Boundary value problem for PDEs and some clases of L^p bounded pseudodifferential operators

In recent years much attention has been extended in the study of differential equations of non-classical types. These articles need, on one hand, fluid mechanics, hydro-and gas dynamics and other applied disciplines, and on the other hand, the actual needs of the mathematical sciences. One of the most important classes of equations of non-classical type is the third-order equation with multiple characteristics which is a generalization of linear Korteweg-de Vries-Burgers equation, special cases which occur in the dissemination of waves in weakly dispersive media, the propagation of waves in a cold plasma, magneto-hydrodynamics, problems of nonlinear acoustics, the hydrodynamic theory of space plasma. A pioneering work in the theory of odd order partial differential equations with multiple characteristics was done by E.Del Vecchio, H.Block, in which they studied the technique of constructing fundamental solutions of these equations. Consequently, the theory of equations with multiple characteristics has been greatly developed by the Italian mathematician L.Cattabriga. In the first part of Ph.D thesis we develop and study boundary value problems for third-order equations with multiple characteristics in areas with curved boundaries, as well as some properties of the fundamental solutions of the equations, when the transition line is a curve. In addition, we construct a solution of the Cauchy problem in the classes of functions growing at infinity, depending on the behaviour of the right-hand side of the equation. Our thesis explores both linear and nonlinear boundary value problems for linear and non-linear third-order equation with multiple characteristics in the domain with curved boundaries. The main result of the first chapter is to prove the unique solvability of the general boundary value problem for the third-order equation with multiple characteristics in curved domains. The proof of the uniqueness theorem of the solution, we use the method of energy integrals. For the existence theorem, we find equivalent systems of Volterra second type integral equations. The next chapter consists of three sections and it investigates the problem with nonlinear boundary conditions for linear and non-linear equations of the third order with multiple characteristics. To prove the existence and uniqueness theorems, we will use methods of integral energy and theory of integral equations. In the last part of the thesis we analyze basic properties of pseudodifferential operators, such as the behaviour of products and adjoins of such operators, their continuity on L^2, L^p and Sobolev spaces. In the thesis we study the L^p - boundedness of vector weighted pseudodifferential operators with symbols which have derivatives with respect to x only up to order k, in the Holder continuous sense.