Solvability of nonlocal elliptic problems in Sobolev spaces

We study elliptic equations of order 2m with nonlocal boundary-value conditions in plane angles and in bounded domains, dealing with the case in which the support of nonlocal terms intersects the boundary. We establish necessary and sufficient conditions under which nonlocal problems are Fredholm in Sobolev spaces and in weighted spaces with small weight exponents, respectively. We also obtain an asymptotics of solutions of nonlocal problems near the conjugation points on the boundary, where solutions can have power singularities.