This monograph provides a rigorous analysis of a wide range of stationary (steady state) boundary value problems for elliptic systems of Stokes and Navier-Stokes type, as encountered in fluid dynamics. Addressing Dirichlet, Neumann, Robin, mixed, and transmission problems in both the isotropic and anisotropic cases, it makes systematic use of the notion of relaxed ellipticity recently introduced by the authors. The problems are treated in Lipschitz domains in the Euclidean setting as well as in compact Riemannian manifolds and in manifolds with cylindrical ends (non-compact manifolds), with given data in a variety of spaces - Lebesgue, standard or weighted Sobolev, Bessel potential, and Besov. A detailed and comprehensive study is provided of the main mathematical properties of boundary value problems related to the Navier-Stokes equations with variable coefficients, such as existence, uniqueness, and regularity of solutions. These are considered in bounded, periodic, and also unbounded domains, in the Euclidean setting as well as on manifolds (compact, or non-compact). The included results represent the authors' contributions to the field of stationary Stokes, Navier-Stokes, and related equations, the main novelty being the analysis of the related boundary problems with anisotropic variable coefficients and on manifolds.

The book is aimed at researchers, graduate and advanced undergraduate mathematics students, physicists, and computational engineers interested in mathematical fluid mechanics, partial differential equations, and geometric analysis. The prerequisites include the basics of partial differential equations, the variational approach and function spaces; some sections need the fundamentals of integral equations, the theory of Riemannian manifolds, and fixed-point techniques.

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**Mirela Kohr is a Full Professor in the Department of Mathematics at Babes-Bolyai University, Cluj-Napoca, Romania. She has an extensive list of publications on various areas of fluid mechanics, PDEs (especially Navier-Stokes), complex analysis, and geometric analysis. She has been the principal investigator on six research grants, an invited speaker to many conferences, and has made several research visits to the University of Toronto, and various universities in Italy, Germany, the UK, Japan and France. Sergey E. Mikhailov is a Full Professor in Applied Mathematics and Analysis at Brunel University, London, UK. He has numerous publications on the analysis and numerics of boundary integral equations, partial differential (especially Navier-Stokes) equations and theoretical solid mechanics. He graduated from the Moscow Institute of Physics and Technology and worked in Moscow before moving to the University of Stuttgart, Germany, in 1993, as a Humboldt Fellow, and then to the UK. Victor Nistor is a Full Professor in the Department of Mathematics at Lorraine University, Metz, France. He has an extensive list of publications covering various areas of partial differential equations, geometric analysis, applied mathematics, and operator algebras. He was an NSF Young Investigator and a Sloan Fellow. Wolfgang L. Wendland studied Mathematics and Fluid Mechanics at TU Berlin. He is Professor Emeritus Dr.-Ing. at University of Stuttgart, where he was a Full Professor of Applied Mathematics (1986-2005). He was a Full Professor of Mathematics at TH Darmstadt (1970-1986), Uni del Chair Prof. at the University of Delaware, USA (1973-1974) and J.G. Herder Professor at Babes-Bolyai University in Cluj, Romania (2005 and 2007). He has numerous publications on analysis and numerical methods for PDEs and boundary integral equations.