Harmonic analysis, additive combinatorics and geometric measure theory

分形是表现出多尺度结构的对象，可以被分配一个不一定是整数的“维数”。它们在自然界中无处不在（植物生长、景观形成），也可以由人类活动（通信系统、基础设施网络）产生。在数学中，它们出现在数学物理或偏微分方程等领域的许多问题中，并在几何测量理论和动力系统中得到广泛研究。分形通常表现出混乱的行为，在某种意义上，通过迭代一个相对简单的复制规则构建的分形对象可以表现出非常高的复杂性，并且以很难提前预测的方式表现出来。******提出的研究的第一个目标是开发新的方法来研究这种结构，基于谐波分析和加性组合。传统上，调和分析在分形研究中的应用相对有限。我计划发展一个分形集合的奇异积分和振荡积分理论，以经典调和分析中的光滑流形的类似理论为模型（包括限制估计、最大和平均算子以及微分定理），但也纳入只出现在分形设置中的新特征。这项工作将借鉴调和分析和最近发展的加性组合学领域的见解和方法。虽然已经获得了一些基础结果，证明这是一个可行而有趣的理论，但许多进一步的问题仍未解决。特别是，分形集可以表现出谐波解析行为，这对于流形或在加性组合学中研究的离散对象都没有精确的类似物，我想研究这些新现象。******第二个目标是将这些发展应用于维度理论、几何测量理论和动力系统的问题。几何测量理论中的一个核心问题家族涉及投影、切片、相交、算术和特定维数集合的差异。特别重要的是在动力系统和数学物理中出现的分形集，如吸引子或不变集。例如，我们可以很容易地说出一个“典型的”投影或分形的切片应该是什么样子，但很难证明一个关于特定投影或切片的类似陈述。类似地，在许多情况下，定性结果很容易证明，但要获得定量估计却困难得多。谐波分析在过去的这种情况下是有用的，我希望我计划开发的新方法将导致进一步的进步

Fractals are objects that exhibit a multiscale structure and can be assigned a "number of dimensions" that is not necessarily integer. They are ubiquitous in nature (plant growth, landscape formation), and can also be produced by human activity (communication systems, infrastructure networks). In mathematics, they arise in many problems in fields such as mathematical physics or partial differential equations, and are studied extensively in geometric measure theory and dynamical systems. Fractals often display chaotic behaviour, in the sense that a fractal object constructed by iterating a relatively simple replication rule can exhibit very high levels of complexity and behave in ways that are very difficult to predict in advance. ******The first goal of the proposed research is to develop new methods of studying such structures, based on harmonic analysis and additive combinatorics. Traditionally, the use of harmonic analysis in the study of fractals has been relatively limited. I plan to develop a theory of singular and oscillatory integrals for fractal sets, modelled after the analogous theory for smooth manifolds in classical harmonic analysis (including restriction estimates, maximal and averaging operators, and differentiation theorems), but also incorporating new features that only occur in the fractal setting. This work will draw on the insights and methods from both harmonic analysis and the more recently developed field of additive combinatorics. While some foundational results have already been obtained, proving that this is a viable and interesting theory, many further questions remain open. In particular, fractal sets can exhibit harmonic-analytic behaviour that does not have exact analogues either for manifolds or for the discrete objects studied in additive combinatorics, and I would like to investigate these new phenomena.******The second goal is to apply these developments to questions in dimension theory, geometric measure theory and dynamical systems. A central family of questions in geometric measure theory concerns projections, slices, intersections, and arithmetic sums and differences of sets of specified dimensionality. Of particular significance are fractal sets that arise in dynamical systems and mathematical physics, such as attractors or invariant sets. For example, it can be very easy to say what a "typical" projection or slice of a fractal should look like, but very difficult to prove a similar statement about a specific projection or slice. Similarly, there are many situations where qualitative results are easy to prove, but it is much harder to obtain a quantitative estimate. Harmonic analysis has been useful in such contexts in the past, and I expect that the new methods I plan to develop will lead to further advances.**