of the audience

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Primordial black holes formation and its origin in inflation - Jianing Wang

October 30 (Thu) 14:15 - 16:00, 2025

Jianing Wang (Project Researcher, Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU), The University of Tokyo)

Primordial black holes (PBHs) are thought to form through gravitational collapse of regions with excessively large density in the early universe, and they could serve as seeds for the formation of galaxies. They are also considered one of the important candidates for cold dark matter (DM). Detecting and constraining the abundance of PBHs can provide an effective constraint on realistic inflationary models. In this talk, I will combine inflation models with gravitational waves (GWs) to discuss cosmological phenomena related to primordial black holes. In particular, I will emphasize a simplified toy model of inflation, which naturally enhances the small-scale scalar perturbations by gluing together two linear potentials with different slopes. The enhanced perturbations can not only generate primordial black holes but also emit gravitational waves through higher-order perturbations. This research demonstrates the significant potential of primordial black hole studies, and it naturally leads to a crucial question of how to accurately estimate the PBH abundance. In the latter part of the talk, I will introduce how to use peaks theory to estimate the abundance of primordial black holes. Our new method works well for any form of the power spectrum, and considering the use of more systematic statistical methods, we believe it is currently the most precise approach in the academic community.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Inferring Phylogenetic Networks in the Genomic Era

October 30 (Thu) 13:00 - 14:00, 2025

Sungsik Kong (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

While phylogenetic trees (i.e., branching diagrams that depict the evolutionary history of different organisms) have been essential for understanding species evolution, they do not fully capture certain evolutionary processes, such as hybridization. In these cases, a phylogenetic network, which extends a phylogenetic tree by allowing two branches to merge into one and create reticulations, is needed. However, existing methods for estimating networks from genomic data become computationally prohibitive as dataset size and topological complexity increase. In this talk, I present the performance of popular computational methods that detect hybridization from genomic data as an alternative to the network inference, discussing their significance and limitations. I then explain how phylogenetic networks generalize trees to represent complex evolutionary histories and explore the biological interpretations that can be drawn from various branching patterns. Finally, I introduce PhyNEST (Phylogenetic Network Estimation using SiTe patterns), a novel method that efficiently and accurately infers phylogenetic networks directly from sequence data using composite likelihood. PhyNEST is implemented as an open-source Julia package and is available at https://github.com/sungsik-kong/PhyNEST.jl.

Venue: #359, 3F, Main Research Building (Main Venue) / via Zoom

Event Official Language: English

Lectures on Neutron Star Structure IV

October 28 (Tue) 15:30 - 17:00, 2025

Mark Alford (Professor, Washington University in St. Louis, USA)

This is a lecture series by Prof. Mark Alford (Washington University in St. Louis) on the structure of neutron stars. Oct. 7 (Tues), 15:30-17:00 Lecture I : Quark matter: the high-density frontier The densest predicted state of matter is color-superconducting quark matter, which has some affinities to electrical superconductors, but a much richer phase structure because quarks come in many varieties. This form of matter may well exist in the core of compact stars, and the search for signatures of its presence is currently proceeding. I will review the nature of color-superconducting quark matter, and discuss some ideas for finding it in nature. Oct. 14 (Tues), 15:30-17:00 Lecture II: Solid quark matter I will review three ways in which quark matter can occur in a solid phase, where translational invariance is broken by some sort of crystalline structure. These include a color superconductor of the Fulde-Ferrell-Larkin-Ovchinnikov type, mixed phases that can arise at a nuclear/quark matter interface, and the strangelet crystal crust of a strange star. Oct. 21 (Tues), 15:30-17:00 Lecture III: Dissipation in neutron star mergers In a neutron star merger, nuclear matter experiences dramatic changes in temperature and density that happen in milliseconds. Mergers therefore probe dynamical properties that may help us uncover the phase structure of ultra-dense matter. I will describe some of the relevant material properties, focusing on flavor equilibration and its consequences such as bulk viscosity and damping of oscillations. Oct. 28 (Tues), 15:30-17:00 Lecture IV: Neutrinos in dense matter: beyond modified Urca Neutrino absorption and emission (the "Urca process") is an essential aspect of the formation and cooling of neutron stars and of the dynamics of neutron star mergers. In this talk I will describe the traditional way of calculating Urca rates, explain its shortfalls, and propose an alternative approach, the nucleon width approximation.

Venue: Seminar Room #359, Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

The 1st Joint General Meeting of RIKEN Quantum and the QII RIKEN User Group

October 24 (Fri) 13:20 - 17:45, 2025

The joint general meeting is scheduled to be held at Okochi Hall from 13:20 to 17:45. The networking reception is scheduled to be held at the RIKEN Cafeteria from 18:00 to 19:40.

Venue: Okochi Hall

Event Official Language: English

Sequence-encoded protein condensation: a statistical physics perspective

October 23 (Thu) 13:00 - 14:00, 2025

Kyosuke Adachi (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))

Event Official Language: English

RIKEN–Berkeley Workshop on Quantum Gravity 2025

October 23 (Thu) - 24 (Fri) 2025

This workshop will serve as the first meeting of the collaboration between the Leinweber Institute for Theoretical Physics (LITP) at the University of California Berkeley and the RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS). Participation is open and researchers from other institutions are welcome to attend. The workshop will feature talks on recent developments in the field of Quantum Gravity and other relevant topics.

Venue: via Zoom / #359 (23rd Morning) & #435-437 (23rd Afternoon & 24th Morning), Main Research Building

Event Official Language: English

Asymptotic gravity waves in core-collapse progenitor stars

October 22 (Wed) 16:30 - 17:30, 2025

Lucy McNeill (Hakubi Assistant Professor, The Hakubi Center for Advanced Research, Kyoto University)

Internal gravity waves (IGW) are generically excited at convective boundaries inside stars. During the final months before a massive stars’ core-collapse, the excited IGW carry energy and angular momentum so large that the wave transport can e.g. completely set the rotation period of the neutron star remnant. In this talk, I present the first three-dimensional simulation of a core-collapse progenitor with which we can characterise IGW generation and transport preceding core-collapse. First I will show that the energy carried by convectively generated IGW in our simulation is described remarkably well by the established asymptotic theory, which utilizes e.g. the WKB approximation. But, the IGW’s subsequent propagation and dissipation depends very sensitively on the rotation. And in 3D, the equilibrium rotation patterns that develop are too complex to be captured in the established asymptotic theory for wave transport. I will present the rich nonlinear wave dynamics in our 3D simulation responsible for angular momentum transport and wave dissipation. I will propose that the angular momentum transport is governed by a “mean flow” interaction between global rotation and IGW transport. Mean flow interactions can explain the periodic Easterly <-> Westerly sudden reversal of winds at the equator on Earth, Saturn and Jupiter. If such reversals are realised in massive stars, it has implications for several exotic phenomena. This includes IGW driven mass loss outbursts observed in the final months before core-collapse supernova, and also gamma ray burst progenitor stars - which require very extreme rotation at core collapse.

Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359

Event Official Language: English

Neural network wavefunctions for SU(2) lattice gauge theory in the Hamiltonian formulation

October 22 (Wed) 15:00 - 16:30, 2025

Tom Spriggs (PostDoc, Kavli Institute of Nanoscience and QuTech, Delft University of Technology, Netherlands)

In this talk I will cover our recent preprint arXiv:2509.12323 where we propose a neural network approach to finding the ground state wavefunction of SU(2) lattice gauge theory. Specifically, we demonstrate that the use of bespoke SU(2)-gauge-equivariant neural network layers increases the extent to which our variational ansatz can represent the ground state of this system. During this talk I will contrast the Hamiltonian and Euclidean formalisms of lattice gauge theories, highlighting the promises that the former offers but also the difficulties: noting briefly the issues of parameterising the continuous Hilbert space that plague tensor network and quantum simulation approaches and how our approach alleviates this. I will try and present our method pedagogically as we are very interested in learning its uses but also the limits of its validity, before closing with some remarks on scaling to larger systems and different gauge groups.

Venue: via Zoom

Event Official Language: English

Lectures on Neutron Star Structure III

October 21 (Tue) 15:30 - 17:00, 2025

Mark Alford (Professor, Washington University in St. Louis, USA)

This is a lecture series by Prof. Mark Alford (Washington University in St. Louis) on the structure of neutron stars. Oct. 7 (Tues), 15:30-17:00 Lecture I : Quark matter: the high-density frontier The densest predicted state of matter is color-superconducting quark matter, which has some affinities to electrical superconductors, but a much richer phase structure because quarks come in many varieties. This form of matter may well exist in the core of compact stars, and the search for signatures of its presence is currently proceeding. I will review the nature of color-superconducting quark matter, and discuss some ideas for finding it in nature. Oct. 14 (Tues), 15:30-17:00 Lecture II: Solid quark matter I will review three ways in which quark matter can occur in a solid phase, where translational invariance is broken by some sort of crystalline structure. These include a color superconductor of the Fulde-Ferrell-Larkin-Ovchinnikov type, mixed phases that can arise at a nuclear/quark matter interface, and the strangelet crystal crust of a strange star. Oct. 21 (Tues), 15:30-17:00 Lecture III: Dissipation in neutron star mergers In a neutron star merger, nuclear matter experiences dramatic changes in temperature and density that happen in milliseconds. Mergers therefore probe dynamical properties that may help us uncover the phase structure of ultra-dense matter. I will describe some of the relevant material properties, focusing on flavor equilibration and its consequences such as bulk viscosity and damping of oscillations. Oct. 28 (Tues), 15:30-17:00 Lecture IV: Neutrinos in dense matter: beyond modified Urca Neutrino absorption and emission (the "Urca process") is an essential aspect of the formation and cooling of neutron stars and of the dynamics of neutron star mergers. In this talk I will describe the traditional way of calculating Urca rates, explain its shortfalls, and propose an alternative approach, the nucleon width approximation.

Venue: Seminar Room #359 (Main Venue) / via Zoom

Event Official Language: English

Simulating nonequilibirum quantum dynamics on Reimei

October 21 (Tue) 10:00 - 12:00, 2025

Tomoya Hayata (Associate Professor, School of Medicine, Keio University)

This is the third quantum computing gathering hold by quantum computing study group.

Venue: via Zoom / Seminar Room #359

Event Official Language: English

Compact Association Schemes and Fourier Analysis

October 17 (Fri) 15:00 - 17:00, 2025

Akifumi Nakada (Ph.D. Student / JSPS Research Fellow DC, Graduate School of Advanced Science and Engineering, Hiroshima University)

Error-correcting codes are a fundamental tool in information and communication technologies. They can be viewed as collections of points in a space that are sufficiently far apart to allow error detection and correction. More broadly, coding theory studies good arrangements of points in spaces. This theory has been particularly developed in the frameworks of association schemes and compact homogeneous spaces, where harmonic analysis plays a central role. In this talk, we will begin with an introduction to error-correcting codes and then present compact association schemes, which we define as a generalization of these spaces in which harmonic analysis can be developed.

Venue: Hybrid Format (3F #359 and Zoom), Main Research Building

Event Official Language: English

Bonded Knotted Structures and Applications

October 16 (Thu) 16:00 - 18:00, 2025

Sofia Lambropoulou (Professor, School of Applied Mathematical and Physical Sciences, National Technical University of Athens, Greece)

We present the theory of bonded knots and bonded knotoids, as well as their algebraic counterparts, the theory of bonded braids and bonded braidoids. We also discuss some applications to the topological study of proteins.

Venue: via Zoom / Seminar Room #359

Event Official Language: English