Fourier series of the $\nabla\,div$ operator and Sobolev spaces II

Abstract

The author studies structure of space $\mathbf{L}_{2}(G)$ of vectors - functions, which are integrable with a square of the module on the bounded domain $G $of three-dimensional space with smooth boundary, and role of the gradient of divergence and curl operators in construction of bases in its orthogonal subspaces $\mathcal{A}$ and $\mathcal{B}$. The ${\mathcal{A}}$ and ${\mathcal{B}}$ are contain subspaces ${\mathcal{A}_{\gamma}}(G)\subset{\mathcal{A}}$ and $\mathbf{V}^{0}(G)\subset{\mathcal{B}}$. The gradient of divergence and a curl operators have continuations in these subspaces, their expansion $\mathcal{N}_d$ and $S$ are selfadjoint and convertible,and their inverse operators $\mathcal{N}_{d}^{-1}$ and $S^{-1}$ are compact. In each of these subspaces we build ortonormal basis. Uniting these bases, we receive complete ortonormal basis of whole space $\mathbf{L}_{2}(G)$, made from eigenfunctions of the gradient of divergence and curl operators . In a case, when the domain $G$ is a ball $B$, basic functions are defined by elementary functions. The spaces $\mathcal{A}^{s}_{\mathcal{K}}(B)$ are defined. Is proved, that condition $ \mathbf{v}\in\mathcal{A}^{s}_{\mathcal {K}}(B)$ is necessary and sufficient for convergence of its Fourier series (on eigenfunctions of a gradient of divergence)in norm of Sobolev space $ \mathbf{H}^{s}(B)$. Using Fourier series of functions $ \mathbf{f}$ and $ \mathbf{u}$, the author investigates solvability(in spaces $\mathbf{H}^{s}(G)$)boundary value problem: $\nabla\mathbf{div}\mathbf{u}+ \lambda\mathbf{u}=\mathbf{f}$ in $G$, $\mathbf{n}\cdot\mathbf{u}|_{\Gamma}=g$ on boundary, under condition of $\lambda\neq0$. In a ball $B$ a boundary value problem: $\nabla\mathbf{div}\mathbf{u}+ \lambda\mathbf{u}=\mathbf{f}$ in $B$, $\mathbf{n}\cdot\mathbf{u}|_S=0$, is solved completely and for any $\lambda$.