## PDE実解析研究会

過去の記録 ～07/20｜次回の予定｜今後の予定 07/21～

開催情報 | 火曜日 10:30～11:30 数理科学研究科棟(駒場) 056号室 |
---|---|

担当者 | 儀我美一、石毛和弘、三竹大寿、米田剛 |

セミナーURL | http://coe.math.sci.hokudai.ac.jp/sympo/pde_ra/ |

目的 | 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。 |

**過去の記録**

### 2018年07月02日(月)

10:30-11:30 数理科学研究科棟(駒場) 056号室

通常の曜日と異なります。

Convex integration in fluid dynamics (English)

通常の曜日と異なります。

**László Székelyhidi Jr. 氏**(Universität Leipzig)Convex integration in fluid dynamics (English)

[ 講演概要 ]

In the talk we present the technique of convex integration for constructing weak solutions to various equations in fluid mechanics.

We will focus on the recent resolution of Onsagers conjecture, but also discuss further directions and in particular the applicability to dissipative systems.

In the talk we present the technique of convex integration for constructing weak solutions to various equations in fluid mechanics.

We will focus on the recent resolution of Onsagers conjecture, but also discuss further directions and in particular the applicability to dissipative systems.

### 2018年05月22日(火)

10:30-11:30 数理科学研究科棟(駒場) 056号室

A discrete game interpretation for curvature flow equations with dynamic boundary conditions (日本語)

**柳 青 氏**(福岡大学)A discrete game interpretation for curvature flow equations with dynamic boundary conditions (日本語)

[ 講演概要 ]

A game-theoretic approach to motion by curvature was proposed by Kohn and Serfaty in 2006. They constructed a family of deterministic discrete games, whose value functions converge to the unique solution of the curvature flow equation. In this talk, we develop this method to provide an interpretation for the associated dynamic boundary value problems by including in the game setting a kind of nonlinear reflection near the boundary. We also discuss its applications to the fattening phenomenon. This talk is based on joint work with N. Hamamuki at Hokkaido University.

A game-theoretic approach to motion by curvature was proposed by Kohn and Serfaty in 2006. They constructed a family of deterministic discrete games, whose value functions converge to the unique solution of the curvature flow equation. In this talk, we develop this method to provide an interpretation for the associated dynamic boundary value problems by including in the game setting a kind of nonlinear reflection near the boundary. We also discuss its applications to the fattening phenomenon. This talk is based on joint work with N. Hamamuki at Hokkaido University.

### 2018年04月17日(火)

10:30-11:30 数理科学研究科棟(駒場) 056号室

Global existence of a strong solution to the Navier-Stokes equations in a curved thin domain (Japanese)

**三浦 達彦 氏**(東京大学)Global existence of a strong solution to the Navier-Stokes equations in a curved thin domain (Japanese)

[ 講演概要 ]

In this talk, we consider the Navier-Stokes equations with Navier's slip boundary conditions in a three-dimensional curved thin domain around a closed surface.

We establish the global-in-time existence of a strong solution for large data when the width of the thin domain is very small.

A key idea is to decompose a three-dimensional vector field into the average part which is almost two-dimensional and the residual part to which we can apply Poincaré type inequalities.

Such decomposition enables us to derive a good estimate for the inner product of the inertia term and the viscous term, which is essential for our arguments.

In this talk, we consider the Navier-Stokes equations with Navier's slip boundary conditions in a three-dimensional curved thin domain around a closed surface.

We establish the global-in-time existence of a strong solution for large data when the width of the thin domain is very small.

A key idea is to decompose a three-dimensional vector field into the average part which is almost two-dimensional and the residual part to which we can apply Poincaré type inequalities.

Such decomposition enables us to derive a good estimate for the inner product of the inertia term and the viscous term, which is essential for our arguments.

### 2017年12月12日(火)

10:30-11:30 数理科学研究科棟(駒場) 056号室

Stochastic Three-Dimensional Navier-Stokes Equations + Waves: Averaging, Convergence, Regularity and Nonlinear Dynamics (English)

**Alex Mahalov 氏**(Arizona State University)Stochastic Three-Dimensional Navier-Stokes Equations + Waves: Averaging, Convergence, Regularity and Nonlinear Dynamics (English)

[ 講演概要 ]

We establish multi-scale stochastic averaging, convergence and regularity theorems in a general framework by bootstrapping from global regularity of the averaged stochastic resonant equations. The averaged covariance operator couples stochastic and wave effects. We also present theoretical results for 3D nonlinear dynamics.

We establish multi-scale stochastic averaging, convergence and regularity theorems in a general framework by bootstrapping from global regularity of the averaged stochastic resonant equations. The averaged covariance operator couples stochastic and wave effects. We also present theoretical results for 3D nonlinear dynamics.

### 2017年11月21日(火)

10:30-11:30 数理科学研究科棟(駒場) 056号室

Optimal isoperimetric inequalities for surfaces in any codimension

in Cartan-Hadamard manifolds (English)

**Felix Schulze 氏**(University College London)Optimal isoperimetric inequalities for surfaces in any codimension

in Cartan-Hadamard manifolds (English)

[ 講演概要 ]

Let $(M^n,g)$ be simply connected, complete, with non-positive sectional

curvatures, and $\Sigma$ a 2-dimensional surface in $M^n$. Let $S$ be an area

minimising 3-current such that $\partial S = \Sigma$. We use a weak mean

curvature flow, obtained via elliptic regularisation, starting from

$\Sigma$, to show that $S$ satisfies the optimal Euclidean isoperimetric

inequality: $|S| \leq 1/(6\sqrt{\pi}) |\Sigma|^{3/2}$. We also obtain the

optimal estimate in case the sectional curvatures of $M$ are bounded from

above by $\kappa < 0$ and characterise the case of equality. The proof

follows from an almost monotonicity of a suitable isoperimetric

difference along the approximating flows in one dimension higher.

Let $(M^n,g)$ be simply connected, complete, with non-positive sectional

curvatures, and $\Sigma$ a 2-dimensional surface in $M^n$. Let $S$ be an area

minimising 3-current such that $\partial S = \Sigma$. We use a weak mean

curvature flow, obtained via elliptic regularisation, starting from

$\Sigma$, to show that $S$ satisfies the optimal Euclidean isoperimetric

inequality: $|S| \leq 1/(6\sqrt{\pi}) |\Sigma|^{3/2}$. We also obtain the

optimal estimate in case the sectional curvatures of $M$ are bounded from

above by $\kappa < 0$ and characterise the case of equality. The proof

follows from an almost monotonicity of a suitable isoperimetric

difference along the approximating flows in one dimension higher.

### 2017年11月15日(水)

10:30-11:30 数理科学研究科棟(駒場) 056号室

※ 通常と曜日が異なります。

Boundary value problems for parabolic equations with measurable coefficients (English)

※ 通常と曜日が異なります。

**Kaj Nyström 氏**(Uppsala University)Boundary value problems for parabolic equations with measurable coefficients (English)

[ 講演概要 ]

In recent joint works with P. Auscher and M. Egert we establish new results concerning boundary value problems in the upper half-space for second order parabolic equations (and systems) assuming only measurability and some transversal regularity in the coefficients of the elliptic part. To establish our results we introduce and develop a first order strategy by means of a parabolic Dirac operator at the boundary to obtain, in particular, Green's representation for solutions in natural classes involving square functions and non-tangential maximal functions, well-posedness results with data in $L^2$-Sobolev spaces together with invertibility of layer potentials, and perturbation results. In addition we solve the Kato square root problem for parabolic operators with coefficients of the elliptic part depending measurably on all variables. Using these results we are also able to solve the $L^p$-Dirichlet problem for parabolic equations with real, time-dependent, elliptic but non-symmetric coefficients. In this talk I will briefly describe some of these developments.

In recent joint works with P. Auscher and M. Egert we establish new results concerning boundary value problems in the upper half-space for second order parabolic equations (and systems) assuming only measurability and some transversal regularity in the coefficients of the elliptic part. To establish our results we introduce and develop a first order strategy by means of a parabolic Dirac operator at the boundary to obtain, in particular, Green's representation for solutions in natural classes involving square functions and non-tangential maximal functions, well-posedness results with data in $L^2$-Sobolev spaces together with invertibility of layer potentials, and perturbation results. In addition we solve the Kato square root problem for parabolic operators with coefficients of the elliptic part depending measurably on all variables. Using these results we are also able to solve the $L^p$-Dirichlet problem for parabolic equations with real, time-dependent, elliptic but non-symmetric coefficients. In this talk I will briefly describe some of these developments.

### 2017年10月17日(火)

10:30-11:30 数理科学研究科棟(駒場) 056号室

Some perspectives on negative index materials (English)

**Hoài-Minh Nguyên 氏**(École Polytechnique Fédérale de Lausanne)Some perspectives on negative index materials (English)

[ 講演概要 ]

Negative index materials (NIMs) are artificial structures whose refractive index has negative value over some frequency range. These materials were first investigated theoretically by Veselago in 1964. The existence of NIMs was confirmed experimentally by Shelby, Smith, and Schultz in 2001. New fabrication techniques now allow the construction of NIMs at scales that are interesting for applications. NIMs have attracted a lot of attention from the scientific community, not only because of potentially interesting applications, but also because of challenges in understanding their peculiar properties. Mathematically, the study of NIMs faces two difficulties. First, the equations describing the phenomenon have sign changing coefficients, hence the ellipticity and the compactness are lost in general. Second, the localized resonance, i.e., the field explodes in some regions and remains bounded in some others as the loss goes to 0, might appear. In this talk I will discuss various mathematics techniques used to understand various applications of NIMs such as cloaking and superlensing and to develop new designs for them.

Negative index materials (NIMs) are artificial structures whose refractive index has negative value over some frequency range. These materials were first investigated theoretically by Veselago in 1964. The existence of NIMs was confirmed experimentally by Shelby, Smith, and Schultz in 2001. New fabrication techniques now allow the construction of NIMs at scales that are interesting for applications. NIMs have attracted a lot of attention from the scientific community, not only because of potentially interesting applications, but also because of challenges in understanding their peculiar properties. Mathematically, the study of NIMs faces two difficulties. First, the equations describing the phenomenon have sign changing coefficients, hence the ellipticity and the compactness are lost in general. Second, the localized resonance, i.e., the field explodes in some regions and remains bounded in some others as the loss goes to 0, might appear. In this talk I will discuss various mathematics techniques used to understand various applications of NIMs such as cloaking and superlensing and to develop new designs for them.

### 2017年07月18日(火)

10:30-11:30 数理科学研究科棟(駒場) 056号室

軸対称非圧縮Euler方程式の或る瞬間爆発について (日本語)

**米田 剛 氏**(東京大学)軸対称非圧縮Euler方程式の或る瞬間爆発について (日本語)

[ 講演概要 ]

本講演では、軸対称非圧縮Euler方程式の瞬間爆発についての結果を報告する。より具体的には、$C^{2,\alpha}$ ($0<\alpha<1$)に入る初期速度場に対応する解が、任意の$T$における$C^1([0,T):C^2)$には入らないという定理を紹介する。定理の証明には、特異積分作用素の$L^\infty$-非有界性は一切使わず、代わりにFrenet-Serret formulasやorthonormal moving frameといった幾何学的概念を本質的に使う。時間があれば、この洞察の物理的背景も紹介したい。

本講演では、軸対称非圧縮Euler方程式の瞬間爆発についての結果を報告する。より具体的には、$C^{2,\alpha}$ ($0<\alpha<1$)に入る初期速度場に対応する解が、任意の$T$における$C^1([0,T):C^2)$には入らないという定理を紹介する。定理の証明には、特異積分作用素の$L^\infty$-非有界性は一切使わず、代わりにFrenet-Serret formulasやorthonormal moving frameといった幾何学的概念を本質的に使う。時間があれば、この洞察の物理的背景も紹介したい。

### 2016年12月20日(火)

10:30-11:30 数理科学研究科棟(駒場) 056号室

On the stability of the 3D Couette Flow (English)

**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.

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

De Giorgi conjecture and minimal surfaces for integro-differential operators (English)

**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.

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

Global solutions to the second boundary value problem of the prescribed affine mean curvature and Abreu's equations (English)

**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.

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

通常の開催曜日、会場と異なります。

The Navier-Stokes equations: stationary existence, conditional regularity, and self-similar singularities (English)

https://www.math.lsu.edu/~pcnguyen/

通常の開催曜日、会場と異なります。

**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 ]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.

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

通常の開催時間と異なります。

Special cases of the planar least gradient problem (English)

通常の開催時間と異なります。

**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.

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

通常の開催時間と異なります。

Global Strong $L^p$ Well-Posedness of the 3D Primitive Equations (English)

通常の開催時間と異なります。

**Amru Hussein 氏**(TU Darmstadt)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.

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

通常の開催時間と異なります。

The role of convection in some Keller-Segel models (English)

通常の開催時間と異なります。

**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.

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

通常の開催時間と異なります。

The total variation flow in $H^{−s}$ (English)

通常の開催時間と異なります。

**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.

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

通常の曜日、時刻と異なります。

Fourier transform versus Hilbert transform (English)

http://u.math.biu.ac.il/~liflyand/

通常の曜日、時刻と異なります。

**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 ]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.

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

通常の曜日、時刻と異なります。

Fluids, vortex membranes, and skew-mean-curvature flows (English)

通常の曜日、時刻と異なります。

**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.

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

Hamilton-Jacobi equations for optimal control on 2-dimensional junction (English)

**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.

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

Well-posedness and stability of the full Ericksen-Leslie system for incompressible nematic liquid crystal flows

**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.

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

Nonlocal self-improving properties (English)

**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.

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

Viscosity solutions of Hamilton-Jacobi equations from a dynamical viewpoint (English)

**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.

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)

**山田澄生 氏**(学習院大学)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).

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

Maximum Principle and generalized principal eigenvalue for degenerate elliptic operators (English)

**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.

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

Global regular solutions to the Navier-Stokes equations which remain close to the two-dimensional solutions (English)

**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.

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.