On homogeneous Sobolev and Besov spaces on the whole and the half space

In this paper, we propose an elementary construction of homogeneous Sobolev spaces of fractional order on R n and R n + in the scope of the treatment of non-linear partial differential equations.This construction extends the construction of homogeneous Besov spaces on S ′ h (R n ) started by Bahouri, Chemin and Danchin on R n .We will also extend the treatment done by Danchin and Mucha on R n + , and the construction of homogeneous Sobolev spaces of integer orders started by Danchin, Hieber, Mucha and Tolksdorf on R n and R n + .Properties of real and complex interpolation, duality, and density are discussed.Trace results are also reviewed.Our approach relies mostly on interpolation theory and yields simpler proofs of some already known results in the case of Besov spaces.The lack of completeness for our function spaces with high regularities will lead to the consideration of the intersection with a complete space to enforce the behavior of low frequencies.From this point one performs decoupled estimates, in order to obtain results similar to the one obtained in the case of low regularities.As standard and simple applications, we treat the problems of Dirichlet and Neumann Laplacians in these homogeneous function spaces.