1. V. A. Ilyin, ‘‘On a connection between the form of the boundary conditions and the basis property and the property of equiconvergence with a trigonometric series of expansions in root functions of a nonselfadjoint differential operator,’’ Differ. Equat. 30, 1402–1403 (1994).
2. A. S. Makin, ‘‘On a nonlocal perturbation of a periodic eigenvalue problem,’’ Differ. Equat. 42, 599–602 (2006).
https://doi.org/10.1134/S0012266106040185
3. M. A. Sadybekov and N. S. Imanbaev, ‘‘On the basis property of root functions of a periodic problem with an integral perturbation of the boundary condition,’’ Differ. Equat. 48, 896–900 (2012).
https://doi.org/10.1134/S0012266112060146
4. N. S. Imanbaev and M. A. Sadybekov, ‘‘Basis properties of root functions of loaded differential operators of the second order,’’ Dokl. NAN RK 2, 11–13 (2010).
5. N. S. Imanbaev, ‘‘On basis of systems of eigenfunctions of a loaded second-order differential operator with antiperiodic boundary value conditions,’’ Kazakh. Math. J. 20 (3), 48–56 (2020).