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 Fractional order Sobolev spaces for the Neumann Laplacian and the vector Laplacian J. Korean Math. Soc.Published online October 2, 2019 Seungil Kim Department of Mathematics, Kyung Hee University Abstract : In this paper we study fractional Sobolev spaces characterized by a norm based on eigenfunction expansions. The goal of this paper is twofold. The first one is to define fractional Sobolev spaces of order $-1\le s\le 2$ equipped with a norm defined in terms of Neumann eigenfunction expansions. Due to the zero Neumann trace of Neumann eigenfunctions on a boundary, fractional Sobolev spaces of order $3/2\le s\le 2$ characterized by the norm are the spaces of functions with zero Neumann trace on a boundary. The spaces equipped with the norm are useful for studying cross-sectional traces of solutions to the Helmholtz equation in waveguides with a homogeneous Neumann boundary condition. The second one is to define fractional Sobolev spaces of order $-1\le s \le 1$ for vector-valued functions in a simply-connected, bounded and smooth domain in $R^2$. These spaces are defined by a norm based on series expansions in terms of eigenfunctions of the vector Laplacian with boundary conditions of zero tangential component or zero normal component. The spaces defined by the norm are important for analyzing cross-sectional traces of time-harmonic electromagnetic fields in perfectly conducting waveguides. Keywords : Fractional Sobolev space, Neumann Laplacian, vector Laplacian, waveguide MSC numbers : 46B70, 46E35 Full-Text :