Spectral Properties of Nonself-Adjoint Nonlocal Boundary-Value Problems for the Operator of Differentiation of Even Order

We study spectral properties of an essentially nonself-adjoint problem generated by nonlocal multipoint conditions for the operator of differentiation of order 2n and analyze the cases of regular and irregular Birkhoff two-point boundary conditions. A system of root functions of the problem and elements of biorthogonal systems are constructed. We also establish sufficient conditions under which these systems are complete and conditions under which they form a Riesz basis (under certain additional assumptions).