# Department Seminars & Colloquia

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A perfect field is said to be Kummer-faithful if the Kummer maps for semiabelian varieties over the field are injective. This notion originates in the study of anabeian geometry. At the same time, our study is also motivated by a conjecture of Frey and Jarden on the structure of Mordell-Weil groups over large algebraic extensions of a number field.
I will begin with a review of known results in this direction, as well as a brief discussion on anabelian geometry. Then I will introduce some recent results on the construction of "large" Kummer faithful fields. This is a joint work with Takuya Asayama.

This is a reading seminar to be given by Mr. Jaehong Kim (a graduate student in the department) on foundations of the intersection theory and the classification theory of complex algebraic surfaces. He will give three 2-hour long talks.

This is a presentation by Mr. Taeyoon Woo, a graduate student in the department, after his reading course on basics on compact Riemann surfaces.
He will concentrate on topics such as degree theory of holomorphic maps, Riemann-Roch theorem, residue theorem, Serre duality, Riemann-Hurtiwz theorem, Hodge decomposition, etc. on the compact Riemann surfaces. If time permits, he will discuss its connections to smooth manifolds and algebraic curves.

Let (R,m_R) be a d-dimensional, excellent, normal local ring. A divisorial filtration {I_n} is determined by a divisor D on a normal scheme X determined by blowing up an ideal on R, so that I_n are the global sections of nD. Associated to an m_R-primary divisorial filtration, we have the Hilbert function f(n)=\lambda_R(R/I_n), where \lambda_R is the length of an R-module. We discuss how close or far this function is from being a polynomial, focusing on examples which are constructed and analyzed geometrically.

Given two relatively prime positive integers, p < q, Kunz and Waldi defined a class of numerical semigroups which we denote by KW(p, q) consisting of semigroups of embedding dimension n and type n−1 and multiplicity p by filling in the gaps of the semigroup < a, b >. We study these semigroups, give a criterion for these in terms of principal matrices or their critical binomials and generalize the notion to KW(p, q, w) and prove
some results and questions. We will discuss their resolutions and Betti Numbers. Most of this is a joint work with Srishti Singh.

In this talk, we will discuss Leray-Hopf solutions to the two-dimensional Navier-Stokes equations with vanishing viscosity. We aim to demonstrate that when the initial vorticity is only integrable, the Leray-Hopf solutions in the vanishing viscosity limit do not exhibit anomalous dissipation. Moreover, we extend this result to the case where the initial vorticity is merely a Radon measure, assuming its singular part maintains a fixed sign. Our proof draws on several key observations from the work of J. Delort (1991) on constructing global weak solutions to the Euler equation. This is a joint work with Luigi De Rosa (University of Basel).

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(E6-1) Room 1501
Partial Differential Equations
Professor Sir John Macleod Ball (Heriot-Watt University)
Distinguished Lectures

(E6-1) Room 1501

Partial Differential Equations

Ist lecture: Understanding material microstructure Abstract Under temperature changes or loading, alloys can form beautiful patterns of microstructure that largely determine their macroscopic behaviour. These patterns result from phase transformations involving a change of shape of the underlying crystal lattice, together with the requirement that such changes in different parts of the crystal fit together geometrically. Similar considerations apply to plastic slip. The lecture will explain both successes in explaining such microstructure mathematically, and how resolving deep open questions of the calculus of variations could lead to a better understanding. 2nd lecture: Monodromy and nondegeneracy conditions in viscoelasticity Abstract For certain models of one-dimensional viscoelasticity, there are infinitely many equilibria representing phase mixtures. In order to prove convergence as time tends to infinity of solutions to a single equilibrium, it is necessary to impose a nondegeneracy condition on the constitutive equation for the stress, which has been shown in interesting recent work of Park and Pego to be necessary. The talk will explain this, and show how in some cases the nondegeneracy condition can be proved using the monodromy group of a holomorphic function. This is joint work with Inna Capdeboscq and Yasemin Şengül.

I will begin by a brief introduction to anabelian geometry.
In particular, I will try to explain the distinction between "bi-" and "mono-anabelian" reconstruction.
Then I review some of the known (elementary) mono-anabelian reconstruction of invariants of mixed characteristic local fields.
Finally, I will explain my (on-going) trial of the mono-anabelian reconstruction of fundamental character and Lubin-Tate character.

In this talk, we consider the self-dual O(3) Maxwell–Chern–Simons-Higgs equation, a semilinear elliptic system, defined on a flat two torus. We discuss about pointwise convergence behavior, which represents the Chern-Simons limit behavior of our system. Building upon this observation, we study the existence, stability, and asymptomatic behavior of solutions.

Virtual element method (VEM) is a generalization of the finite element method to general polygonal (or polyhedral) meshes. The term ‘virtual’ reflects that no explicit form of the shape function is required. The discrete space on each element is implicitly defined by the solution of certain boundary value problem. As a result, the basis functions include non-polynomials whose explicit evaluations are not available. In implementation, these basis functions are projected to polynomial spaces. In this talk, we briefly introduce the basic concepts of VEM. Next, we introduce mixed virtual volume methods (MVVM) for elliptic problems. MVVM is formulated by multiplying judiciously chosen test functions to mixed form of elliptic equations. We show that MVVM can be converted to SPD system for the pressure variable. Once the primary variable is obtained, the Darcy velocity can be computed locally on each element.

In this talk, I will discuss the expansion of the free energy of two-dimensional Coulomb gases as the size of the system increases. This expansion plays a central role in proving the law of large numbers and central limit theorems. In particular, I will explain how potential theoretic, topological, and conformal geometric information of the model arises in this expansion and present recent developments.

The Gromov-Wasserstein (GW) distance is a generalization of the standard Wasserstein distance between two probability measures on a given ambient metric space. The GW distance assumes that these two probability measures might live on different ambient metric spaces and therefore implements an actual comparison of pairs of metric measure spaces. A metric-measure space is a triple (X,dX,muX) where (X,dX) is a metric space and muX is a fully supported Borel probability measure over X.
In Machine Learning and Data Science applications, this distance is estimated either directly via gradient based optimization approaches, or through the computation of lower bounds which arise from distributional invariants of metric-measure spaces. One particular such invariant is the so-called ‘global distance distribution’ which precisely encodes the distribution of pairwise distances between points in a given metric measure space. This invariant has been used in many applications yet its classificatory power is not yet well understood.
This talk will overview the construction of the GW distance, the stability of distributional invariants, and will also discuss some results regarding the injectivity of the global distribution of distances for smooth planar curves, hypersurfaces, and metric trees.

Distances such as the Gromov-Hausdorff distance and its Optimal Transport variants are nowadays routinely invoked in applications related to data classification. Interestingly, the precise value of these distances on pairs of canonical shapes is known only in very limited cases. In this talk, I will describe lower bounds for the Gromov-Hausdorff distance between spheres (endowed with their geodesic distances) which we prove to be tight in some cases via the construction of optimal correspondences. These lower bounds arise from a certain version of the Borsuk-Ulam theorem for discontinuous functions.

Let $C$ be a general rational curve of degree $d$ in a Grassmannian $G(k, n)$. The natural expectation is that its normal bundle is balanced, i.e., isomorphic to $\bigoplus O(e_i)$ with all $|e_i - e_j| \leq 1$. In this talk, I will describe several counterexamples to this expectation, propose a suitably revised conjecture, and describe recent progress towards this conjecture.

https://kaist.zoom.us/j/82606384650?pwd=fmtYmqREcLFZ2qDMF1TBhG80z4Y51f.1 회의 ID: 826 0638 4650 암호: syzygies

https://kaist.zoom.us/j/82606384650?pwd=fmtYmqREcLFZ2qDMF1TBhG80z4Y51f.1 회의 ID: 826 0638 4650 암호: syzygies

The lecture series gives a view on computational methods and their some applications to existence and classfication problems. In the first lectures I will introduce Groebner basis and their basic applications in commutative algebra such as computing kernel and images of morphism between finitely presented modules over polynomial rings. As a theoretical application of Groebner basis I will give Petri's analysis of the equations of a canonical curve. The second topic will be Computer aided existence and unirationality proofs of algebraic varieties and their moduli spaces. In case of curves liaison theory is needed, which will be developed. For existence proofs random searches over finite fields is a technique that has not been exploited very much. I will illustrate this technique in a number of examples, in particular for the construction of certain surfaces. Classification of non-minimal surfaces uses adjunction theory. We will discuss this from a computational point of view.

This talk presents a novel and efficient approach to solving incompressible Navier-Stokes flows by combining a projection scheme with the Axial Green Function Method (AGM). AGM employs one-dimensional Green functions tailored for axially split differential operators, enabling the resolution of intricate multidimensional challenges. Our methodology integrates the projection method with a predictor-corrector mechanism, thereby ensuring stable and accurate velocity corrections. By transforming complex differential equations into simpler one-dimensional integral equations along axis-parallel lines within the flow domain, a notable enhancement in computational efficiency is achieved.
A significant innovation of our approach is the use of axial Green functions that have been specifically designed for the reaction-diffusion ordinary differential operator. This enables the effective handling of discrete-time derivatives and viscous terms in the momentum equation. The flexibility of constructing axis-parallel lines at will allows for a detailed analysis of critical flow regions and even permits a random distribution of these lines, thereby enhancing adaptability. The efficacy of our methodology is validated through numerical examples involving benchmark flow scenarios, such as lid-driven cavity flow and flow past an obstacle, which illustrate convergence, adaptability to arbitrary domain geometries, and potential applicability to three-dimensional flow problems.

In the N-body problem, choreographies are periodic solutions where N equal masses follow each other along a closed curve. Each mass takes periodically the position of the next after a fixed interval of time. In 1993, Moore discovered numerically a choreography for N = 3 in the shape of an eight. The proof of its existence is established in 2000 by Chenciner and Montgomery. In the same year, Marchal published his work on the most symmetric family of spatial periodic orbits, bifurcating from the Lagrange triangle by continuation with respect to the period. This continuation class is referred to as the P12 family. Noting that the figure eight possesses the same twelve symmetries as the P12 family, the author claimed that it ought to belong to P12. This is known as Marchal’s conjecture. In this talk, we present a constructive proof of Marchal’s conjecture. We formulate a one parameter family of functional equations, whose zeros correspond to periodic solutions satisfying the symmetries of P12; the frequency of a rotating frame is used as the continuation parameter. The goal is then to prove the uniform contraction of a mapping, in a neighbourhood of an approximation of the family of choreographies starting at the Lagrange triangle and ending at the figure eight. The contraction is set in the Banach space of rapidly decaying Fourier-Chebyshev series coefficients. While the Fourier basis is employed to model the temporal periodicity of the solutions, the Chebyshev basis captures their parameter dependence. In this framework, we obtain a high-order approximation of the family as a finite number of Fourier polynomials, where each coefficient is itself given by a finite number of Chebyshev polynomials. The contraction argument hinges on the local isolation of each individual choreography in the family. However, symmetry breaking bifurcations occur at the Lagrange triangle and the figure eight. At the figure eight, there is a translation invariance in the normal direction to the eight. We explore how the conservation of the linear momentum in this direction can be leveraged to impose a zero average value in time for the choreographies. Lastly, at the Lagrange triangle, its (planar) homothetic family meets the (off-plane) P12 family. We discuss how a blow-up (as in “zoom-in”) method provides an auxiliary problem which only retains the desired P12 family.

Resonance varieties are algebro-geometric objects that emerged from the geometric group theory. They are naturally associated to vector subspaces in second exterior powers and carry natural scheme structures that can be non-reduced. In algebraic geometry, they made an unexpected appearance in connection with syzygies of canonical curves. In this talk, based on works with G. Farkas, Y. Kim, C. Raicu, A. Suciu, and J. Weyman I report on some recent results concerning the geometry of resonance schemes in the vector bundle setup.

https://kaist.zoom.us/j/81427312084?pwd=arF7jyUZ3aVbnQoKv74adW2Bx4Nh6g.1 Meeting ID: 814 2731 2084 Password: syzygies

https://kaist.zoom.us/j/81427312084?pwd=arF7jyUZ3aVbnQoKv74adW2Bx4Nh6g.1 Meeting ID: 814 2731 2084 Password: syzygies

We examine the dynamics of short-range interacting Bose gases with varying diluteness and interaction strength. Using a combination of mean-field and semiclassical methods, we show that, for large numbers of particles, the system’s local mass, momentum, and energy densities can be approximated by solutions to the compressible Euler system (with pressure P = gρ2 ) up to a blow-up time. In the hard-core limit, two key results are presented: the internal energy is derived solely from the many-body kinetic energy, and the coupling constant g = 4πc0 where c0 the electrostatic capacity of the interaction potential. The talk is based on our recent work arXiv:2409.14812v1. This is joint work with Shunlin Shen and Zhifei Zhang. The talk will be delivered in English and is meant for the general audience.

In the 19th century, Kummer extensively studied quartic surfaces in the complex projective 3-space containing 16 nodes(=ordinary double points). One of his notable results states that a quartic surface cannot contain more than 16 nodes. This leads to a classic question: how many nodes may a surface of degree d contain? The answer to this question is known only for a very low degrees, namely, degrees 5 and 6. To find the optimal answer(31) for quintics, Beauville introduced the concept of "even sets of nodes," which turned out to be highly influential in the study of nodal surfaces. Based on the structure theorem of even sets by Casnati and Catanese, we will discuss some structure theorems of nodal quintics and sextics with maximal number of nodes. This talk is based on joint works with Fabrizio Catanese, Stephen Coughlan, Davide Frapporti, Michael Kiermaier, and Sascha Kurz.

2-linear varieties are a rich topic. Sijong Kwak initiated the study of 3-regular varieties. In this talk I report on joined work Haoang Le Truong on the classification of smooth 3-regular varieties of small codimension 3. Some of these varieties are analogously to the 2-regular case determinantal. This first non-determinantal cases occurs in codimension 3. In this talk I report on the classification of varieties with Betti table
$$
\begin{matrix}
& 0 & 1 & 2 & 3\\ \hline
0: & 1 & . & . & .\\
1: & . & . & . & .\\
2: & . & 10 & 15 & 6
\end{matrix}
$$
Our approach consist of studying extension starting from curves. Let $X \subset \mathbb P^n$ be a variety. An e-extension $Y \subset \mathbb P^{n+e}$ of $X$ is a variety, which is not a cone, such that there exists a regular sequence $y_1,\ldots,y_e$ of linear forms for the homogeneous coordinate ring $S_Y$ of $Y$ such that $S_Y/(y_1,\ldots,y_e) = S_X$ is the coordinate ring of $X$. Using a computationally easy deformation theoretic method to compute extensions, we classify the extensions of 3-regular curves in $\mathbb P^4$ to surfaces in $\mathhbb P^5$ completely.

Syzygies of algebraic varieties have long been a topic of intense interest among algebraists and geometers alike. Starting with the pioneering work of Mark Green on curves, numerous attempts have been made to extend these results to higher dimensions. Ein and Lazarsfeld proved that if A is a very ample line bundle, then K_X + mA satisfies property N_p for any m>=n+1+p. It has ever since been an open question if the same holds true for A ample and basepoint free. In recent joint work with Purnaprajna Bangere we give a positive answer to this question.

Let L be a ample line bundle on a projective scheme X. We say that (X,L) satisfies property QR(k) if the homogeneous ideal can be generated by quadrics of rank less than or equal to k. In the previous paper, we show that the Veronese embedding satisfies property QR(3). Let (X,L) be a Segre-Veronese embedding where X is a product of P^{a_i} with i=1,...,l and L is a very ample lines bundle O_X(d_1,d_2,...,d_l). In the paper [Linear determinantal equations for all projective schemes, SS2011], they prove that (X,L) satisfies QR(4) and it is determinantally presented if at least l-2 entries of d_1,...,d_l are at least 2. in this talk, we prove that (X,L) satisfies Qr(3) if and only if all the entries of d_1,...,d_l are at least 2. For one direction, we compute the radical ideal of 4 by 4 minors of a big matrix with linear forms, and for the other direction, we use the inducution on the sum of entries of (d_1,...,d_l).

A fundamental problem at the confluence of algebraic geometry, commutative algebra and representation theory is to understand the structure and vanishing behavior of the cohomology of line bundles on flag varieties. Over fields of characteristic zero, this is the content of the Borel-Weil-Bott theorem and is well-understood, but in positive characteristic it remains wide open, despite important progress over the years. By embedding smaller flag varieties as Schubert subvarieties in larger ones, one can compare cohomology groups on different spaces and study their eventual asymptotic behavior. In this context I will describe a sharp stabilization result, and discuss some consequences and illustrative examples. Joint work with Keller VandeBogert.

A vector bundle on projective space is called "Steiner" if it can be recognized simply as the cokernel of a map given by a matrix of linear forms. Such maps arise from various geometric setups and one can ask: from the Steiner bundle, can we recover the geometric data used to construct it? In this talk, we will mention an interesting Torelli-type result of Dolgachev and Kapranov from 1993 that serves as an origin of this story, as well as other work that this inspired. We'll then indicate our contribution which amounts to analogous Torelli-type statements for certain tautological bundles on the very ample linear series of a polarized smooth projective variety. This is joint work with R. Lazarsfeld.

In this talk, we study initial value problem for the Einstein equation with null matter fields, motivated by null shell solutions of Einstein equation. In particular, we show that null shell solutions can be constructed as limits of spacetimes with null matter fields. We also study the stability of these solutions in Sobolev space: we prove that solutions with one family of null matter field are stable, while the interaction of two families of null matter fields can give rise to an instability.

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(E2) Room1223 or online (see URL below)
Topology, Geometry, and Data Analysis
Enhao Liu (Kyoto University)
Computing the interval rank invariant of persistence modules

(E2) Room1223 or online (see URL below)

Topology, Geometry, and Data Analysis

In this talk, I will first review the story about single/multi-parameter persistent homology and its algebraic abstraction, persistence modules, from the perspective of representation theory. Then, I will define the so-called interval rank invariant of persistence modules. This invariant can be computed easily by utilizing our proposed formula though its definition is purely algebraic, which will become the main part of this talk. One direct application of the formula is to show the relation between our invariant and the generalized rank invariant proposed by Kim-Memoli. If time permits, I will introduce some other applications and related content.

We use the geometry of symmetric products of curves to construct rank one symmetric Ulrich sheaves on the (higher) secant varieties. Time permitting, we will also give an application towards an algebraic theory of knots. This is joint work with M. Kummer and J. Park.

In this talk, we report some results on equations and the ideal of $\sigma_k(v_d(\mathbb{P}^n))$, the $k$-th secant variety of $d$-uple Veronese embedding of a projective space, in case of the $k$-th secant having a relatively small degree. Knowledge on defining equations of higher secant varieties is fundamental in the study of algebraic geometry and in recent years it also has drawn a strong attention in relation to tensor rank problems.
We first recall known results on the equation of a $k$-th secant variety and introduce key notions for this work, which are '$k$-secant variety of minimal degree' and 'del Pezzo $k$-secant variety', due to Ciliberto-Russo and Choe-Kwak, respectively. Next, we focus on the case of $\sigma_4(v_3(\mathbb{P}^3))$ in $\mathbb{P}^{19}$ as explaining our method and considering its consequences. We present more results which can be obtained by the same method. This is a joint work with K. Furukawa (Josai Univ.).

It is believed that one can attach a smooth mod-p representation of a general linear group to a mod-p local Galois representation in a natural way that is called mod-p Langlands program. This conjecture is quite far from being understood beyond GL₂(ℚₚ). However, for a given mod-p local Galois representation one can construct a candidate on the automorphic side corresponding to the Galois representation for mod-p Langlands correspondence via global Langlands. In this talk, we introduce automorphic invariants on the candidate that determine the given Galois representation for a certain family of mod-p Galois representations.

I will explain the notion of projection of syzygies, which was originally given by Ehbauer and later was much used by many mathematicians and then give two applications of it. We will firstly explain how it can be used to study the syzygies of canonical curves and in particular explain its application to a conjecture by Schreyer on the ranks of generating linear syzygies for general canonical curves. We will then explain an application of it to the study of linear syzygies of Veronese varieties.

In this talk, I will take you on a journey from mathematical concepts to their applications in brain and cognitive sciences. We will explore how differential equations and nonlinear dynamical systems can be employed to model complex biological systems, including the brain. I will also discuss how these models help in understanding higher-level processes such as sleep and circadian rhythms, offering a deeper glimpse into how the brain operates as a complex, dynamic system. Additionally, I will share my personal journey from my student days to my current role as a professor in brain and cognitive sciences, illustrating how my research path has evolved over time.

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자연과학동(E6) Room 1401 (최석정 강의실)
Algebraic Geometry
Doyoung Choi (KAIST)
Degree of the 3-secant variety

자연과학동(E6) Room 1401 (최석정 강의실)

Algebraic Geometry

We present a formula for the degree of the 3-secant variety of a nonsingular projective variety embedded by a 5-very ample line bundle. The formula is provided in terms of Segre classes of the tangent bundle of a given variety. We use the generalized version of double point formula to reduce the calculation into the case of the 2-secant variety. Due to the singularity of the 2-secant variety, we use secant bundle as a nonsingular birational model and compute multiplications of desired algebraic cycles. Additionally, we compute the Hilbert-Samuel multiplicity of 2-secant variety along given variety.

In this talk we present a construction of quadratic equations and their weight one syzygies of tangent varieties using 4-way tensors of linear forms. This is in line with the 2-minor technique for quadratic equations of projective varieties and with the Oeding-Raicu theorem on equations of tangent varieties to Segre-Veronese varieties. We also discuss generalizations of the method if time permits. This is an early stage research.

The study of monomial ideals is central to many areas of commutative algebra and algebraic geometry, with Stanley-Reisner theory providing a crucial bridge between algebraic invariants and combinatorial structures. We explore how the syzygies and Betti diagrams of Stanley-Reisner ideals can be understood through combinatorial operations on simplicial complexes.
In this talk, we focus on the regularity of Stanley-Reisner ideals. We introduce a graph decomposition that bounds the regularity and a decomposition of simplicial complexes with respect to facets. In addition, we introduce secant complexes corresponding to the joins of varieties defined by Stanley-Reisner ideals and investigate the secant variety of minimal degree defined by the Stanley-Reisner ideals. This talk includes multiple collaborative works with G. Blekherman, J. Choe, J. Kim, M. Kim, and Y. Kim.