Seminar
996 events
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Seminar
Testing the quantum nature of gravity "ab absurdo"
March 18 (Wed) 14:00 - 16:00, 2026
Emanuele Panella (Postdoctoral Researcher, Tor Vergata University of Rome, Italy)
The quest for a quantum description of gravity has been long, diverse, and productive. Yet, despite decades of theoretical progress, there is still no direct experimental evidence for the quantum nature of spacetime. In this talk, I explore an alternative, indirect route to probing quantum gravity by assuming the fundamental classicality of the gravitational field and examining the resulting observational conflicts. In particular, I will discuss a key consistency condition—known as the decoherence–diffusion trade-off—that any theory of fundamentally classical gravity coupled to quantum matter must satisfy. By analysing a toy model of a linearised classical–quantum (CQ) gravity–matter system, I will explicitly show how this trade-off implies unavoidable, measurable effects, such as a fundamental stochastic gravitational-wave background, which cannot be eliminated by fine-tuning the model parameters.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Quantum modular form and quantum invariants
March 13 (Fri) 14:00 - 16:00, 2026
Yuya Murakami (Research Scientist, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
Quantum invariants are invariants of knots and 3-manifolds which relate deeply to mathematical physics and representation theory. In recent years, it has become increasingly clear that it is also deeply related to number theory, that is, quantum modularity for quantum invariants. This topic is interesting from a topological viewpoint since this is a refinement of establishing asymptotic expansions of quantum invariants, which is an important problem in quantum topology, and is interesting from a number-theores[tic viewpoint since this gives examples of quantum modular forms, which are mysterious objects in number theory. I obtained two linked results on topology and number theory: Establishing explicit asymptotic expansions of quantum invariants for negative definite plumbed 3-manifolds and establishing quantum modularity of false theta functions in full generality. In this talk, I will outline previous progress on quantum modularity for quantum invariants and my results.
Venue: via Zoom / Seminar Room #359, Seminar Room #359
Event Official Language: English
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Seminar
Chronotaxicity and Dynamic Stability: From Theory to Quantitative Measures
March 12 (Thu) 13:00 - 14:00, 2026
Aneta Stefanovska (Professor, Lancaster University, UK)
Living systems operate far from equilibrium under continuous time-varying forcing across multiple temporal and spatial scales. From neural and cardiovascular rhythms to microcirculatory dynamics and circadian cycles, physiological processes are inherently nonautonomous. Classical stability concepts based on autonomous attractors and stationary limit cycles are therefore insufficient to explain how such systems remain robust yet adaptable. In this talk, I will introduce chronotaxicity as a framework for nonautonomous oscillatory systems possessing time-dependent point attractors and contraction regions. Chronotaxic systems maintain stability under continuous forcing, providing a rigorous theoretical description of dynamic robustness. To illustrate the generality of this concept, I will show how chronotaxicity can be observed in a controlled physical experiment. I will then present a new order parameter based on angular velocity for quantifying phase dynamics in numerical simulations of coupled nonautonomous oscillators, along with the methods collected in the Multiscale Oscillatory Dynamics Analysis (MODA) toolbox for analysing time-dependent oscillatory behaviour. This approach provides a unified perspective on dynamic stability in complex systems, highlighting how living systems remain robust yet adaptable and suggesting quantitative signatures of dysfunction in health and disease. While the focus is on physiological and numerical models, it is broadly applicable to complex nonautonomous systems, underscoring its generality as a dynamical principle.
Venue: via Zoom / Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Causality Constraints on Black Hole Thermodynamics in Nonlinear Electrodynamics
March 6 (Fri) 15:30 - 17:00, 2026
Kaho Yoshimura (Ph.D. Student, Graduate School of Arts and Sciences, The University of Tokyo)
Black holes exhibit thermodynamic properties and provide an important window into the quantum aspects of gravity. In this context, nonlinear electrodynamics (NLED) offers a useful framework for constructing and analyzing charged black-hole solutions beyond Maxwell theory. Requiring causality - namely, excluding superluminal signal propagation - imposes nontrivial constraints on the allowed form of the NLED Lagrangian. In this talk, we focus on two quantities: the charge-to-mass ratio and the entropy density (entropy-to-mass squared ratio). The charge-to-mass ratio is expected to obey a monotonic behavior consistent with the Weak Gravity Conjecture, while the entropy density is also anticipated to be monotonic, reflecting the expectation that higher-energy effective theories contain more degrees of freedom. We show that these monotonic behaviors follow directly from the causality constraints on the NLED sector.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
How does the brain compute the value of odors and trigger adaptive behavior?
March 5 (Thu) 13:00 - 14:00, 2026
Hokto Kazama (Team Director, Laboratory for Circuit Mechanisms of Sensory Perception, RIKEN Center for Brain Science (CBS))
The world is filled with numerous odors that are impossible to experience all in our lifetime. Perhaps to cope with this situation, the brain is equipped with an ability to recognize whether an odor is attractive or aversive even from the first encounter and guide adaptive behavior. However, how information about the innate value of odors (attractiveness/aversiveness) is computed and transformed into appropriate behavioral outputs in the brain remains poorly understood. We are addressing this question in the olfactory circuit of fruit flies by combining behavioral analysis in virtual reality, comprehensive neuronal activity imaging, neuronal connectivity analysis, and computational modeling. In this talk, I will present our latest efforts to decipher how odor value is computed and how this information is transformed into motor-related signals in a tiny brain.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
The minimal absolute value of sums of fifth-roots of unity
March 5 (Thu) 11:00 - 12:00, 2026
Guillermo Núñez Ponasso (JSPS Research Fellow, Graduate School of Information Sciences, Tohoku University)
The minimal absolute value \sigma_{\ell}(n) of a weight-n sum of \ell-th roots of unity, for all n and a fixed \ell, is an interesting value in the study of maximal determinant matrices. In the cases where \ell=2,3,4, or 6, this minimal absolute value is either 0 or 1. Thus \ell=5 constitutes the smallest non-trivial case. In this talk I will discuss recent results in collaboration with Akihiro Munemasa, where we determined \sigma_5(n) for all n\geq 1. This problem turns out to be related to the Diophantine approximation of the golden ratio, and can be tackled using the theory of continued fractions.
Venue: via Zoom / SUURI-COOL (Kyushu)
Event Official Language: English
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Seminar
Non-perturbative geodesic length in JT gravity and universal time evolution of holographic complexity
March 2 (Mon) 16:00 - 17:00, 2026
Shono Shibuya (Ph.D. Student, Nagoya University)
The interplay between black hole interior dynamics and quantum chaos provides a crucial framework for probing quantum effects in quantum gravity. According to the holographic "Complexity=Volume" proposal, we investigated non-perturbative generating function of geodesic length in Jackiw-Teitelboim (JT) gravity to uncover universal signatures of quantum chaos and quantum complexity. We observed that the generating function interpolates between two major probes of quantum chaos - spectral form factor and complexity - highlighting its utility as a probe of chaotic spectrum in quantum gravity. Generalizing the result to general chaotic systems, we demonstrated that time evolution of the complexity is universally governed by a certain pole structure of observables, suggesting a validity of wide class of observables as a probe of quantum chaos in quantum gravity.
Venue: via Zoom / Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
The career talk: From Quarks to Cinematic Sparks
February 27 (Fri) 15:00 - 16:30, 2026
Agnes Mocsy (Professor, Department of Mathematics and Science, Pratt Institute, USA)
While my career began in a linear way, it gradually opened into a non-traditional path through unexpected mergings, where theoretical nuclear physics, filmmaking, and creative public and academic engagement intertwined. I will share how scientific inquiry, artistic practice, and storytelling began shaping one another, opening new ways to explore complexity, emotion, and connection. Drawing on work from my physics research to cinema projects like Rare Connections, I will reflect on how curiosity and creative thinking move freely across science and art, deepening each and expanding how we understand the human experience. My aim is to offer a perspective on the possibilities that emerge when we allow our multitudes to meet and transform one another.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Scattering in de Sitter space
February 26 (Thu) 14:00 - 15:30, 2026
Jason Kristiano (Program-Specific Researcher, Yukawa Institute for Theoretical Physics, Kyoto University)
The analytic structure of scattering amplitudes provides a framework for mapping the fundamental properties of a high-energy (UV) theory onto non-perturbative constraints for low-energy (IR) effective field theories. While this structure is well understood in flat space, its extension to de Sitter space is hindered by the expanding background, which complicates the definition of asymptotic states and breaks time-translation symmetry. In this talk, I will outline a foundational approach to bridging this gap. I will demonstrate how the analytic properties of flat-space amplitudes are imprinted on their de Sitter counterparts. The ultimate goal of this program is to derive Swampland-type constraints for cosmological EFTs, ensuring they admit a consistent UV completion.
Venue: #345-347, Main Research Building, RIKEN Wako Campus (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Sex ratio theory for facultative parthenogens: from fortuitously optimal stick insects to the origin of haplodiploidy in Hymenoptera
February 26 (Thu) 13:00 - 14:00, 2026
Kora Klein (Visiting Researcher, Division of Fundamental Mathematical Science, RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS))
First, I will present one of my PhD papers which focuses on sex ratios when females can reproduce both sexually and asexually. This paper features two models: an optimality model and an evolutionary invasion analysis. More generally, it shows my style of approaching evolutionary theory. After this, I will briefly discuss the work I plan to do in the next 6 months during my JSPS fellowship where I will focus on the evolution of more conventional X-chromosomes. Short Bio: I studied biology at the Free University in Berlin. My biology studies were mostly empirical but I attended several Bachelor courses for mathematicians (calculus and stochastics) and did my Master thesis in Zurich with Hanna Kokko, a theoretical evolutionary ecologist who studies a broad range of topics. In 2020, I then continued working in Hanna Kokko’s group for my PhD and moved with her from Zurich to Mainz in 2023. During my time with Hanna Kokko, worked on various topics centered around intraspecific diversity, including projects on sexual dimorphism, sex ratio theory, intralocus sexual conflict, and a female-limited color polymorphism in a butterfly. Since then, I started my first PostDoc in 2024 with Laura Ross in Edinburgh (UK) where I modelled how the unusual genetic systems of Scarid flies could have evolved, and have now started a 6 month JSPS fellowship with Ryosuke Iritani.
Venue: #435-437, Main Research Building
Event Official Language: English
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Seminar
Testing the quantum nature of gravity with optomechanical systems
February 26 (Thu) 10:00 - 12:00, 2026
Yuta Michimura (Assistant Professor, Department of Physics, Graduate School of Science, The University of Tokyo)
Quantum gravity remains one of the major challenges in modern physics. Even at the most fundamental level, there is no experimental confirmation of whether a mass placed in a spatial superposition generates a corresponding superposition of gravitational fields. In recent years, experiments aiming to create gravity-induced quantum entanglement have attracted significant attention as a way to probe the quantum nature of non-relativistic gravity. In particular, optomechanical systems, which exploit the interaction between light and mechanical oscillators, provide a promising platform for such studies. We are pursuing experiments at the milligram scale, which lies between the smallest mass scale at which classical gravity has been tested and the largest mass scale at which quantum states of mechanical oscillators have been realized [1]. In this seminar, I will discuss experimental approaches to testing the quantum nature of gravity using suspended and levitated mirrors. I will also discuss our recent proposal to use inverted oscillators to enhance gravity-induced entanglement exponentially [2].
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Noninvertible symmetry protected topological phases on lattice
February 25 (Wed) 10:30 - 11:30, 2026
Weiguang Cao (PD, Centre for Quantum Mathematics, University of Southern Denmark, Denmark)
The recent discovery of noninvertible symmetries—a radical extension of conventional symmetry—has challenged long-standing paradigms in condensed matter physics and quantum information and opened new territory in both theory and technology. Unlike ordinary symmetries, which can be inverted, these symmetries behave like projections (one-way operations) yet still strongly constrain quantum dynamics and enable new classes of phases and phase transitions. However, their role in organizing and stabilizing novel quantum phases remains poorly understood. One important example is a symmetry protected topological (SPT) phase, characterized by nontrivial edge modes and potential applications in quantum information. In this talk, I will discuss the classification of noninvertible symmetry-protected topological (NISPT) phases in both closed and open quantum systems using a duality-based method, and present concrete lattice realizations. These lattice models provide controlled playgrounds in which the physics of noninvertible symmetry can be explored numerically and, potentially, experimentally.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Quantitative phylogenomics
February 24 (Tue) 13:00 - 14:00, 2026
Hector Banõs (Assistant Professor, Department of Mathematics, California State University, USA)
Benjamin Teo (Postdoc, Mathematical Analysis of Cellular Systems, University of Melbourne, Australia)This session features two speakers: Hector Banos, Assistant Professor of Mathematics at California State University, whose research focuses on phylogenetic inference and network models, and Benjamin Teo, a Postdoctoral Researcher at the University of Melbourne, working on probabilistic and computational methods for continuous trait evolution on phylogenetic networks. See below for details. 【Talk 1】 Speaker: Hector Banos Title: Bringing a Knife to a Gunfight: Pitfalls of Phylogenetic Inference under Model Misspecification Abstract: Phylogenetic networks provide a flexible framework for representing evolutionary histories that include hybridization, introgression, and other reticulate processes. However, inferring such networks remains computationally and statistically difficult. Many current methods often scale only to restricted classes of networks. Consequently, researchers frequently analyze their data using simpler models (most commonly phylogenetic trees) even when there is strong evidence that the underlying evolutionary history is more complex. In this talk, we examine the impact of model misspecification on phylogenetic inference, focusing on situations in which data are generated by a complex network but are analyzed using simpler tree or network models. I then show how this mismatch can influence the topology of inferred trees, as well as the structure of inferred networks. These results highlight the limitations and the practical consequences of using simplified models for phylogenetic inference. 【Talk2】 Speaker: Benjamin Teo Title: Adapting cluster graphs for inference of continuous trait evolution on phylogenetic networks Abstract: I consider a new approach ("loopy belief propagation") for fitting Gaussian models on a phylogenetic network to explain the data observed across present-day species for a continuous univariate or multivariate trait. We previously showed [1] that a trait evolution model coupled to a network can be readily cast as a probabilistic graphical model, so that the likelihood can be efficiently computed using a dynamic programming framework ("belief propagation") defined on an auxiliary graph ("cluster graph") that is tree-structured. Even so, maximum likelihood estimation can grow computationally prohibitive for large complex networks. Belief propagation can be applied more generally to non-tree ("loopy") cluster graphs to compute a factored energy approximation to the log-likelihood. "Loopy" belief propagation may provide a more practical trade-off between estimation accuracy and runtime. However, the influence of cluster graph structure on this trade-off is not precisely understood. We conduct a simulation study using our Julia package PhyloGaussianBeliefProp [2] to investigate how varying the maximum cluster size of a cluster graph affects this trade-off. We discuss recommended choices for maximum cluster size, and prove the equivalence of likelihood-based and factored-energy based estimates for the homogeneous Brownian motion trait model. The talk is based on our preprint [3]. I will introduce the key concepts from the ground up.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Basic Conceptual and Mathematical Problems of QFT
February 20 (Fri) 14:00 - 15:30, 2026
Christy Koji Kelly
In this talk we discuss some of the most basic conceptual and mathematical difficulties that arise in the standard physics analysis of QFT. In particular we shall discuss the origin of UV divergences in QFT—pointing out that there is both a kinematic and a dynamic aspect to this problem, and that the standard physics explanation (’new physics’) only considers the latter—and suggest that despite the notoriety of the problem, UV divergences are essentially under control. Secondly we discuss Haag’s theorem—which ensures the nonexistence of the interaction picture and the triviality of the perturbative S-Matrix—and indicate how this is the most elementary manifestation of a series of infrared problems in QFT. Finally we will outline why the rigorous construction of path-integral measures is difficult. If we have time, we may discuss some difficulties associated with gauge theories such as the infraparticle problem of QED and the mass-gap problem of Yang-Mills theory.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
Tunneling with physics-informed renormalisation group flows in the anharmonic oscillator
February 20 (Fri) 10:00 - 11:00, 2026
Friederike Ihssen (Postdoctoral Fellow, Physics, Ruhr University Bochum, Germany)
The resolution of strongly correlated physical systems is computationally hard, but can be simplified enormously by a formulation in terms of suitable dynamical degrees of freedom. Within the functional renormalisation group framework, physics-informed renormalisation group flows (PIRG flows) [1] implement scale-dependent coordinate transformations that can be used to devise optimal expansion schemes around such degrees of freedom. Recently, we have applied PIRG flows to the anharmonic oscillator, with an emphasis on the weak coupling regime with its instanton-dominated tunnelling processes [2]. We show that the instanton physics behind the exponential decay of the energy gap is already covered in the first order of the derivative expansion of the PIRG. The crucial new ingredients in the present analysis are the use of the ground state expansion within PIRG flows, as well as precision numerics based on Galerkin methods. Our result a_inst = 1.910(2) for the decay constant is in quantitative agreement with the analytic one, a_inst = 1.886 with a deviation of 1%. This illustrates very impressively the capacity of the PIRG for fully capturing non-perturbative physics already in relatively simple approximations.
Venue: via Zoom / #359, Main Research Building
Event Official Language: English
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Seminar
Spectral Codes : A Geometric Formalism for QEC
February 19 (Thu) 15:00 - 16:30, 2026
Satoshi Kanno (Researcher, Research Institute of Advanced Technology, SoftBank Corp.)
In this talk, I will introduce a novel geometric framework for quantum error correction based on spectral triples in noncommutative geometry. In this formulation, quantum error-correcting codes are described as spectral projections onto the low-energy eigenspaces of Dirac-type operators, where the separation between logical information and local errors is captured geometrically. This approach provides a unified spectral and geometric understanding of key properties such as code distance and error thresholds. Moreover, it accommodates various existing codes, including classical linear codes, stabilizer codes, GKP codes, and topological codes. This geometric perspective also suggests intriguing connections to deformation quantization and holographic quantum error correction, offering promising directions for future research.
Venue: #359, Seminar Room #359
Event Official Language: English
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Seminar
Binary neutron Star Merger as a Probe of Hadron-Quark Transition
February 19 (Thu) 14:00 - 15:00, 2026
Yongjia Huang (Research Associate, Purple Mountain Observatory, Chinese Academy of Sciences, China)
This seminar is a joint seminar between GWX-EOS and the iTHEMS-ABBL Joint Astro SG. The recent rise of multi-messenger astronomy—including radius measurements from NICER, tidal deformability constraints from gravitational-wave events GW170817, and first-principles calculations from chiral effective field theory (χEFT) and perturbative QCD—has significantly tightened constraints on the neutron star equation of state. These advances consistently point to a non-monotonic sound speed in dense matter, suggesting that the cores of massive neutron stars may host exotic phases such as quark matter. However, the masquerade effect in static neutron stars makes it difficult to directly probe the nature of the transition (e.g., a smooth crossover or a sharp phase transition) near the core through observation alone.
Venue: #345, 3F, Main Research Building, RIKEN Wako Campus (Main Venue) / via Zoom
Event Official Language: English
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Seminar
The sample complexity of species tree estimation: How many genes does it take to infer a species tree?
February 19 (Thu) 13:00 - 14:00, 2026
Max Hill (Assistant Professor, University of Hawaiʻi, USA)
In this talk, I will discuss the problem of inferring an evolutionary tree from DNA sequence data. The main focus will be on the sample complexity of this problem---i.e., the question of how much data is required to achieve high probability of correct inference. After introducing a standard stochastic model of gene and DNA evolution, I will highlight some surprising features of DNA sequence data that complicate inference. Finally, I will present an impossibility result which takes the form of an information-theoretic lower bound on the minimum amount of data needed for accurate inference when genes exhibit variation in mutation rates. No prior knowledge of phylogenetics or information theory is assumed. Based on joint work with Sebastien Roch.
Venue: Seminar Room #359 (Main Venue) / via Zoom
Event Official Language: English
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Seminar
Taming the Butterfly: A New "Duality Principle" Turns Chaos into Control
February 18 (Wed) 13:00 - 14:00, 2026
Takemasa Miyoshi (Team Principal, Data Assimilation Research Team, RIKEN Center for Computational Science (R-CCS))
Data Assimilation (DA) is the backbone of modern weather forecasting. It integrates observational data into computer simulations to synchronize the model with nature. The Duality Principle posits that chaos control is mathematically the "twin" (dual) of DA. Data Assimilation: Uses observations to synchronize the Model to Nature. Chaos Control: Uses interventions to synchronize Nature to a desired Model ("target trajectory"). "The butterfly effect has long been a symbol of unpredictability," says Dr. Miyoshi. "But I asked a simple question: If a butterfly's wings can change the future, does that not imply that with the right, tiny push, we could choose a better future?" Instead of suppressing the chaotic system with massive force, this method acts like mathematical judo—leveraging the system's inherent instability. By applying minute, calculated "interventions" (analogous to the butterfly's flap), the system can be guided toward a "target trajectory"—for instance, shifting real-world conditions just enough to align with a model-simulated scenario where a typhoon causes no damage. Once synchronized, control becomes much easier to maintain. This study establishes the theoretical foundation for "Control Simulation Experiments" (CSE), a framework previously proposed by Miyoshi’s team. It provides a roadmap for future disaster prevention research, moving beyond passive prediction to active mitigation. Beyond meteorology, this general framework is expected to serve as a universal tool for studying interventions in various chaotic systems, from ecosystems to economics. Following the seminar, we will hold an informal discussion (brainstorming) on data assimilation with quantum computing in the same room from 2-4 pm.
Venue: Hybrid Format (3F #359 and Zoom), Seminar Room #359
Event Official Language: English
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Seminar
The Rectangular Peg Problem and microlocal sheaf theory
February 17 (Tue) 14:00 - 15:00, 2026
Yuichi Ike (Associate Professor, Graduate School of Mathematical Sciences, The University of Tokyo)
The Square Peg Problem asks whether every Jordan curve in the plane contains four distinct points that form the vertices of a square. This problem was proposed by Toeplitz in 1911 and remains unsolved in full generality. It can be generalized to the Rectangular Peg Problem, which concerns the existence of inscribed rectangles with a prescribed aspect ratio. Recently, Greene and Lobb successfully applied techniques in symplectic geometry to the problem and obtained new results. In this talk, I will explain how microlocal sheaf theory allows us to further extend their approach and affirmatively solve the Rectangular Peg Problem for a large class of Jordan curves, including all curves of finite length. This is joint work with Tomohiro Asano.
Venue: Seminar Room #359
Event Official Language: English
996 events