Offering in-depth analyses of current theories and approaches related to Sobolev-type equations and systems, this reference is the first to introduce a classification of equations and systems not solvable with respect to the highest order derivative, and studies boundary-value problems for these classes of equations. Offering 2200 equations, this volume studies specific partial differential equations for applications in mechanics, physics, hydrodynamics, and oceanography and presents previously unpublished results, solvability theorems, and research. It includes mathematical results from S.L. Sobolev's study on the small oscillations of a rotating fluid and more.