Structure of the spectrum of a nonselfadjoint Dirac operator

For the Dirac operator with two-point boundary conditions and an arbitrary complex-valued $L_2$-integrable potential $V(x)$ the spectral problem is considered. Necessary and sufficient conditions on an entire function to be the characteristic function of such a boundary value problem are obtained. Necessary and sufficient conditions on the spectrum of the above operator are established in the case when the boundary conditions are regular.
Bibliography: 16 titles.