On Divergence of Spectral Expansions Corresponding to 2 × 2‐Dirac‐Type Operator

The paper is concerned with spectral problems for the Dirac‐type operator with arbitrary two‐point boundary conditions and complex‐valued potential summable over a finite interval. It is proved that under certain conditions the root function system of the considered operator with degenerate boundary conditions does not form a basis. We also establish a necessary condition of convergence of the corresponding spectral expansions.