# Seminars & Colloquia

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구글 Calendar나 iPhone 등에서 구독하면 세미나 시작 전에 알림을 받을 수 있습니다.

The limits of converging sequences of graphs are natural objects from extremal graph theory that are also representable as measure-theoretic objects (graphons) or as algebraic objects (flag algebra homomorphisms). I will give an introduction to this theory via a geometric relative: its adaptation from graphs to a combinatorial encoding of planar point sets. This is based on joint work with Alfredo Hubard, Rémi de Joannis de Verclos, Jean-Sébastien Sereni and Jan Volec (http://drops.dagstuhl.de/opus/volltexte/2015/5126/). The talk will not assume any specific knowledge.

Host: 엄상일     To be announced     2017-09-22 09:15:21

The main aim of the lectures is to show some recent applications of
positive characteristic version of non-abelian Hodge theory to study
of algebraic varieties and their invariants in positive and zero characteristics.
I will also review classical Simpson's correspondence and relate
it to recent developments.

Host: 이용남     To be announced     2017-09-07 10:39:40

빅데이터(Big-Data), 단어는 많이 들어보았는데 과연 빅데이터의 본질은 무엇일까? Volume, Velocity, Variety의 3V로 정의되는 빅데이터는 과연 우리의 미래를 예측할 수 있을까? 이번 강연에서는 우리가 살고 있는 복잡한 사회를 이해하는 도구로서의 빅데이터와 네트워크 과학에 관한 설명과 함께, 그 성공적 응용사례로 구글의 검색엔진을 이용한 선거의 예측, N-gram 프로젝트를 통한 과거의 검색엔진 개발, 알파고의 AI 알고리즘, 대규모 특허분석을 통한 기술예측, 인문학 및 예술에서의 빅데이터 분석사례 등을 소개하고자 합니다. 또한 성공의 이면에 숨겨있던 빅데이터의 한계점과 어두운 그림자까지 빅데이터의 모든 것을 함께 고민하고 이를 통해 우리의 미래의 가능성을 살짝 엿보고자 합니다.

Host: 임미경     Korean     2017-09-01 16:36:42

The main aim of the lectures is to show some recent applications of
positive characteristic version of non-abelian Hodge theory to study
of algebraic varieties and their invariants in positive and zero characteristics.
I will also review classical Simpson's correspondence and relate
it to recent developments.

Host: 이용남     English     2017-09-07 10:38:36

The main aim of the lectures is to show some recent applications of
positive characteristic version of non-abelian Hodge theory to study
of algebraic varieties and their invariants in positive and zero characteristics.
I will also review classical Simpson's correspondence and relate
it to recent developments.

Host: 이용남     English     2017-09-07 10:37:11

In these two talks, I will introduce geometric problems related to mass in general relativity, such as a series of geometric inequalities, and
conjectures regarding a notion of quasi-local mass proposed by Bartnik.

The geometric inequalities we consider include the angular momentum-mass
inequality for axially symmetric initial data for the Einstein equations.
Note that the special cases treating maximal data have been proved by Dain
et al. Here I will explain how to reduce the general (non-maximal) case to
the known maximal case, and then discuss the solvability of the system of
Elliptic PDEs arose in the process, for near maximal case.

The second part of the talk will mainly provide an introduction to the
static/stationary metric extension conjectures, related to Bartnik
quasi-local mass. I will briefly discuss some known results for the static
metric extension conjecture by Anderson, Anderson/Khuri, Miao et al., and
show a local existence theorem for the solutions of axially symmetric,
stationary vacuum Einstein equations.

Host: 백형렬     English     2017-09-18 08:51:12

We show how to compute for n-vertex planar graphs in roughly O(n^(11/6)) expected time the diameter and the sum of the pairwise distances. These are the first algorithms for these problems using time O(n^c) for some constant c<2, even when restricted to undirected, unweighted planar graphs.

Host: 엄상일     English     2017-09-19 09:20:41
In this talk, we study a dynamical instability of liquid crystals under applied magnetic fields, which is the so-called Freedericksz transition. More precisely, we show that if the magnetic field strength exceeds the threshold, any global solution to the simplified Ericksen-Leslie system subject to the magnetic field converges exponentially to a unique nontrivial equilibrium, provided that a nontrivial initial orientation angle lies between $0$ and $\pi$. It is also proved that below the threshold, the equilibrium of the orientation angle is the zero state. This is based on joint work with Yuan Chen and Yong Yu.

Host: 권순식     To be announced     2017-09-20 10:30:02
We will discuss some basic idea of the Deligne-Rapoport model of modular curves.  Then, we will introduce the special fiber of the Neron model of modular Jacobian variety at some prime p. Mainly, we will focus on the component group of this special fiber. (It can be computed by the monodromy exact sequence due to the semistable reduction theorem by Grothendieck.) The structure of this component group is already known but some Hecke actions are missing.
We will complete this understanding, namely, we will compute all the Hecke actions on this group.
(This is a joint work with Taekyung Kim.)

Host: Bo-Hae Im     To be announced     2017-09-04 11:32:53

In combinatorial and computational geometry, the complexity of a system of sets is often studied via its shatter function. I will introduce these functions, and discuss how their asymptotic growth rate is governed from a single of its values, in the spirit of the classical notion of “Vapnik-Chernonenkis dimension” of hypergraphs. In particular, I will describe a probabilistic construction that refutes a conjecture of Bondy and Hajnal. This is joint work with Boris Bukh (https://arxiv.org/abs/1701.06632). The talk will start from first principles.

Host: 엄상일     To be announced     2017-09-18 09:31:35

대표적인 심혈관 질환인 관상동맥질환은 유럽에서는 5명중 1명이, 미국에서는 6명중 1명이
사망에 이르는 고위험 질병으로 선진국병으로 불린다. 대한민국에서도 생활수준의 향상과
서구화된 식습관에 따라 관상동맥질환이 중장년층에 크게 발병하고 있으며, 3대
사망원인(뇌혈관질환, 심혈관질환, 암)중에 하나이다. 심장질환의 경우 2006년에는 인구
10만명당 41.1명이던 사망률이 2017년에는 55.6명으로 증가하는 추세에 있다. 이러한
증가추세를 둔화시키기 위해서는 다각도적인 접근이 필요하겠으나, 진단과 치료의 영역에서는
정확한 혈관상태를 가시화된 영상정보로 의료진에게 제공할 수 있는 것이 필수적이다. 하지만
현재까지 표준화된 기술(gold standard)은 C-arm을 이용한 2차원 혈관 영상이 유일하다. X-ray를
이용한 실시간성이 가장 큰 장점인 2차원 영상이지만, 투사영상이 가지고 있는 궁극적인
한계(투사각도, 혈관의 중첩상태, 비틀림, 이미지 왜곡등)는 피할 수 없다. 이러한 문제점 때문에,
혈관의 스텐트 확장술등에 있어서 적정한 스텐트의 크기를 찾는데 어려움이 따르게 되며, 이는
수술시의 확정할 수 없는 위험요소가 된다.
본 발표에서는 C-arm 기반 2차원 조영영상을 이용하여 3차원 또는 시간의 움직임까지 고려한
4차원 관상동맥 영상복원방법에서 요구되는 수학적인 문제들에 대해서 이야기하고, 현재까지
연구된 결과를 논의 하고자 한다.

Host: 이창옥     To be announced     2017-09-21 17:58:18

A coherent sheaf F on a projective variety X is Ulrich if its pushforward by a finite degree map is trivial. Since they naturally appears in several different theories, the study of Ulrich bundles becomes important. In this talk, I will discuss two different approaches to construct Ulrich bundles on the intersection of two 4-dimensional quadrics: via Serre correspondence and via derived categories. I will also briefly explain an unexpected connection between generalized theta series. This is a joint work with Y. Cho and K.-S.Lee.

Host: 곽시종     To be announced     2017-09-20 14:19:29

In these two talks, I will introduce geometric problems related to mass in general relativity, such as a series of geometric inequalities, and
conjectures regarding a notion of quasi-local mass proposed by Bartnik.

The geometric inequalities we consider include the angular momentum-mass
inequality for axially symmetric initial data for the Einstein equations.
Note that the special cases treating maximal data have been proved by Dain
et al. Here I will explain how to reduce the general (non-maximal) case to
the known maximal case, and then discuss the solvability of the system of
Elliptic PDEs arose in the process, for near maximal case.

The second part of the talk will mainly provide an introduction to the
static/stationary metric extension conjectures, related to Bartnik
quasi-local mass. I will briefly discuss some known results for the static
metric extension conjecture by Anderson, Anderson/Khuri, Miao et al., and
show a local existence theorem for the solutions of axially symmetric,
stationary vacuum Einstein equations.

Host: 백형렬     English     2017-09-15 09:54:51

It was shown by Schaffer that for a dense class of compactly supported smooth initial data,the number of shock curves is finite for large time. It was not known if there is a smooth data for which the number of shock curves is unbounded. In this talk, using the structure of entropy solutions, one can construct a smooth initial data for which the number of shocks curves is infinite.

Host: 김용정     English     2017-09-11 10:42:15

We classify minimal projective $3$-folds of general type with the geometric genus $p_g= 1, 2$ or $3$ by studying the birationality of $\varphi_{16},\ \varphi{6}$ and $\varphi{5}$. This is a joint work with Meng Chen and Matteo Penegini.

Host: 신용주     English     2017-08-21 08:58:33

For a motivation of stochastic integrals, we start with stochastic (random) versions of deterministic systems and then we discuss  the Ito integral as a non-anticipating (adapted) stochastic integral.

Based on the quantum decomposition of Brownian motion, we study the Skorohod integral and Stratonovich integral as anticipating (non-adapted) stochastic integrals.

Note that the Skorohod integral is the adjoint action of the Malliavin derivative.

Host: 황강욱     Korean     2017-09-01 16:16:42

I discuss recent progress in the classification of simply
connected Godeaux surfaces. There are two parts. The first
describes remarkable recent work of Isabel Stenger
(Kaiserslautern). She constructs an 8-dimensional rationally
parametrised moduli family of "general" surfaces, for which
the bicanonical pencil has no hyperelliptic fibres. Her
family contains a 7-dimensional subfamily with a single
hyperelliptic fibre and a 6-dimensional subfamily with two
hyperelliptic fibres. The second part describes my different
attack on the same problem, on which there has been some
progress over the last 25 years.

Host: 이용남     English     2017-09-05 13:51:16

Backward stochastic differential equation (BSDE) is a generalization of martingale representation theorem  and it has been widely used for financial derivative pricing and stochastic optimization. Traditionally, most of well-posedness result of BSDE were based on contraction mapping theorem on the space of stochastic processes.
In our work, we were able to transform BSDEs into fixed point problems in the space of Lp random variables. The simplicity of our framework enables us to apply various kind of fixed point theorems which have not been tried in previous literature. In particular, this enables to remove infinite dimensionality arise from time and we were able to use white noise analysis to use topological fixed point theorems. As a result, we were able to generalize previous well-posedness results: e.g. time-delayed type, mean-field type, multidimensional super-linear type.
The talk is aimed for those who are not familiar with BSDE and it is based on BSE's, BSDE's and fixed point theorem (joint work with Patrick Cheridito)

Host: 강완모     Korean English if it is requested     2017-08-09 12:59:07

I will explain the construction of coordinates on the moduli spaces of maximal globally hyperbolic 3 dimensional spacetimes generalising the shear coordinates on the Teichmüller space. I will then discuss the Poisson structure and the mapping class group action on these moduli spaces. These constructions are motivated by the problem of quantum gravity, so I’ll also give a brief description of some of the ideas behind quantisation and how they apply in the context of 3d gravity. This talk is based on joint work with Catherine Meusburger and ongoing work with Hyun Kyu Kim.

Host: 백형렬     To be announced     2017-08-24 10:00:15

What is the probability that in a telecommunication system, an atypically large proportion of users experiences a bad quality of service induced by lack of capacity? Although today's wireless networks exhibit nodes embedded in the Euclidean plane, large deviations have so far predominantly been analyzed in mean-field approximations. We explain how to prove a large deviation principle for a random spatial telecommunication network featuring capacity-constrained relays. This talk is based on joint work with Benedikt Jahnel and Robert Patterson (WIAS Berlin)

Host: Paul Jung     English     2017-08-23 10:09:52
Barycentre is the geometric mean of a distribution on a metric space; it is a point that minimizes its average distance squared to the given distribution.  Such a point is highly non-unique in general, though we have uniqueness when the underlying space is the Euclidean space or more generally a space of nonpositive curvature with trivial topology. We consider such a notion from the viewpoint of optimal transport which gives a natural distance structure between probability measures.
This allows us to uniquely interpolate many probability distributions, called the Wasserstein barycentre, as initiated by Agueh and Carlier. It also leads to a uniquely defined canonical barycentre, which is obtained by relaxing the notion of barycentre point to a barycentre measure. We will explain these developments, based on join work with Brendan Pass
Host: 강완모     Korean English if it is requested     2017-08-09 12:57:10

In this talk, I will talk on transverse stability line solitary waves for KP-II and the Benney Luke equation. Both equations are long wave long wave models for 3D water waves with weak surface tension. I will explain that the resonant continuous eigenmodes which we can find in an exponentially weighted space have to do with modulation of line solitary waves.

Host: Soonsik Kwon     Contact: Hyeonsu Kim (8545)     English     2017-08-21 10:31:40
In this talk, we consider continuous and discrete nonlinear Schr\"odinger equations (NLS) with potential.
Under the assumption that Schr\"odinger operator has several eigenvalues, it is well known that there exist small bound states (localized in space, periodic in time solutions).
We will show that for the continuous NLS, even though the linear Schr\"odinger equaiton has quasi-periodic (in time) solutions, there exists no quasi-periodic (in time) solutions and for discrete NLS, there can exists quasi-periodic (in time) solutions.
Similar result holds for nonlinear Klein Gordon equations.
Host: Soonsik Kwon     Contact: Hyeonsu Kim (8545)     English     2017-08-21 10:32:39

We study the dynamical properties of the topological generalized beta transformations, which generalizes the concept of generalized beta transformations defined by Gora. In particular, we generalize the result on admissible sequence for unimodular maps to the case of generalized beta maps, and also study the properties of the topological entropy and its Galois conjugates, generalizing some results by Tiozzo. This talk represents an ongoing collaboration with Diana Davis, Kathryn Lindsey and Harry Bray.

Host: 백형렬     To be announced     2017-08-17 08:59:06
We will study relations between Fully nonlinear PDEs, Embedding problems, Isoperimetric inequality, Curvature flows, and Optimal transportation.
In particular, we will discuss about fully degenerate or singular elliptic equations in geometric problems and its applications curvature derivative controls in singularity analysis.
We will also divergence structures in certain fully nonlinear PDEs and their geometric background.
Host: 강완모     Korean English if it is requested     2017-08-09 12:55:41

A novel high-order numerical scheme is proposed to solve the shallow water equations (SWEs) on arbitrary rotating curved surfaces. Based on the method of moving frames (MMF), the proposed scheme not only has the smallest dimensionality of two in space, but also does not require either of (i) metric tensors, (ii) composite meshes, or (iii) the surrounding space. The MMF-SWE formulation is numerically discretized using the discontinuous Galerkin method of arbitrary polynomial order p in space and an explicit Runge-Kutta scheme in time. In this talk, we start with the fundamental concepts of the innovational moving frames for Riemannian geometry developed by the famous French mathematician Elie Cartan in the early 20th century. Then, we discuss its adaptation and validity in the discrete space for scientific computing by overviewing the past works on conservational laws and diffusion equations. Applications to SWEs  will be explained in details in views of algorithmic novelty to overcome the classical issues of PDEs on the closed surface such as geometric singularities and rotational effects. Results of six standard tests on the sphere will be displayed with the optimal order of convergence of p+1. Also, its general applicability and stability on arbitrary rotating surfaces such as ellipsoid, irregular, and non-convex surfaces will be demonstrated.

Host: 김용정     Korean     2017-08-09 16:26:38

We proved a Kazhdan type theorem for the canonical metrics of finite graphs. Namely, we show that the canonical metric of finite normal coverings of the graph converges when the covering converges, and the limit depends only on the limit of the coverings. We also generalize the argument to higher dimensional simplicial complexes. The proof is mostly based on an analogous argument in the case of Riemann surfaces and Lück's approximation theorem for L^2 cohomology. This is joint work with Farbod Shokrieh.

Host: 백형렬     English     2017-08-04 09:30:46

High-dimensionality is one of the major challenges in stochastic simulation of realistic physical systems. The most appropriate numerical scheme needs to balance accuracy and computational complexity, and it also needs to address issues such as multiple scales, lack of regularity, and long-term integration.

In this talk, I will review state-of-the-art numerical techniques for high-dimensional systems including low-rank tensor approximation, sparse grid collocation, and ANOVA decomposition. The presented numerical methods are tested and compared in the joint response-excitation PDF equation that generalizes the existing PDF equations and enables us to do kinetic simulations with non-Gaussian colored noise. The alternative to the numerical approach, I will discuss dimension reduction techniques such as Mori-Zwanzig approach and moment closures that can obtain reduced order equations in lower dimensions. I will also present numerical results including stochastic Burgers equation and Lorenz-96 system.

English     2017-07-21 11:30:09