Integro-differential equations : regularity theory and Pohozaev identities
The main topic of the thesis is the study of Elliptic PDEs. It is divided into three parts: (I) integro-differential equations, (II) stable solutions to reaction-diffusion problems, and (III) weighted isoperimetric and Sobolev inequalities. Integro-differential equations arise naturally in the study...
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| Tipo de recurso: | tesis doctoral |
| Estado: | Versión publicada |
| Fecha de publicación: | 2014 |
| País: | España |
| Institución: | CBUC, CESCA |
| Repositorio: | TDR. Tesis Doctorales en Red |
| OAI Identifier: | oai:www.tdx.cat:10803/279289 |
| Acceso en línea: | http://hdl.handle.net/10803/279289 https://dx.doi.org/10.5821/dissertation-2117-95453 |
| Access Level: | acceso abierto |
| Palabra clave: | 51 |
| Sumario: | The main topic of the thesis is the study of Elliptic PDEs. It is divided into three parts: (I) integro-differential equations, (II) stable solutions to reaction-diffusion problems, and (III) weighted isoperimetric and Sobolev inequalities. Integro-differential equations arise naturally in the study of stochastic processes with jumps, and are used in Finance, Physics, or Ecology. The most canonical example of integro-differential operator is the fractional Laplacian (the infinitesimal generator of the radially symmetric stable process). In the first Part of the thesis we find and prove the Pohozaev identity for such operator. We also obtain boundary regularity results for general integro-differential operators, as explained next. In the classical case of the Laplacian, the Pohozaev identity applies to any solution of linear or semilinear problems in bounded domains, and is a very important tool in the study of elliptic PDEs. Before our work, a Pohozaev identity for the fractional Laplacian was not known. It was not even known which form should it have, if any. In this thesis we find and establish such identity. Quite surprisingly, it involves a local boundary term, even though the operator is nonlocal. The proof of the identity requires fine boundary regularity properties of solutions, that we also establish here. Our boundary regularity results apply to fully nonlinear integro-differential equations, but they improve the best known ones even for linear ones. Our work in Part II concerns the regularity of local minimizers to some elliptic equations, a classical problem in the Calculus of Variations. More precisely, we study the regularity of stable solutions to reaction-diffusion problems in bounded domains. It is a long standing open problem to prove that all stable solutions are bounded, and thus regular, in dimensions n<10. In dimensions n>=10 there are examples of singular stable solutions. The question is still open in dimensions 4<n<10. We prove here that, in domains of double revolution, all stable solutions are bounded in dimensions n<8. Except for the radial case, our result is the first partial answer valid for all nonlinearities. While studying this, we were led to some weighted Sobolev inequalities with monomial weights that were not treated in the literature. We establish them in Part III of the thesis. Our proof of such Sobolev inequalities is based on a new weighted isoperimetric inequality in R^n. It is quite surprising that, even if the weight is not radially symmetric, Euclidean balls (centered at the origin) solve this isoperimetric problem. Also in Part III, we study more general weights, and also anisotropic perimeters. We obtain a family of new isoperimetric inequalities with homogeneous weights satisfying a concavity condition. As a particular case of our results, we provide with totally new proofs of two classical results: the Wulff inequality, and the Lions-Pacella inequality. |
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