We consider the nonself-adjoint Sturm-Liouville operator with and either periodic or antiperiodic boundary conditions. We obtain necessary and sufficient conditions for systems of root functions of these operators to be a Riesz basis in in terms of the Fourier coefficients of .
"On the Riesz Basisness of Systems Composed of Root Functions of Periodic Boundary Value Problems." Abstr. Appl. Anal. 2015 1 - 7, 2015. https://doi.org/10.1155/2015/945049