The project is concerned with development of the mathematical theory for several fundamental inverse problems arising in science and technology, as well as with achieving advances in significant questions of spectral theory coming from problems of electromagnetism and quantum mechanics. Broadly speaking, in an inverse problem, one wishes to determine internal properties of a medium by performing measurements along the boundary of the medium. For instance, in electrical impedance tomography, one attempts to recover the conductivity of a body by making voltage and current measurements at the boundary. Since inverse problems are at the core of a variety of engineering and scientific investigations, including medical imaging, seismography, oil prospection, radar imaging, and non-destructive testing, any further progress in the mathematical theory of such problems will undoubtedly have real world applications. Spectral theory deals with the investigation of vibrations and their frequencies for a variety of different objects, ranging from atoms and molecules in chemistry to obstacles in acoustic waveguides. The fundamental issues, which are of great significance in many problems of science and engineering, from celestial to quantum mechanics, include deciding when such vibrations occur, how to go about computing their frequencies, as well as understanding the size and localization of the vibrations. The aim of the project is to advance our understanding of these issues by concentrating on model problems of quantum mechanics, specifically in the physically relevant regime of high frequencies.
The project addresses the following significant topics: the mathematical theory of inverse boundary problems for elliptic partial differential equations (PDE), harmonic analysis for elliptic PDE, and spectral theory of elliptic PDE with periodic coefficients. Although these topics have originated in distinct mathematical communities, recent work has shown that techniques and insights in the various topics are closely related and interact in a fruitful way. A novel idea of the project is to expand this interaction to solve significant problems in all of these areas. Despite an impressive body of results in the field of inverse problems obtained within the last 30 years, many fundamental questions still remain unsolved, including inverse problems for PDE with irregular coefficients, partial data problems when measurements are performed only on a portion of the boundary, and inverse problems on manifolds. The goal of the first part of the project is to attack these problems for several fundamental elliptic PDE, in particular the conductivity, magnetic Schroedinger, and polyharmonic equations, as well as the Maxwell system. The second topic is concerned with estimates for resolvents of elliptic operators on compact and non-compact manifolds, in Lebesgue spaces. Apart from their intrinsic significance in spectral and scattering theory, such estimates are crucial in control theory and inverse problems. The aim here is to understand how the dynamics of the underlying Hamilton flow of the operator and regularity of the coefficients each impacts on the spectral estimates. The third topic deals with the spectral theory of Schroedinger type operators with periodic coefficients, coming from solid state physics. A central question concerns the nature of the spectra of such operators, which one conjectures to be purely absolutely continuous. Long known in the Euclidean case, it is still wide open for general Laplace-Beltrami type operators. The objective is to resolve this conjecture in several significant special cases of Riemannian metrics.
