Elliptic problems with nonclassical boundary conditions in an extended Sobolev scale

Abstract: We consider elliptic problems with nonclassical boundary conditions that contain additional unknown functions on the border of the domain of the elliptic equation and also contain boundary operators of higher orders with respect to the order of this equation. We investigate the solvability of the indicated problems and properties of their solutions in an extended Sobolev scale. It consists of Hilbert generalized Sobolev spaces for which the order of regularity is a general radial function $\mathrm{RO}$-varying in the sense of Avakumovi\'{c} at infinity. We establish a theorem on the Fredholm property of the indicated problems on appropriate pairs of these spaces and theorems on the regularity and a priori estimate of the generalized solutions to the problems. We obtain exact sufficient conditions for components of these solutions to be continuously differentiable.