On two-point boundary-value problems for the Sturm–Liouville and Dirac operators

The problems of completeness and basic property of systems of eigenfunctions and root functions are important questions of the spectral theory of non-self-adjoint differential operators with discrete spectra. In this paper, we give a brief survey of results on this topic for the Sturm–Liouville and Dirac operators with arbitrary two-point boundary conditions and arbitrary complex-valued summable potentials.