Harmonic Analysis and Geometric Partial Differential Equations

调和分析与几何偏微分方程

• 批准号: 0202139
• 负责人: Andrea Nahmod
• 金额: $ 10.2万
• 依托单位: University of Massachusetts Amherst
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-07-01 至 2006-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0202139&HistoricalAwards=false
• 关键词: Harmonic; Analysis; Geometric; Partial; Differential

Nahmod's research lies in the overlap of harmonic analysis, geometry and partial differential equations. It aims at studying the behavior of nonlinear waves arising in geometry, ferromagnetism and gauge field theories; and that of functions along vector fields whose integral curves lack sufficient curvature. In the first part the focus is in geometric partial differential equations. Of special interest are Schroedinger maps, Wave maps and other gauge field theories such as the Yang Mills equationsin Minkowski space-time. All of these equations model `wave like phenomena'. Their solutions arise as minimizers of the corresponding energy functionals. To conform with natural physical situations, it is of interest to study their existence, uniqueness under minimal regularity assumptions. These are difficult issues because the nonlinearities of these equations involve not just the solutions but also their derivatives. Nahmod will address these questions and plans to show that in the scale invariant set up solutions to the Cauchy initial value problem exist globally provided that the data is sufficiently small when measured relative to the critical regularity norm. She also plans stability issues; e.g. whether such a system remains close to its initial state as time evolves when the data has small energy. From a physical viewpoint the latter models whether such systems are close to equilibrium. The techniques exploit geometric aspects of these equations to extract crucial information -such as special structures in the nonlinearity- which is then used in the analysis. The method combines deep Fourier analysis with gauge theoretic geometric tools. The goal of the second part is the study of the Hilbert transform along vector fields and its associated maximal operator in two dimensions. Their treatment departs from the classical study of singular integrals for in the present situation, the singularity lives on some variety that is changing at each point. Nahmod will investigate how to develop time frequency techniques to study operators under no curvature assumptions. This is the case, for example, in studying differentiability properties of functions along vector fields. Partial differential equations are the mathematical models to the laws governing much of the phenomena in our physical world. The wave equation models the propagation of different kind of waves -such as light waves- in homogeneous media. Nonlinear models of conservative type arise in quantum mechanics while other variants appear for example in the study of vibrating systems and semiconductors. The nonlinear Schroedinger equation arises in various physical contexts in the description of nonlinear waves- such as propagation of a laser beam in a medium whose index of refraction is sensitive to the wave amplitude, water waves at the free surface of an ideal fluid as well as in plasma waves. Some of the interesting questions are those about local and global existence of solutions, uniqueness as well as long time behavior of global solutions. The role of mathematical analysis is to understand the behavior of the solutions to these equations, provide the tools to extract their quantitative and qualitative information and lay the foundations upon which methods to accurately approximate the solutions are developed. Fourier analysis and more generalized adapted frequency decompositions such as time-frequency analysis' consists in decomposing complex objects via `modulated waveforms' into basic building blocks which are localized and easy to understand, and then piecing them back together in a straightforward manner. It works very similarly to a musical score. The modulated waveforms have four attributes: amplitude (loudness), scale (duration), frequency (pitch) and position (instant it is played). The objects could be speech, radar signals, as well as oscillatory expressions arising in optics, AC ousting scattering, wave propagation and other phenomena of nonlocal nature.

Nahmod的研究在于调和分析，几何和偏微分方程的重叠。它的目的是研究几何学、铁磁性和规范场论中产生的非线性波的行为，以及积分曲线缺乏足够曲率的沿着向量场的函数的行为。在第一部分的重点是在几何偏微分方程。特别感兴趣的是薛定谔映射，波映射和其他规范场理论，如闵可夫斯基时空中的杨米尔斯方程。所有这些方程都模拟了“类波现象”。他们的解决方案出现作为相应的能量泛函的极小。为了符合自然物理的实际情况，在极小正则性假设下研究它们的存在性、唯一性是很有意义的。这些都是困难的问题，因为这些方程的非线性不仅涉及解决方案，而且还涉及其衍生物。Nahmod将解决这些问题，并计划表明，在规模不变设置柯西初值问题的解决方案存在全球性的数据是足够小的时候，测量相对于临界正则性规范。她还计划稳定性问题;例如，当数据具有小能量时，随着时间的推移，这样的系统是否保持接近其初始状态。从物理学的角度来看，后者模拟这样的系统是否接近平衡。这些技术利用这些方程的几何方面来提取关键信息-例如非线性中的特殊结构-然后将其用于分析。该方法结合了深度傅里叶分析和规范理论的几何工具。第二部分的目标是研究二维向量场的沿着希尔伯特变换及其相关的极大算子。他们的治疗背离了经典的研究奇异积分在目前的情况下，奇异性生活在一些品种，是在每一点的变化。Nahmod将研究如何开发时频技术，以研究无曲率假设下的算子。例如，在研究沿沿着向量场的函数的可微性时就是这种情况。偏微分方程是我们物理世界中许多现象的数学模型。波动方程模拟了不同种类的波（如光波）在均匀介质中的传播。保守型非线性模型出现在量子力学中，而其他变体出现在例如振动系统和半导体的研究中。非线性薛定谔方程在描述非线性波的各种物理背景中出现-例如激光束在折射率对波振幅敏感的介质中的传播，理想流体自由表面处的水波以及等离子体波。一些有趣的问题是关于局部和整体解的存在性，唯一性以及长期行为的整体解决方案。数学分析的作用是理解这些方程的解的行为，提供提取其定量和定性信息的工具，并为开发精确近似解的方法奠定基础。傅立叶分析和更广义的适应频率分解，如时间-频率分析，在于通过“调制波形”将复杂对象分解为局部化和易于理解的基本构件，然后以直接的方式将它们拼凑在一起。它的工作原理与乐谱非常相似。调制波形有四个属性：振幅（响度）、音阶（持续时间）、频率（音高）和位置（播放的瞬间）。对象可以是语音，雷达信号，以及光学，AC驱逐散射，波传播和其他非局部性质的现象中产生的振荡表达式。