## PDE実解析研究会

開催情報 火曜日　10:30～11:30　数理科学研究科棟(駒場) 056号室 儀我美一、高棹圭介、米田剛 http://coe.math.sci.hokudai.ac.jp/sympo/pde_ra/ 首都圏の偏微分方程式、実解析の研究をさらに活発にするために本研究会を東大で開催いたします。偏微分方程式研究者と実解析研究者の討論がより日常的になることを目指しています。そのため、講演がその分野の概観をもわかるような形になるよう配慮いたします。また講演者との1対1の討論がしやすいように講演は火曜の午前とし、午後に討論用の場所を用意いたします。この研究会を通して皆様に気楽に東大を訪問していただければ幸いです。北海道大学のHPには、第1回(2004年9月29日)～第38回(2008年10月15日)の情報が掲載されております。

### 2017年07月18日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室

[ 講演概要 ]

### 2016年12月20日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Nader Masmoudi 氏 (Courant Institute, NYU)
On the stability of the 3D Couette Flow (English)
[ 講演概要 ]
We will discuss the dynamics of small perturbations of the plane, periodic Couette flow in the 3D incompressible Navier-Stokes system at high Reynolds number. For sufficiently regular initial data, we determine the stability threshold for small perturbations and characterize the long time dynamics of solutions near this threshold. For rougher data, we obtain an estimate of the stability threshold which agrees closely with numerical experiments. The primary linear stability mechanism is an anisotropic enhanced dissipation resulting from the mixing caused by the large mean shear. There is also a linear inviscid damping similar to the one observed in 2D. The main linear instability is a non-normal instability known as the lift-up effect. There is clearly a competition between these linear effects. Understanding the variety of nonlinear resonances and devising the correct norms to estimate them form the core of the analysis we undertake. This is based on joint works with Jacob Bedrossian and Pierre Germain.

### 2016年11月22日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Yannick Sire (Johns Hopkins University) 氏
De Giorgi conjecture and minimal surfaces for integro-differential operators (English)
[ 講演概要 ]
I will review the classical De Giorgi conjecture and its link with minimal surfaces. Then I will move on recent results for flatness of level sets of solutions of semi linear equations involving anomalous diffusion. First I will deal with the fractional laplacian; second with quite general integral operators in 2 dimensions.

### 2016年10月11日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Nam Quang Le 氏 (Indiana University)
Global solutions to the second boundary value problem of the prescribed affine mean curvature and Abreu's equations (English)
[ 講演概要 ]
The second boundary value problem of the prescribed affine mean curvature equation is a nonlinear, fourth order, geometric partial differential equation. It was introduced by Trudinger and Wang in 2005 in their investigation of the affine Plateau problem in affine geometry. The previous works of Trudinger-Wang, Chau-Weinkove and the author solved this global problem under some restrictions on the sign or integrability of the affine mean curvature. In this talk, we explain how to remove these restrictions and obtain global solutions under optimal integrability conditions on the affine mean curvature. Our analysis also covers the case of Abreu's equation arising in complex geometry.

### 2016年08月29日(月)

10:30-11:30   数理科学研究科棟(駒場) 268号室

Nguyen Cong Phuc 氏 (Louisiana State University)
The Navier-Stokes equations: stationary existence, conditional regularity, and self-similar singularities (English)
[ 講演概要 ]
In this talk, both stationary and time-dependent Navier-Stokes equations are discussed. The common theme is that the quadratic nonlinearity and the pressure are both treated as weights generally belonging to a Sobolev space of negative order. We obtain the unique existence of solutions to stationary Navier-Stokes equations with small singular external forces that belong to a critical space. This result can be viewed as the stationary counterpart of an existence result obtained by H. Koch and D. Tataru for the free non-stationary Navier-Stokes equations with small initial data in $BMO^{-1}$. In another direction, some new local energy bounds are obtained for the time-dependent Navier-Stokes equations which imply the regularity condition $L_{t}^{\infty}(X)$, where $X$ is a non-endpoint borderline Lorentz space $X=L_{x}^{3, q}, q\not=\infty$. The analysis also allows us to rule out the existence of Leray's backward self-similar solutions to the Navier–Stokes equations with profiles in $L^{12/5}(\mathbb{R}^3)$ or in the Marcinkiewicz space $L^{q, \infty}(\mathbb{R}^{3})$ for any $q \in (12/5, 6)$.
This talk is based on joint work with Tuoc Van Phan and Cristi Guevara.
[ 講演参考URL ]
https://www.math.lsu.edu/~pcnguyen/

### 2016年07月12日(火)

10:20-11:00   数理科学研究科棟(駒場) 056号室

Piotr Rybka 氏 (University of Warsaw)
Special cases of the planar least gradient problem (English)
[ 講演概要 ]
We study the least gradient problem in two special cases:
(1) the natural boundary conditions are imposed on a part of the strictly convex domain while the Dirichlet data are given on the rest of the boundary; or
(2) the Dirichlet data are specified on the boundary of a rectangle. We show existence of solutions and study properties of solution for special cases of the data. We are particularly interested in uniqueness and continuity of solutions.

### 2016年07月12日(火)

14:20-15:00   数理科学研究科棟(駒場) 056号室

Global Strong $L^p$ Well-Posedness of the 3D Primitive Equations (English)
[ 講演概要 ]
Primitive Equations are considered to be a fundamental model for geophysical flows. Here, the $L^p$ theory for the full primitive equations, i.e. the three dimensional primitive equations coupled to the equation for temperature and salinity, is developed. This set of equations is globally strongly well-posed for arbitrary large initial data lying in certain interpolation spaces, which are explicitly characterized as subspaces of $H^2/p$, $p$, $1 < p < \infty$, satisfying certain boundary conditions. Thus, the general $L^p$ setting admits rougher data than the usual $L^2$ theory with initial data in $H^1$.

In this study, the linearized Stokes type problem plays a prominent role, and it turns out that it can be treated efficiently using perturbation methods for $H^\infty$-calculus.

### 2016年07月12日(火)

12:10-12:50   数理科学研究科棟(駒場) 056号室

Elio Espejo 氏 (National University of Colombia)
The role of convection in some Keller-Segel models (English)
[ 講演概要 ]
An interesting problem in reaction-diffusion equations is the understanding of the role of convection in phenomena like blow-up or convergence. I will discuss this problem through some Keller-Segel type models arising in mathematical biology and show some recent results.

### 2016年07月12日(火)

11:20-12:00   数理科学研究科棟(駒場) 056号室

Monika Muszkieta 氏 (Wroclaw University of Science and Technology)
The total variation flow in $H^{−s}$ (English)
[ 講演概要 ]
In the talk, we consider the total variation flow in the Sobolev space $H^{−s}$. We explain the motivation to study this problem in the context of image processing applications and provide its rigorous interpretation under periodic boundary conditions. Furthermore, we introduce a numerical scheme for an approximate solution to this flow which has been derived based on the primal-dual approach and discuses some issues concerning its convergence. We also show and compare results of numerical experiments obtained by application of this scheme for a simple initial data and different values of the index $s$.
This is a join work with Y. Giga.

### 2016年04月27日(水)

15:00-16:00   数理科学研究科棟(駒場) 056号室

Elijah Liflyand 氏 (Bar-Ilan University, Israel)
Fourier transform versus Hilbert transform (English)
[ 講演概要 ]
We present several results in which the interplay between the Fourier transform and the Hilbert transform is of special form and importance.
1. In 50-s (Kahane, Izumi-Tsuchikura, Boas, etc.), the following problem in Fourier Analysis attracted much attention: Let $\{a_k\},$ $k=0,1,2...,$ be the sequence of the Fourier coefficients of the absolutely convergent sine (cosine) Fourier series of a function $f:\mathbb T=[-\pi,\pi)\to \mathbb C,$ that is $\sum |a_k|<\infty.$ Under which conditions on $\{a_k\}$ the re-expansion of $f(t)$ ($f(t)-f(0)$, respectively) in the cosine (sine) Fourier series will also be absolutely convergent?
We solve a similar problem for functions on the whole axis and their Fourier transforms. Generally, the re-expansion of a function with integrable cosine (sine) Fourier transform in the sine (cosine) Fourier transform is integrable if and only if not only the initial Fourier transform is integrable but also the Hilbert transform of the initial Fourier transform is integrable.
2. The following result is due to Hardy and Littlewood: If a (periodic) function $f$ and its conjugate $\widetilde f$ are both of bounded variation, their Fourier series converge absolutely.
We generalize the Hardy-Littlewood theorem (joint work with U. Stadtmüller) to the Fourier transform of a function on the real axis and its modified Hilbert transform. The initial Hardy-Littlewood theorem is a partial case of this extension, when the function is taken to be with compact support.
3. These and other problems are integrated parts of harmonic analysis of functions of bounded variation. We have found the maximal space for the integrability of the Fourier transform of a function of bounded variation. Along with those known earlier, various interesting new spaces appear in this study. Their inter-relations lead, in particular, to improvements of Hardy's inequality.
There are multidimensional generalizations of these results.
[ 講演参考URL ]
http://u.math.biu.ac.il/~liflyand/

### 2016年03月16日(水)

16:00-17:00   数理科学研究科棟(駒場) 056号室

Boris Khesin 氏 (University of Toronto)
Fluids, vortex membranes, and skew-mean-curvature flows (English)
[ 講演概要 ]
We show that an approximation of the hydrodynamical Euler equation describes the skew-mean-curvature flow on vortex membranes in any dimension. This generalizes the classical binormal, or vortex filament, equation in 3D. We present a Hamiltonian framework for dynamics of higher-dimensional vortex filaments and vortex sheets as singular 2-forms (Green currents) with support of codimensions 2 and 1, respectively.

### 2016年01月26日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Salomé Oudet 氏 (University of Tokyo)
Hamilton-Jacobi equations for optimal control on 2-dimensional junction (English)
[ 講演概要 ]
We are interested in infinite horizon optimal control problems on 2-dimensional junctions (namely a union of half-planes sharing a common straight line) where different dynamics and different running costs are allowed in each half-plane. As for more classical optimal control problems, ones wishes to determine the Hamilton-Jacobi equation which characterizes the value function. However, the geometric singularities of the 2-dimensional junction and discontinuities of data do not allow us to apply the classical results of the theory of the viscosity solutions.
We will explain how to skirt these difficulties using arguments coming both from the viscosity theory and from optimal control theory. By this way we prove that the expected equation to characterize the value function is well posed. In particular we prove a comparison principle for this equation.

### 2016年01月19日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Hao Wu 氏 (Fudan University)
Well-posedness and stability of the full Ericksen-Leslie system for incompressible nematic liquid crystal flows
[ 講演概要 ]
In this talk, the general Ericksen-Leslie (E-L) system modelling the incompressible nematic liquid crystal flow will be discussed.
We shall prove the well-posedness and long-time behavior of the E-L system under proper assumptions on the viscous Leslie coefficients.
In particular, we shall discuss the connection between Parodi's relation and stability of the E-L system.

### 2015年09月29日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Tuomo Kuusi 氏 (Aalto University)
Nonlocal self-improving properties (English)
[ 講演概要 ]
The classical Gehring lemma for elliptic equations with measurable coefficients states that an energy solution, which is initially assumed to be $W^{1,2}$-Sobolev regular, is actually in a better Sobolev space space $W^{1,q}$ for some $q>2$. This is a consequence of a self-improving property that so-called reverse Hölder inequality implies. In the case of nonlocal equations a self-improving effect appears: Energy solutions are also more differentiable. This is a new, purely nonlocal phenomenon, which is not present in the local case. The proof relies on a nonlocal version of the Gehring lemma involving new exit time and dyadic decomposition arguments. This is a joint work with G. Mingione and Y. Sire.

### 2015年07月14日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Lin Wang 氏 (Tsinghua University)
Viscosity solutions of Hamilton-Jacobi equations from a dynamical viewpoint (English)
[ 講演概要 ]
By establishing an implicit variational principle for contact Hamiltonian systems, we detect some properties of viscosity solutions of Hamilton-Jacobi equations of certain Hamilton-Jacobi equations depending on unknown functions, including large time behavior and regularity on certain sets. Besides, I will talk about some connections with contact geometry, thermodynamics and nonholonomic mechanics.

### 2015年05月19日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室

Convex bodies and geometry of some associated Minkowski functionals (日本語)
[ 講演概要 ]
In this talk, we will investigate the construction of so-called Hilbert metric, as well as Funk metric, defined on convex set from a new variational viewpoint. The local and global aspects of the geometry of the resulting Minkowski functionals will be contrasted. As an application, some remarks on the Perron-Frobenius theorem will be made. Part of the project is a joint work with Athanase Papadopoulos (Strasbourg).

### 2015年01月20日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Italo Capuzzo Dolcetta 氏 (Università degli Studi di Roma "La Sapienza")
Maximum Principle and generalized principal eigenvalue for degenerate elliptic operators (English)
[ 講演概要 ]
In my presentation I will report on a joint paper with H. Berestycki, A. Porretta and L. Rossi to appear shortly on JMPA.
We characterize the validity of the Maximum Principle in bounded domains for fully nonlinear degenerate elliptic operators in terms of the sign of a suitably defined generalized principal eigenvalue. Here, the maximum principle refers to the property of non-positivity of viscosity subsolutions of the Dirichlet problem.
The new notion of generalized principal eigenvalue that we introduce here allows us to deal with arbitrary type of degeneracy of the elliptic operators.
We further discuss the relations between this notion and other natural generalizations of the classical notion of principal eigenvalue, some of which have been previously introduced for particular classes of operators.

### 2015年01月13日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Wojciech Zajączkowski 氏 (Institute of Mathematics Polish Academy of Sciences)
Global regular solutions to the Navier-Stokes equations which remain close to the two-dimensional solutions (English)
[ 講演概要 ]
We consider the motion of the Navier-Stokes equations in a cylinder with the Navier-boundary conditions. First we prove global existence of regular two-dimensional solutions non-decaying in time. Next we show stability of these solutions. In this way we have existence of global regular solutions which remain close to the two-dimensional solutions. We prove the results for nonvanishing external force in time.

### 2015年01月06日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室
Elio Eduardo Espejo 氏 (National University of Colombia / Osaka University)
Global existence and asymptotic behavior for some Keller-Segel systems coupled with Navier-Stokes equations (英語)
[ 講演概要 ]
There are plenty of examples in nature, where cells move in response to some chemical signal in the environment. Biologists call this phenomenon chemotaxis. In my talk I will approach the problem of describing mathematically the phenomenon of chemotaxis when it happens surrounded by a fluid. This is a new research topic bringing the attention of many scientists because it has given rise to many interesting questions having relevance in both biology and mathematics. In particular, I will present some new mathematical models arising from my current research that have given rise to Keller-Segel type systems coupled with Navier-Stokes systems. I will present some results of global existence and asymptotic behavior. Finally I will discuss some open problems.

### 2014年12月02日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室

Remark on single exponential bound of the vorticity gradient for the two-dimensional Euler flow around a corner (JAPANESE)
[ 講演概要 ]
In this talk, the two dimensional Euler flow under a simple symmetry condition with hyperbolic structure in a unit square $D=\{(x_{1}, x_{2}): 0 < x_{1} + x_{2} < \sqrt{2},\ 0<-x_{1} + x_{2} < \sqrt{2}\}$ is considered.
It is shown that the Lipschitz estimate of the vorticity on the boundary is at most single exponential growth near the stagnation point.
(Joint work with Tsuyoshi Yoneda and Hideyuki Miura.)

### 2014年09月17日(水)

16:30-17:30   数理科学研究科棟(駒場) 128号室

On the abstract evolution equations of hyperbolic type (JAPANESE)
[ 講演概要 ]
This talk deals with the abstract Cauchy problem for linear evolution equations of hyperbolic type in a Hilbert space. We will discuss the existence and uniqueness of its classical solution and apply the results to linear Schrödinger equations with time dependent potentials.

### 2014年06月24日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室

Piotr Rybka 氏 (University of Warsaw)
Sudden directional diffusion: counting and watching facets (ENGLISH)
[ 講演概要 ]
We study two examples of singular parabolic equations such that the diffusion is so strong that is leads to creation of facets. By facets we mean flat parts of the graphs of solutions with singular slopes. In one of the equations we study there are two singular slopes. The other equation has just one singular slope and the isotropic diffusion term. For both problems we watch and count facet.

For the system with two singular slopes a natural question arises if any solution may have an infinite number of oscillations. We also show that the solutions we constructed are viscosity solutions. This in turn gives estimates on the extinction time based on the comparison principle.

### 2014年04月15日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室

An application of weighted Hardy spaces to the Navier-Stokes equations (JAPANESE)
[ 講演概要 ]
The purpose of this talk is to investigate decay orders of the L^2 energy of solutions to the incompressible homogeneous Navier-Stokes equations on the whole spaces by the aid of the theory of weighted Hardy spaces. The main estimates are two weighted inequalities for heat semigroup on weighted Hardy spaces and a weighted version of the div-curl lemma due to Coifman-Lions-Meyer-Semmes. It turns out that because of the use of weighted Hardy spaces, our decay orders of the energy can be close to the critical one of Wiegner.

### 2013年11月13日(水)

10:30-11:30   数理科学研究科棟(駒場) 056号室

Mark Wilkinson 氏 (École normale supérieure - Paris)
Eigenvalue Constraints and Regularity of Q-tensor Navier-Stokes Dynamics (ENGLISH)
[ 講演概要 ]
The Q-tensor is a traceless and symmetric 3x3 matrix that describes the small-scale structure in nematic liquid crystals. In order to be physically meaningful, its eigenvalues should be bounded below by -1/3 and above by 2/3. This constraint raises questions regarding the physical predictions of theories which employ the Q-tensor; it also raises analytical issues in both static and dynamic Q-tensor theories of nematic liquid crystals. John Ball and Apala Majumdar recently constructed a singular map on traceless, symmetric matrices that penalises unphysical Q-tensors by giving them an infinite energy cost. In this talk, I shall present some mathematical results for a coupled Navier-Stokes system modelling nematic dynamics into which this map is built, including the existence, regularity and so-called `strict physicality' of its weak solutions.

### 2013年11月12日(火)

10:30-11:30   数理科学研究科棟(駒場) 056号室

Reinhard Farwig 氏 (Technische Universität Darmstadt)
Optimal initial values and regularity conditions of Besov space type for weak solutions to the Navier-Stokes system (ENGLISH)
[ 講演概要 ]
In a joint work with H. Sohr (Paderborn) and W. Varnhorn (Kassel) we discuss the optimal condition on initial values for the instationary Navier-Stokes system in a bounded domain to get a locally regular solution in Serrin's class.
Then this result based on a description in Besov spaces will be used at all or almost all instants to prove new conditional regularity results for weak solutions.